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Effect of hot electrons on the polar wind
Abstract
A semikinetic model is used to describe the steady state collisionless flow of H(+), O(+), and electrons along diverging geomagnetic field lines in the high-latitude topside ionosphere. The effect that hot electron populations have on the polar wind is emphasized. Several such populations are considered, including the polar rain, polar showers, and polar squall. Hot electron densities and temperatures are calculated from the characteristic energy and flux measurements. The results indicate that the hot/cold electron temperature ratio varies from 10 to 10,000 and that the hot/cold electron density ratio varies from 0.001 to 0.1 at the baropause. For higher hot electron temperatures and a greater percentage of hot electrons, there is a discontinuity in the kinetic solution, which indicates the presence of a sharp transition corresponding to a contact surface between the hot and cold electrons. Along this surface, a double-layer potential barrier exists which reflects the cold ionospheric electrons and prevents their penetrations to higher altitudes.
... [4] On the theoretical investigation side, the polar cap density distributions have been modeled by using hydrodynamic, generalized transport, and kinetic polar wind models [Moore et al., 1999;Ganguli, 1996, and references therein]. Most of those models have predicted a crossover from O + dominant to H + dominant at altitudes of 3000-7500 km with a very low O + density at high altitudes in the polar cap [e.g., Banks and Holzer, 1969;Lemaire and Scherer, 1973;Barakat and Schunk, 1984;Mitchell et al., 1992]. This is because the ambipolar electric field alone is usually not strong enough to overcome the gravitational barrier for escape of the O + ions. ...
... It is noted that in the simulations we used relatively high electron temperature at the upper boundary, T e? = 14,000 K and the ratio T e? /T ek = 2. This gave rise to a total electron temperature of about 11,600 K at the upper boundary, which at a rudimentary level introduced the possible polar rain/shower effects in a manner somewhat similar to the treatment of Ho et al. [1992] and Barakat and Schunk [1984]. Yet the effects of high electron temperature alone are not as significant as the cleft ion fountain, as suggested by Figure 8. ...
... Furthermore, even if the polar rain effects are incorporated in a rigorous manner, the present simulations results will not be significantly affected since the polar rain effects have been shown to be only effective at altitudes above $3-4 R E [Barakat and Schunk, 1984]. Nevertheless, more investigations are needed to clarify the relative importance of the other local processes with detailed information available. ...
... This has been demonstrated by numerous simulations (e.g., see review by Yamada 2007) as well as in laboratory experiments (Ono et al. 1997; Ji et al. 2004; Egedal et al. 2007). When such heated electrons disperse outside the localized diffusion region in the kinetic regime of reconnection and mix with the cold chromospheric plasma and expand upward in the highly stratified TR toward the corona, space charge separation could result in a double layer (DL), as demonstrated theoretically (Morse & Nielson 1973; Bezzerides et al. 1978; Ahedo & Sa'nchez 2009), experimentally (Diebold et al. 1987; Hairapetian & Stenzel 1991), and by modeling and simulations (Barakat & Schunk 1984; Ergun et al. 2000; Singh & Khazanov 2003; Singh et al. 2005; Singh 2011). Figure 1schematically shows a scenario involving the reconnection between the emerging and existing open magnetic fields (Moore et al. 2011). ...
... Bezzerides et al. (1978) advanced the theory, giving the specific conditions for DL formation. It has been studied in laboratory experiments (Diebold et al. 1987; Hairapetian & Stenzel 1991) and applied to explain the outflow of polar wind from the Earthʼs ionosphere (Barakat & Schunk 1984) and acceleration of ions from the ionosphere into the auroral density cavities (Ergun et al. 2000; Singh 2011). Here we apply the same physics for the supply of plasma from the chromosphere to the corona across the TR. ...
... The DL forms with sharp transitions in both density and electric potential when the temperature condition in Equation (2) is met, namely, T h > 10T c . The conditions in Equation (2) for DL formation (Bezzerides et al. 1978) have been verified with theoretical study (Ahedo & Sa'nchez 2009), geophysical-scale modeling (Barakat & Schunk 1984; Ergun et al. 2000), small-scale kinetic simulations (Singh & Khazanov 2003; Singh et al. 2005), and also by lab experiments (Hairapetian & Stenzel 1991), as summarized in a review article (Singh 2011). We briefly discuss results from laboratory experiments showing density and electric potential structures of the DL (Hairapetian & Stenzel 1991). ...
A novel mechanism for the supply of hot plasma into the corona from the chromosphere is suggested here; the mechanism involves collisionless magnetic reconnection (CMR) in the transition region (TR) followed by double layer (DL) formation in the enhanced expansion of the chromospheric cold plasma mixed with CMR-heated hot electrons. It is well known that (i) the CMR produces energetic electrons and (ii) DLs naturally form in expanding dense plasmas containing a minor population of hot electrons. We apply these plasma physics facts to the dynamics of stratified plasma in the TR. In the TR where densities fall below ∼1016 m-3, all collisional mean-free paths, electron-ion, ion-neutral, and electron-neutral, become long enough to render plasma collisionless at kinetic scale lengths, making CMR and DL formation possible. The DLs accelerate the chromospheric cold ions to energies comparable to the energy of the hot electrons. When the upflowing energized ions neutralized by the escaping hot electrons thermalize, the resulting hot tenuous plasma supplies an energy flux ∼3 × 105 erg cm-2 s-1 = 3 × 102 J m-2 s-1 into the corona. The CMR-DL mechanism introduces sudden transitions in the TR as microstructures in both density and energy. The global transition in the TR could be a fractal structure containing such microscopic features. If not impossible, it is difficult to measure such microstructures, but it seems that the coronal heating begins in the nearly collisionless TR by CMR and DL formation. © 2015. The American Astronomical Society. All rights reserved..
... We discuss here the LASPE in a coronal funnels having the scenarios of both bulk and tail heated electrons. We first highlight the features of LASPE in a diverging flux tube using prior studies on the acceleration of both light and heavy ions in the terrestrial polar wind (Barakat & Schunk 1983, 1984) followed by a discussion on coronal funnels. ...
... During the 1980s it emerged that LASPE with a hot/warm electron population could be an effective mechanism for the ion acceleration in the terrestrial polar wind (Singh & Schunk 1982; Barakat & Schunk 1983, 1984). Barakat & Schunk (1983) modeled the outflow of H + and O + ions from an ionospheric boundary at a geocentric distance r = r o = 10.8 ...
... The increased T e enhances the ambipolar electric fields (Equation (1)), which accelerate the heavy ions upward against the gravity. In another study Barakat & Schunk (1984) modeled the effects of a minor hot electron population on the outflow of ions, instead of the bulk heated electrons as discussed above. In this model, the densities of the cold (n c ) and hot (n h ) electrons are given by n c = n co exp(eϕ/KT ec ); n h = n ho exp(eϕ/KT eh ), (2) the total electron density n e = n c + n h = n i , the total ion density. ...
Tu et al. measured outflow of Ne7+ at a height of 20 Mm above the photosphere demonstrating that the solar wind originates in coronal funnels footed in the chromospheric magnetic networks. We suggest that when the bottom of a coronal flux tube is populated by the chromospheric plasma consisting of protons as the major ions, Ne7+ as minor ions, and heated electrons in the magnetic networks, large-scale plasma expansion could accelerate the Ne7+ ions to a velocity ~10 km s–1 at a height ~ 20 Mm as measured. Two scenarios are discussed here, one with the bulk heating of the electrons to a temperature T
e > 64 eV and another with a small fraction of the electrons heated to high temperatures T
eh > 158 eV, appearing as an energetic tail to the electrons' velocity distribution function. In the former scenario, the expansion produces weak ambipolar electric fields distributed along the entire length of the funnel. In the latter one the electric field is concentrated in a double layer. The electric fields accelerate the ions against the solar gravity. The required electron temperatures in the above scenarios are large enough to ionize neon atoms yielding Ne7+.
... Lemaire and Scherer (1978) suggested that this effect could lead to a double layer above the ionosphere. The formation of a double layer was also reported in simulations of Barakat and Schunk (1984). They have shown that where the hot electron density is substantial and the energy is of the order of several ke V, there exists a discontinuity in the kinetic solution. ...
... This discontinuity corresponds to a contact surface between the hot and cold electrons, at which a double-layer potential barrier reflects the cold ionospheric electrons and prevents their escape. Barakat and Schunk (1984) have also shown that if the hot/cold electron temperature ratio is small, the polar wind solutions are similar to those obtained previously without hot electrons. When the temperature ratio is high, the supersonic H+ and 0+ ion outflow velocities are increased on passage through the surface. ...
... 2. The collisionless region where the individual-ion characteristics dominate the ion motion. Different models were used to study the behavior of H in the that region, hydromagnetic (4) , kinetic (5)(6)(7) and semi kinetic (8)(9) . The above two regions are separated by a narrow transition layer (10)(11)(12) . ...
In view of the fact that Feminine “Ha'a” has a vast prescence in many language issues, this research was intended to display that prescence in a single issue i.e., plural. It was found that Ha’a has an apparent role and a meaningful value in plural, and many of language concepts which are related to plural are connected with it and may be made uninflected in case of its insertion or not.
... [4] The major proposed mechanisms for driving these upflows include (1) frictional heating caused by convection that causes plasma expansion and outflow [e.g., Heelis et al., 1993], (2) ionospheric electron temperature enhancement with the consequent increased upward ambipolar electric field [e.g., Whitteker, 1977], (3) convection shear-driven ion instabilities that can induce heating [Ganguli et al., 1994], and (4) ring current ion precipitation [Yeh and Foster, 1990]. In addition, for high-altitude polar wind and ion outflow effects involving hot polar rain electrons, soft electron precipitation, topside ion heating, and photoelectrons have also been considered [Barakat and Schunk, 1984;Li et al., 1988;Tam et al., 1995;Seo et al., 1997]. ...
We have examined the vertical ion flux in the topside high-latitude
ionosphere from measurements of the vertical ion drift and ion number
density made by the DMSP F13 satellite. We found that the average
vertical flux over the entire high-latitude region near 800 km altitude
is quite small. This suggests that most of the vertical ion flux is
associated with a large-scale ``breathing'' of the upper ionosphere. In
the polar cap the vertical ion flux is uniformly downward at all
locations. However, in the auroral zone the ion flux is highly
structured, and a net upward flux is produced only by spatially and
temporally confined events containing upward fluxes in excess of
1010 cm-2 s-1 that have no downward
counterparts.
... Several ''non-classical'' mechanisms were investigated in the second phase. The elevated electron temperature and photoelectrons were found to help significant fluxes of the ionospheric O + to reach the magnetosphere [Barakat and Schunk, 1983, 1984; Lemaire, 1972]. A double layer was shown to form due to the presence of energetic electrons of magnetospheric origin (such as polar rain) or ionospheric origin (photoelectrons) in addition to the thermal ionospheric electrons. ...
The effects of low-altitude energization (LAE) of ions on the dynamic behavior of the high-latitude plasma was investigated using a macroscopic particle-in-cell (mac-PIC) model. The model simulates the behavior of a plasma-filled flux tube as it drifts across the different high-latitude regions (cusp, polar cap, auroral, and subauroral regions). In addition to the LAE, the model properly accounts for gravity, electrostatic field, magnetic mirror force, ion-ion collisions, wave-particle interactions, magnetospheric electrons, and centrifugal acceleration. However, the focus here is on the effects of the LAE and their seasonal dependence. The LAE was emulated by uniform energization of the ions in the perpendicular direction as they pass through a narrow domain (200 km in altitude) that is embedded within the cusp/auroral oval region. The roles that season, solar activity, and the altitude of the LAE play, with regard to the effects of the LAE on the plasma characteristics, were studied. In particular, several simulation runs were performed for different seasons (summer/winter), for different solar activity levels, and for different altitudes of the LAE region. Comparing the results from these runs, the following conclusions can be drawn: (1) When the LAE occurs at high altitudes, where less O+ exists, it does not appreciably enhance the O+ escape flux. The O+ escape flux for LAE occurring above ~{3000} km is almost identical to the case with no LAE; (2) In the absence of LAE, the dominant source of escaping O+ occurs in the polar cap due to magnetospheric electrons; (3) Both upward and downward O+ fluxes occur at low altitudes, while only upward O+ fluxes occur at high altitudes; (4) As the plasma drifts from the polar cap into the auroral region, it is (first) depleted due to the rapid energization associated with wave-particle interactions (WPI) and then it is slowly replenished due to the effect of the LAE; (5) In general, the cases of summer-solar maximum and winter-solar minimum produce the two extreme results, while the other two cases (summer-solar minimum and winter-solar maximum) produce intermediate results. For example, the largest O+ escape fluxes were found for the case of (summer-solar maximum) and the smallest fluxes were found for the case of (winter-solar minimum).
... In the last ten years, several non-classical effects were included in order to study their effect on the plasma outflow. The escape flux of O + was found to be enhanced due to the effect of high electron temperature (Barakat and Schunk, 1983), high ion temperature (Li et al., 1988), and energetic magnetospheric electrons (Barakat and Schunk, 1984). The effects of ion-acceleration at high altitudes, and of chemical and collisional H + − O + coupling, on the composition of the ion escape flux were investigated by . ...
The energization of charged particles, due to interaction with the ambient electromagnetic turbulence, has a significant influence on the plasma transport in space. The effect of wave-particle interactions on the outflow characteristics of polar wind plasma was investigated. The theoretical model included gravitational acceleration (g), polarization electrostatic field (Ep), and divergence of the geomagnetic field. Within the simulation region (1.7 to 10 earth radii, Re) the ions were assume to be collisionless and the electrons to obey a Boltzmann relation. Profiles of altitude-dependent diffusion coefficients [D⊥ (O+) and D⊥ (H+)] were computed from the wave spectral density (S) observed by the Plasma Wave Instrument (PWI) on board DE-1. The effects of WPI were introduced via a Monte Carlo technique, and an iterative approach was used in order to converge to self-consistent results. The main conclusions of this study were the following. As a result of perpendicular heating, the temperature anisotropy (T| /T⊥) was reduced and even reversed (T| < T⊥) at high altitudes. The O+ velocity distribution function developed a conic shape at high altitudes. The altitudes above which the WPI influences the O+ ions were lower than those for the H+ ions. The escape flux of O+ could be enhanced by more than an order of magnitude while the H+ flux remains constant. The O+ ions are heated more efficiently than the H+ ions, especially at low altitudes due to the 'pressure cooker' effect. As the ions are heated and move to higher altitudes, the ion's Larmor radius a
L
may become comparable to the perpendicular wavelength λ⊥. As the ratio aL /λ⊥ becomes > 1, the heating rate becomes self-limited and the ion distribution displays toroidal features. This result is consistent with the observation of O+ toroidal distribution in the high altitude ionosphere. Finally, the large variability in the wave spectral density S was studied. This variability was found to change our results only in a quantitative manner, while our conclusions remained qualitatively unchanged.
... In addition, high-altitude polar wind and ion outflow can possibly be driven by effects involving hot polar rain electrons, soft electron precipitation, topside ion heating, and photoelectrons. [Barakat and Schunk, 1984;Wilson et al., 1988;Tam et al., 1995;Seo et al., 1997]. A good overview of possible processes is provided by André and Yau [1997]. ...
We have examined characteristics of the vertical O+ flux in the topside high-latitude ionosphere from measurements of the vertical ion drift and ion number density made by the Defense Meteorological Satellite Program (DMSP) F13 spacecraft from June 1996 to January 1997, June 1998 to January 1999, and June 2001 to January 2002. In the polar cap the vertical ion flux is, on average, downward at all locations. However, in the auroral zone the ion flux is highly structured, and a net upward flux is produced primarily by spatially and temporally confined events containing upward fluxes in excess of 109 cm−2s−1. These dominant high-flux events tend to be produced more commonly by high densities rather than by high velocities. The range of vertical velocity and number density changes with season and solar cycle with greater variability in the vertical velocity occurring during winter and at lower levels of solar activity. There is also evidence that the vertical fluxes are a function of the interplanetary magnetic field, with upward fluxes in the auroral zones for negative Bz and upward fluxes in the polar cap for positive Bz.
... [4] The major proposed mechanisms for driving these upflows include (1) frictional heating caused by convection that causes plasma expansion and outflow [e.g., ; (2) ionospheric electron temperature enhancement with the consequent increased upward ambipolar electric field [e.g., Whitteker, 1977]; (3) convection sheardriven ion instabilities that can induce heating [Ganguli et al., 1994;Liu and Lu, 2004]; and (4) ring current ion precipitation [Yeh and Foster, 1990]. In addition, for highaltitude polar wind and ion outflow, effects involving hot polar rain electrons, soft electron precipitation, topside ion heating, and photoelectrons have also been considered [Barakat and Schunk, 1984;Li et al., 1988;Tam et al., 1995;Seo et al., 1997]. A good overview of possible processes is provided by André and Yau [1997]. ...
We have examined characteristics of the vertical ion flux of thermal
O+ in the topside high-latitude ionosphere before and during
a number of geomagnetic storms, including the October and November 2003
events, using measurements of the vertical ion drift and ion number
density made by the DMSP F13 and F15 spacecraft. Prior to storm onset
typical upward fluxes of approximately 108-109
cm-2 s-1 are observed in the auroral zones with
somewhat smaller downward fluxes in the polar caps. Immediately
following storm onset upward fluxes reach and sometimes exceed
1010 cm-2 s-1 and are observed with
vertical velocities of 500-1500 m s-1. At the same time,
downward fluxes at the higher latitudes reach unusually high values of
109 cm-2 s-1. Separately integrating
the upward and downward fluxes over the high-latitude region (auroral
zone and polar cap) for each spacecraft pass allows the observation of
variations in the total upflow/downflow during the progression of a
geomagnetic storm. A superposed epoch analysis of events from 1995 to
2004 reveals a pattern of sudden onset of upward flux (averaging
approximately 14 hours) with a gradually increasing and more extended
period of downward flux (lasting about 24 hours). The upward fluxes are
stronger when the coincident fluctuations in the z component of the IMF
are larger. Strong upward flux events during storm times tend to occur
when the solar wind velocity is elevated and the solar wind density is
high. Upward flux events can also occur without a coincident geomagnetic
storm if the solar wind velocity is above average.
... 2. The collisionless region where the individual-ion characteristics dominate the ion motion. Different models were used to study the behavior of H in the that region, hydromagnetic (4) , kinetic (5)(6)(7) and semi kinetic (8)(9) . The above two regions are separated by a narrow transition layer (10)(11)(12) . ...
The polar wind is an ambipolar plasma outflow from the terrestrial ionosphere at high latitudes. As the ions drift upward along geomagnetic flux tubes, they move from collision-dominated to collisionless regions. A Monte Carlo simulation was used to calculate the temperature and Coulomb collision frequency in the polar wind. The simulation properly accounted for the divergence of geomagnetic field lines, the gravitational force, the polarization electric field, and Coulomb collisions. The temperature was found to increase with altitude and then decreases due to the interplay between frictional heating due to Coulomb collisions and adiabatic cooling (due to diverging geomagnetic field). The Coulomb collision frequency was found to decrease with altitude. As altitude increases, the ions are accelerated by the upward directed ambipolar electric field and become less coupled with the background ions. One of the objectives is to study the consequences of a velocity distribution function with an enhanced high energy tail for the injected ions. As the number of high energy ions increases in the tail of the velocity distribution at the injection point (i.e. kappa parameter decreases), the temperature increases and decreases.
إن بلازما الرياح القطبية تكون مؤلفة أساسا من أيونات الأكسجين والهيدروجين بالإضافة إلى الإلكترونات، حيث تتمكن هذه الأيونات من التغلب على قوة الجاذبية الأرضية لها وتتمكن من الإفلات إلى ارتفاعات أعلى. وهذه الحركة بسبب التسارع الذي تكتسبه بواسطة المجال الكهربائي المتواجد في المنطقة القطبية. استهدفت هذه الدراسة حساب تردد التصادم بين أيونات الهيدروجين وأيونات الأكسجين في الرياح القطبية باستخدام تقنية مونت كارلو حيث تمت محاكاة حركة أيون الهيدروجين في وسط تتوزع فيه أيونات الأكسجين تبعا لقانون توزيع السرعة لماكسويل. وتوصلت الدراسة إلى أن تردد التصادم يعتمد على الكثافة العددية لأيونات الأكسجين المتواجدة في الرياح القطبية ودرجة حرارة كل من أيونات الهيدروجين وأيونات الأكسجين حيث وجد أن تردد التصادم يتناقص مع الارتفاع
Upflowing ions of ionospheric origin are an important source of magnetospheric plasma at all latitudes, particularly during geomagnetic storms and substorms. Upflowing ions from the cusp populate the mantle/ boundary layer and plasma sheet regions. Polar wind ions escaping from the polar cap can populate both the plasma sheet and magnetotail lobes. Ionospheric ions energized in the nocturnal auroral oval can populate the plasma sheet and ring current regions. Unfortunately, at the present time, there are no time-dependent global models that can properly account for all of the sources of escaping ionospheric plasma. Howeyer, we have recently conducted several time- dependent global simulations of the coupled ionosphere-polar wind system that show how this system reacts to changing geomagnetic activity. The system’s response is found to be nonlinear, and the temporal response of O⁺ can be opposite to that of H⁺. Also, the temporal response of the ions at high altitudes can be opposite to that at low altitudes, depending on the seasonal and solar cycle conditions. In general, the simulation results are in good agreement with the available measurements. These global ionosphere-polar wind simulations, and related results, are reviewed in this paper.
The energization of charged particles, due to interaction with electromagnetic turbulence, has an important influence on the plasma outflow in space. The effect of wave-particle interaction (WPI) on O+ and H+ velocity distributions in the polar wind was investigated by using Monte Carlo method. The Monte Carlo simulation included the effect of WPI, gravity, polarization electrostatic field, and the divergence of geomagnetic field within the simulation tube (1.7-10 earth radii, Re) as the ions are heated due to WPI and move to higher altitudes, the ion's Larmor radius a(L) may become comparable to the perpendicular wave length lambda(perpendicular to) of the electromagnetic turbulence. As the ratio a(L)/lambda(perpendicular to) becomes (>) over tilde 1 the quasi-linear perpendicular diffusion coefficient becomes velocity dependent, the heating rate becomes self-limited and the ion ditribution displays toroidal features. This result is consistent with observations of O+ toroidal distribution in the auroral region.
Four studies were considered to simulate the ion behavior in the auroral region and
the polar wind.
In study I, a Monte Carlo simulation was used to investigate the behavior of o +
ions that are E x B-drifting through a background of neutral 0, with the effect of
o+(Coulomb) self-collisions included_ Wide ranges of the ion-to-neutral density ratio
n; I n. and electrostatic field E were considered in order to investigate the change of ion
behavior with respect to the solar cycle and altitude. For low altitudes and/or solar
minimum ( n; I n. :<;; w-s ), the effect of self-collisions is negligible. For higher values of
n; In. , the effect of self-collisions becomes significant and, hence, the non-Maxwellian
features of the o+ distributions are reduced.
In study II, the steady-state flow of the polar wind protons through a background
of o+ ions was studied. Special attention was given to using an accurate collision model.
The Fokker-Planck expression was used to represent H+-o+ Coulomb collision s. The
transition layer between the collision-dominated and the collision less regions plays a
pivotal role in the behavior of the H+ flow. In the transition region, the shape of H+
di stribution changes in a complicated manner from Maxwellian to "kidney bean". The
flow also changes from subsonic to supersonic within the transition region. The heat
fluxes of parallel and perpendicular energies change rapidly from their maximum
(positive) to their minimum (negative) values within the same transition region.
In study III, a Monte Carlo simulation was developed in order to study the effect
of the wave-particle interactions (WPI) on o+ and H+ ions outflow in the polar wind.
The simulation also considered the other mechanisms included in the classical polar wind
studies such as gravity, polarization electrostatic field, and divergence of geomagnetic
field lines. Also, an altitude dependent wave spectral density was adopted. The main
conclusions are (I) the o+ velocity distribution develops conic features at high altitudes;
(2) the o+ ions are preferentially energized; (3) the escape flux of o+ increased by a factor
of 40, while the escape flux of H+ remained constant; (4) including the effect of a fmite
ion Larmor radius produced toroidal features for o+ and H+ distributions at higher
altitudes.
In study IV, a comparison between the effect of WPI on H+ and o+ ion outflow
in the polar wind and in the auroral regions was studied. It was concluded that: (I) o+ is
preferentially energized in both regions; (2) both ions (H+ and o+) are more energetic in
the auroral region at most altitudes; (3) in the auroral region, the ion conics formed at
lower altitudes, at 1.6 R, foro+ and 2.5 R, for H+, while in the polar wind H+ did not
form conics and o+ formed conics at high altitudes; ( 4) the effects of body forces are
more important in the polar wind than in the auroral region, and for o+ than H+.
(160 pages)
Photoelectrons escape from the ionosphere on sunlit polar cap field lines. In order for those field lines to carry zero current without significant heavy ion outflow or cold electron inflow, field-aligned potential drops must form to reflect a portion of the escaping photoelectron population back to the ionosphere. Using a 1-D ionosphere-polar wind model and measurements from the Resolute Bay Incoherent Scatter Radar (RISR-N), this paper shows that these reflected photoelectrons are a significant source of heat for the sunlit polar cap ionosphere. The model includes a kinetic suprathermal electron transport solver, and it allows energy input from the upper boundary in three different ways: thermal conduction, soft precipitation, and potentials that reflect photoelectrons. The simulations confirm that reflection potentials of several 10s of eV are required to prevent cold electron inflow and demonstrate that the flux tube integrated change in electron heating rate (FTICEHR) associated with reflected photoelectrons can reach 109eVcm− 2s− 1. Soft precipitation can produce FTICEHR of comparable magnitudes, but this extra heating is divided among more electrons as a result of electron impact ionization. Simulations with no reflected photoelectrons and with downward field-aligned currents (FAC) primarily carried by the escaping photoelectrons have electron temperatures which are ~ 250 − 500 K lower than the RISR-N measurements in the 300-600 km region; however, simulations with reflected photoelectrons, zero FAC, and no other form of heat flux through the upper boundary can satisfactorily reproduce the RISR-N data.
Research concerning the transport and distribution of ionospheric plasma in the magnetosphere are reviewed, stressing the dichotomy in explanations given for the low plasma densities outside the plasmasphere. The convection/hot solar plasma model and the convection/loss model are considered. Observations of global ionospheric outflows are compared with theoretical studies. It is suggested that there is a need for a hybrid model of magnetospheric plasma in which terrestrial plasma is both lost into the solar wind and energized and trapped within the magnetosphere, inflating the geomagnetic field and excluding cold plasma from conjugate regions.
The polar wind is an ambipolar outflow of thermal plasma from the
terrestrial ionosphere at high latitudes to the magnetosphere along
geomagnetic field lines. The polar wind plasma consists mainly of
H+, He+, and O+ ions and electrons.
Although it was initially believed that O+ ions play a major
role only at low altitudes, it is now clear from observations that
relatively large amounts of suprathermal and energetic O+
ions are present in the polar magnetosphere. Recently, thermal
O+ outflow has been observed at altitudes of 5000-10,000 km
together with H+ and He+ ions. The polar wind
undergoes four major transitions as it flows from the ionosphere to the
magnetosphere: (1) from chemical to diffusion dominance, (2) from
subsonic to supersonic flow, (3) from collision-dominated to
collisionless regimes, and (4) from heavy to light ion composition. The
collisions are important up to about 2500 km, after which the ions and
electrons exhibit temperature anisotropies. The direction of the
anisotropy varies with geophysical conditions. The polar wind outflow
varies with season, solar cycle, and geomagnetic activity. The
O+ flux exhibits a summer maximum, while the H+
flux reaches a maximum in the spring. The He+ flux increases
by a factor of 10 from summer to winter. At both magnetically quiet and
active times the integrated H+ ion flux is largest in the
noon sector and smallest in the midnight sector. The integrated upward
H+ ion flux exhibits a positive correlation with the
interplanetary magnetic field. In the sunlit polar cap the
photoelectrons can increase the ambipolar electric field, which in turn
increases the polar wind ion outflow velocities. The outflowing polar
wind plasma flux tubes also convect across the polar cap. When the flux
tubes cross the cusp and nocturnal auroral regions, the plasma can be
heated and become unstable. Similar mixing of hot magnetospheric plasma
with cold polar wind may result in instabilities. A number of free
energy sources in the polar wind, including temperature anisotropy,
relative drift between species, and spatial inhomogeneities, feed
various fluid and kinetic instabilites. The instabilities can produce
plasma energization and cross-field transport, which modify the
large-scale polar wind outflow.
Flux, density, velocity, and temperature (parallel and perpendicular)
are calculated by using the polar wind acceleration process in the
dayside and nightside ion exosphere of Mars at exospheric plasma
temperatures 1000 K, 1500 K, and 2000 K. The results are compared with
the measurements made by the solar wind plasma instrument (TAUS) and
automatic space plasma experiment with a rotating analyzer (ASPERA)
experiments in the magnetotail of Mars. For these calculations the
ionospheric plasma is transported into the Martian tail along the
magnetic field line in the presence of charge separation electric
fields, which are calculated by using a quasi-neutrality condition such
that both flux and density of ions are equal at every point in the
exosphere to flux and density of electrons. It has been found that the
ions O+, O+2, and NO+ are
present in the distant tail at 2000 K. The ion
CO+2 seems to be absent at high altitude because
of the polar wind acceleration mechanism. Among these ions,
O+ is the dominant species in the Martian tail. The
exospheric plasma temperature is the most important parameter in the
polar ion exosphere. As it increases from 1000 K to 2000 K, the results
are found to be changed by several orders of magnitude.
Some results are presented from initial VHF observations which
illustrate the ability of the EISCAT Svalbard radar to determine polar
wind characteristics; but the limitations of these studies, due to the
geomagnetic location of the EISCAT facility at the southern boundary of
the auroral zone, are discussed. The EISCAT Svalbard radar, located near
or within the polar cap and under the polar cusp region in the noon
sector, will offer outstanding new opportunities; particular emphasis is
placed on the effects of closed-open magnetic flux tube conditions on
polar wind characteristics and their observation with this new radar.
Data assimilation models like the Utah State University (USU) Global
Assimilation of Ionospheric Measurements (GAIM) models use physics-based
models of the ionosphere, ionosphere-plasmasphere, or thermosphere and a
Kalman filter as a basis for assimilating a diverse set of measurements.
With a sufficient amount of data and with multiple data types, the data
assimilation models can provide reliable specifications and near-term
forecasts. However, for long-term forecasts (5 days or longer)
stand-alone or coupled physics-based models are needed. Unfortunately,
the various physics-based models contain several uncertain parameters
and processes as well as missing physics. Further complications arise
for coupled physics-based models because of coupling issues and error
propagation from model to model. Some of the problems are associated
with the magnetosphere and lower atmosphere drivers, the adopted set of
physics-based equations, the parameterization of physical processes, the
values adopted for the transport coefficients, the numerical techniques
used, the spatial and temporal resolutions adopted, and the
uncertainties in the initial and boundary conditions. Examples of the
type of problems the space weather community faces in its attempt at
long-term ionosphere-thermosphere forecasting are given.
A model for the polar ion exosphere of Mars is developed to calculate the escape flux and density of oxygen ions and electrons through the plasma sheet of Mars at different exospheric electron temperatures (600K-2300K) along the magnetic field lines originating from the baropause at latitude 75 deg and longitude zero degree. The intensity of the magnetic field lines in the noon-midnight meridian plane at all latitudes are calculated by assuming that Mars has weak magnetic field of dipolar nature. To calculate the electric potential in the region of magnetic field, quasi-neutrality condition is satisfied in such a way that both the flux and density of oxygen ions are equal at every point to that of electrons. It has been found that above 2300 K, escape flux and density of O(+) do not increase. The total escape rate of O(+) at this temperature is approximately 3.5 x 1024 ions/s, which shows close agreement with the observations taken by Solar Wind Plasma Instrument (TAUS) and Automatic Space Plasma Experiment with Rotating Analyser (ASPERA) experiments in the plasma sheet of Mars. The present calculation concludes that O(+) ions in the plasma sheet of Mars are mainly due to the escape of oxygen ions from the ionosphere in presence of charge separation electric field.
Differential directional ionospheric electron measurements at energies from <1 to 60 eV have been obtained in the Earth's geomagnetic cleft ionosphere on NASA's Sounding of the Cleft Region Ion Fountain Energization Region (SCIFER) rocket. We introduce the electron data set, present electron temperature measurements, and show examples of phase space distributions measured near and in the cleft ionosphere. Topside electron temperatures ranged from near 0.15 eV south of the high energy electron trapping boundary to near 1 eV within precipitation regions in the auroral cleft. Field-aligned electron bursts extend down to thermal energies and merge with the core, yielding core heating and anisotropy in the derived electron temperature. One interval is identified where enhanced electron temperature is collocated with a deep density depletion, low frequency electrostatic waves, and transverse ion heating, consistent with operation of the current-driven ion cyclotron instability. On the downleg, in what may be polar cap, the shape of the thermal electron energy distribution was observed to be quite uniform, with temperatures near 0.25 eV.
Anomalous electron heat fluxes and recent observations of day-night asymmetries in polar wind features indicate that photoelectrons may affect polar wind dynamics. These anomalous fluxes require a global kinetic description (i.e., mesoscale particle phase space evolution involving microscale interactions); their impact o the polar wind itself requires a selfconsistent description. In this Letter, we discuss results of a selfconsistent hybrid model that explains the dayside observations. This model represents the first global kinetic collisional description for photoelectrons in a selfconsistent classical polar wind picture. In this model, photoelectrons are treated as test particles, ion properties are based on global kinetic collisional calculations, thermal electron features and the ambipolar field are determined by fluid calculations. The model provides the first global steadystate polar wind solution that is continuous from the subsonic collisional regime at low altitude to the supersonic collisionless regime at high altitude. Also, the results are consistent with experiments in several aspects, such as order of magnitude of the ambipolar electric potential, qualitative features of the ion outflow characteristics, electron anisotropy and upwardly directed electron heat flux on the dayside.
The parallel dynamics of the plasma along high-latitude field lines is a critical feature of the ionosphere-magnetosphere coupling problem. Results are reported from a time-dependent large-scale simulation of these dynamics in a region from the ionosphere well into the magnetosphere. Including multiple electron species has made it possible to dynamically simulate the diodelike response of the field-line plasma to the parallel currents coupling the ionosphere and magnetosphere. Downward (return) currents flow with small resistance, while upward currents produce kilovolt-sized potential drops along the field due to the effect of the converging magnetic field on the high-energy magnetospheric electrons.
In this paper, we show how centrifu.fal acceleration and frictional heating due to ionospheric convection affects O⁺ and H transport along open field lines from a. collisional to a. collisionless region (200 - 6000 km). The model we use is a. generalized semikinetic model similar to that of Wilson [1994] who used it to study the effects of convection frictional heating of O⁺ ions in the 300 - 1100 km altitude polar region. The present study is complementary to the work of Horwitz et al. [1994] on the effects of centrifugal acceleration of the collisionless polar wind. We found that in the altitude range considered here, centrifugal acceleration makes very little direct contribution to the overall upward force on the H⁺ ions, but has a. pronounced effect on them through the centrifugally accelerated O⁺ ions. While the effect of centrifugal acceleration on the O⁺ ions is macroscopic in nature, i.e., acceleration increases the bulk flow speeds and thus cools the ions adiabatically, the effect of centrifugal acceleration on the H⁺ ions is to a. large extent dominated by microscopic processes - mainly H⁺ -O⁺ collisions. Because the collisional mean free path of a. H⁺ ion decreases rapidly with the decrease in the relative speed between the H ion and the average O⁺ flow speed, when the upward O⁺ flow speed is increased by centrifugal acceleration, a. normally collisionless high speed H⁺ ion can now exchange energy with the O⁺ ions. The result is an overall decrease in the H⁺ flow velocity and a. large upward or downward H⁺ heatflux. The H⁺ heatflux is upward or downward depending on whether the majority of the H⁺ ions are held back by the collisional drag force that comes from the O⁺ions. During strong convection, when there is enhanced O⁺ ion loss, the drop in O⁺ density decreases the collisional mean free path of the H⁺ ions. The majority of H⁺ ions therefore stream upwards and leave a tail of collisional ions, resulting in a large downward heatflux. We therefore conclude that in moving through the collisional/collisionless transition region, the velocity space characteristics of the H ions in the polar region are greatly dependent on the effects of centrifugal acceleration on the O⁺ ions and the loss of O⁺ ions by enhanced chemical reactions with the neutrals.
A Monte Carlo simulation was used to study the effects of wave-particle interactions (WPI) on ion outflow at high latitudes (the auroral region and the polar cap). As the ions drift upward along the geomagnetic field lines, they interact with the electromagnetic turbulence and, consequently, get heated in the direction perpendicular to the geomagnetic field. The mirror force converts some of the gained ion energy in the perpendicular direction into parallel kinetic energy. These effects combine to form an ion-conic distribution. Previous studies of WPI in the auroral region neglected the body forces (i.e., gravitational and polarization electrostatic) and the altitude dependence of the spectral density. In contrast, this work includes the effect of body forces and an altitude-dependent spectral density. The ion distribution function, the profiles of ion density, drift velocity, and parallel and perpendicular temperatures are presented for both H+ and O+ ions. These results are compared with the ones corresponding to polar wind conditions. The main conclusions are as follows: (1) the effect of body forces is more important in the polar wind case and for the O+ ions than it is for the auroral region and the H+ ions, respectively; (2) the O+ ions are preferentially energized in both regions; (3) both ions (H+ and O+) are more energetic in the auroral region at most altitudes; and (4) the results of the Monte Carlo simulations agree with the ``analytical'' results of the mean particle theory.
Transverse acceleration by waves and parallel acceleration by field-aligned electric fields are important processes in the transport of ionospheric ions along auroral field lines. In order to study the transport of ionospheric plasma in this environment we have developed a generalized semikinetic model which combines the tracking of ionospheric ion gyrocenters with a generalized fluid treatment of ionospheric electrons. Large-scale upward and downward directed electric fields are generated within the model by introducing magnetospheric plasma whose components have differing temperature anisotropies. We study the effects of such potentials when combined with the effect of ion heating by a distribution of waves along the flux tube. We find that the combination of wave heating and an upward electric field results in an order of magnitude increase in O+ outflow (compared to a case with an upward electric field and no wave heating). Under these conditions we observe the formation of bimodal conics. When a downward electric field is added to a case with wave heating, the energy gained by the ions from the waves increases by a factor of 2 or 3 (over the scenario with wave heating and no hot plasma-driven electric field) owing to their slower transit of the heating region. Typically, the velocity distributions under these conditions are toroids and counterstreaming conics. We also find that the upflowing, dense, heated ionospheric plasma acts to reduce the potential set up by the anisotropies in the magnetospheric components.
It has long been recognized that photoelectrons can enhance the ambipolar electric fields affecting polar wind outflows [e.g., Axford, 1968; Lemaire, 1972]. Since ionospheric ions and electrons are produced in large part by photoionization of the neutral atmosphere at lower altitudes, and the maximum photoelectron production rate occurs in the 130-140 km altitude range, it is essential to model this photoelectron-driven polar wind self-consistently from the E region to an altitude of several Earth radii. Here we describe a new steady state coupled fluid-semikinetic model to efficiently couple the source region to the high-altitude regions. This model couples a fluid treatment for the 120-800 km altitude range, a generalized semikinetic (GSK) treatment for the altitude range 800 km to 2 RE, and a steady state collisionless semikinetic method for the altitude range 2-9 RE. We apply this model to investigate the photoelectron-driven polar wind with ionospheric conditions ranging from solar minimum (F10.7=90) to solar maximum (F10.7=200). The O+ and H+ densities are found to increase by factors of approximately 5 and 2, respectively, from solar minimum to solar maximum below 3 RE altitude. However, the parallel bulk velocities display little variation with increased F10.7 for altitudes below 3 RE. An electric potential layer of the order of 40 V develops above 3 RE altitude, when the included downward magnetosheath electron fluxes (such as polar rain) are insufficient to balance the ionospheric photoelectron flux. Such potential layers accelerate the ionospheric ions to supersonic speeds at high altitudes, above 3 RE, but not at low altitudes. We also found that the potential layer decreases from 40 to 8.5 V for solar minimum conditions and from 46 to 12 V for solar maximum conditions when the magnetospheric electron density is increased from 0.05 to 2cm-3.
A Monte Carlo simulation was used to study the steady state flow of the polar wind protons through a background of O+ ions. The simulation region included a collision-dominated region (barosphere), a collisionless region (exosphere), and the transition layer embedded between these two regions. Special attention was given to using an accurate collision model, i.e., the Fokker-Planck expression was used to represent H+-O+ collisions. The model also included the effects of gravity, the polarization electric field, and the divergence of the geomagnetic field. For each simulation, 105 particles were monitored, and the collected data were used to calculate the H+ velocity distribution function fH+, density, drift velocity, parallel and perpendicular temperatures, and heat fluxes for parallel and perpendicular energies at different altitudes. The transition region plays a pivotal role in the behavior of the H+ flow. First, the shape of the distribution function is very close to a slowly drifting Maxwellian in the barosphere, while a ``kidney bean'' shape prevails in the exosphere. In the transition region, the shape of fH+ changes in a complicated and rapid manner from Maxwellian to kidney bean. Second, the flow changes from subsonic (in the barosphere) to supersonic (in the exosphere) within the transition region. Third, the H+ parallel and perpendicular temperatures increase with altitude in the barosphere due to frictional heating, while they decrease with altitude in the exosphere due to adiabatic cooling. Both temperatures reach their maximum values in the transition region. Fourth, the heat fluxes of the parallel and perpendicular energies are positive and increase with altitude in the barosphere, and they change rapidly from their maximum (positive) values to their minimum (negative) values within the transition region. The results of this simulation were compared with those found in previous work in which a simple (Maxwell-molecule) collision model was adopted. It was found that the choice of the collision model can alter the results significantly. The effect of the body forces was also investigated. It was found that they can also alter the results significantly. Both the body forces and collision model have a large effect on the heat flux, while they have only a small quantitative effect on the lower-order moments (density, drift velocity, and temperature).
Recent polar wind measurements between 5000 and 9000 km altitude by the
Akebono satellite indicate that both H+ and O+
ions can have remarkably higher outflow velocities in the sunlit region
than on the nightside. In addition, electrons also display an asymmetric
behavior: the dayside difference in energy spread, greater for
upward-moving than downward-moving electrons, is absent on the
nightside. We use a self-consistent hybrid model [Tam et al., 1995b]
that was developed for the polar wind outflow to address these observed
day-night asymmetric features. The model takes into account the
evolution of the polar wind self-consistently by properly recognizing
the global, kinetic, collisional effects of the sunlit photoelectrons.
By studying the effects of the presence and absence of photoelectrons on
the polar outflow, we compare the daytime and night-time polar wind
results, and demonstrate the asymmetries observed by the Akebono
satellite.
In February 1996, the POLAR spacecraft was placed in an elliptical orbit with a 9 RE geocentric distance apogee in the northern hemisphere and 1.8RE perigee in the southern hemisphere. The Thermal Ion Dynamics Experiment (TIDE) on POLAR has allowed sampling of the three-dimensional ion distribution functions with excellent energy, angular, and mass resolution. The Plasma Source Instrument (PSI), when operated, allows sufficient diminution of the electric potential to observe the polar wind at very high altitudes. In this paper, we describe the results of a survey of the polar wind characteristics for H+, He+, and O+ as observed by TIDE at ~5000 km and ~8RE altitudes over the polar cap during April-May 1996. At 5000 km altitude, the H+ polar wind exhibits a supersonic outflow, while O+ shows subsonic downflow, which suggests a cleft ion fountain origin for the O+ ions in the polar cap region. Dramatic decreases of the 5000 km altitude H+ and O+ ion densities and fluxes are seen as the solar zenith angle increases from 90° to 100° for the ionospheric base, which is consistent with solar illumination ionization control. However, the polar cap downward O+ flow and density decline from dayside to nightside in magnetic coordinates suggest a cleft ion fountain origin for the polar cap O+. Cleft ion fountain origin O+ density plumes could also be partially responsible for a similar day-night asymmetry in H+, owing to the charge-exchange reaction. At 8RE altitude, both H+ and O+ outflows are supersonic and H+ is the highly dominant ion species. The average bulk ion field-aligned velocities are in the typical ratio VO+:VHe+:VH+~2:3:5, which may suggest a tendency toward comparable energy gains, such as via an electric potential layer.
There is increasing observational evidence that photoelectrons may affect polar wind dynamics. For example, suprathermal electron pitch-angle distributions in the photoelectron energy range have been observed in the high-altitude polar wind. These distributions contribute little to the polar wind density, but carry an appreciable outward heat flux. Evidence of such reflected photoelectron distributions at low altitudes have been attributed to field-aligned potential drop. More recently, measurements of day-night asymmetries in electron temperature and ion outflow provide further indications of the photoelectrons’ impact on the polar wind. Such non-thermal fluxes can be explained by a mechanism relying on the earth’s decreasing magnetic field, the field-aligned potential drop, and the energy dependence of the Coulomb collisional cross-sections. The description of this mechanism requires a kinetic approach. Such an approach was used in a testparticle simulation of this mechanism, in agreement with the measured suprathermal fluxes. However, the effects of these fluxes on the polar wind itself require a self-consistent description. Unfortunately, a fully kinetic self-consistent description is at present not achievable. Instead, we suggest a hybrid approach, in which the background features of the polar wind are described by well-established fluid models, while the suprathermal features are described using a kinetic model. This approach retains the expediency of fluid theory while in effect extending its applicability. In this paper, we will review the physics underlying the mechanism mentioned earlier, discuss how the kinetic-fluid synthesis can best be achieved, and present our latest results. Our initial calculations show, for example, that the suprathermal electrons carry much of the polar wind heat flux, and may significantly increase the ambipolar electric field. This increase in the electric field can change the dynamics of the polar wind outflow.
The outflow of the polar wind along diverging geomagnetic field lines has been the subject of many modeling studies for the past 25 years. As the plasma drifts up and out of the topside ionosphere, it undergoes several transitions; for instance, its velocity changes from subsonic to supersonic and its velocity distribution changes from Maxwellian to non-Maxwellian. The complexity of the flow led to the development of several modeling approaches, such as the generalized moment, the kinetic, and the semikinetic models. Recently, a ``macroscopic'' particle-in-cell (PIC) model was adopted to study the polar wind. However, because one is always restricted to a finite number of particles, the validity of the approach must be established when it is applied to macroscopic flows. In this study the polar wind predictions obtained from a macroscopic PIC simulation were compared to those obtained from the more rigorous semikinetic model for steady state conditions. The study also shows the sensitivity of the PIC simulation to the adopted modeling parameters for both time-dependent and steady state conditions, including the number of simulation particles, the time step, the spatial bin size, etc. The study indicates that (1) the PIC model can be a powerful simulation tool if special attention is given to its potential pitfalls; (2) because of the finite number of particles the PIC technique is subject to a considerable amount of noise; (3) the noise level is higher for the higher-order moments, such as heat flow, and for the velocity distribution function; (4) the use of a time step that is too large leads to a modulation of the results; (5) an insufficient number of spatial bins yields a poor spatial resolution, while too many spatial bins leads to more noise; (6) the noise level can be reduced by averaging over time and/or space, but this affects the spatial and/or temporal resolution; (7) the bin size in velocity space must be carefully chosen to balance numerical noise and velocity space resolution; (8) in the steady state the PIC technique can achieve the same accuracy as the semikinetic model if all of the PIC modeling parameters are optimized; and (9) a few hundred thousand simulation particles, as used in some previous studies, are not adequate to resolve the tail of the velocity distribution, even in the steady state when time averaging is possible. For 106 particles the noise in the tail is still appreciable; and (10) when poor spatial resolution is used, important features can be missed, as was the case in some previous studies.
The distribution of current-driven electrostatic potentials along auroral magnetospheric flux tubes is studied analytically, based on the principles of quasi-neutrality and kinetic orbital motion of electrons and ions governed by inertia, electric forces, and magnetic mirror forces. When certain particle accessibility conditions are fulfilled, the kinetics determines the electron and ion number densities as unique functions of position along the flux tube and local potential. These functions are used to define potential profiles for which electron and ion number densities everywhere match, or alternatively positions where potential jumps (double layers) may occur. Magnetospheric and ionospheric particle sources in the form of Maxwellians with loss cones depending on the total voltage drop are considered, as well as trapped particles. The potential profile generally comprises a potential jump that accounts for a sizeable fraction of the total voltage drop (only by assuming unrealistic velocity distributions of the magnetospheric electron source can solutions without a jump be found). The jump may typically occur around an altitude of 1 RE, an altitude that increases with density of the ionospheric source and that decreases with increasing density of trapped, secondary, and backscattered particles and total voltage drop along the flux tube. Below the potential jump, only small potential variations occur. Above the potential jump, the potential falls off gradually, asymptotically linear in magnetic field intensity. Only a small percentage of the potential drop occurs above an altitude of a few RE, but the weak field extending to higher altitudes is essential for quasi-neutrality and for the current-voltage relationship (discussed in a previous paper). Above the potential jump, the number density of particles is reduced as compared to regions without current flow and potential drops. Depending on the density of the ionospheric source, solutions are possible for total potential drops much larger than the temperature of the magnetospheric electron source. While full accessibility for the mirroring electrons tends to be locally violated, accessibility for all particles within the source cone is fulfilled in these solutions, so the current-voltage relationship does not depend on the shape of the potential profile. Some comments are also made on transient conditions when there is no time for transport of neutralizing ions from the ionosphere.
Superthermal electrons are one of the major energy players in the inner magnetosphere, and are responsible for the formation
of self-consistent electric potentials in some space plasma regions. Numerous processes are involved in the determination
of their distribution function, and only a kinetic approach provides the proper tool to treat this component of the inner
magnetosphere. Because of the relative complexity of the kinetic equation solution, analytical investigations of some simplified
kinetic problems are very useful because they help us gain physical insight into how the system responds to various physical
processes and external boundary conditions. Solutions to these simplified problems also provide us a convenient method to
test the validity of complicated numerical models where superthermal electrons are involved.
The core plasma of the inner magnetosphere is cold and, potentially, could be handled based on the solutions of the variety
of hydrodynamic equations discussed in Chap. 4. The cold plasma of the ionosphere and plasmasphere is the mediator of wave–particle
interaction processes and provides the extremely important coupling element between the terrestrial ring current and Earth's
radiation belts. The different parts of the inner magnetosphere also strongly couple to each other and create an additional
complexity in the analysis of the numerical models of this system. The use of different sets of transport equations to describe
the cold plasma distribution can also lead to different results and create an additional complexity in the interpretation
of the role of the physical processes that control the behavior of the core plasma in the ionosphere–magnetosphere system.
These difficulties illustrate why approximate analytic solutions of cold plasma transport phenomena are very useful, i.e.,
because they help us gain physical insight into how the system responds to varying sources of mass, momentum, and energy and
also to various external boundary conditions. They also provide a convenient method to test the validity of complicated numerical
models.
A time-dependent macroscopic particle-in-cell (mac-PIC) model was used to study the temporal evolution of the polar wind under the influence of a hot electron population. First, the steady state results of the mac-PIC model were found for a wide range of hot/cold electron temperature ratios and compared with the results of the well-established time-independent semikinetic model, and excellent agreement was found. Second, simulations were conducted to study the temporal evolution of a plasma that was originally in a steady state condition, and then a hot electron population was suddenly introduced. The profiles of the plasma moments again displayed discontinuities, which oscillated with a decreasing amplitude until they reached their steady state values. As the hot electron temperature increased, the oscillation amplitude increased, and the altitude of the discontinuity decreased, while the period of oscillation and decay rate remained essentially unchanged. Third, simulations were conducted for plasma flux tubes as they drifted across the subauroral, cusp, polar cap, and auroral regions. It was found that as soon as the plasma entered the polar cap, the signatures of the hot electrons were observed. The strength of these signatures varied with time owing to the variation in the instantaneous values of the density and temperature of the thermal electrons. After the plasma exited the polar cap the signatures of the hot electrons persisted for a while, and a density bump formed. For more energetic hot electrons the signatures of the hot electrons became more pronounced in the polar cap and persisted longer after the flux tube left the polar cap. The results of this study were shown to explain some interesting features of the polar wind that were observed by the POLAR satellite.
We present a general solution to the collisionless Boltzmann (Vlasov) equation for a free-flowing plasma along a magnetic field line using Liouville's theorem, allowing for an arbitrary field-aligned potential energy structure including nonmonotonicities. The constraints of the existing collisionless kinetic transport models are explored, and the need for a more general approach to the problem of self-consistent potential energy calculations is described. Then a technique that handles an arbitrary potential energy distribution along the field line is presented and discussed. For precipitation of magnetospherically trapped hot plasma, this model yields moment calculations that vary by up to a factor of 2 for various potential energy structures with the same total potential energy drop. The differences are much greater for the high-latitude outflow scenario, giving order of magnitude variations depending on the shape of the potential energy distribution. Self-consistent calculations for the photoelectron-driven polar wind are compared with previous results, and it is shown that even a photoelectron concentration of 0.03% at the base of the simulation (500 km) will cause the potential energy distribution to violate the constraints of the existing models.
We present a method for estimating the thermal ion drift velocity, ion temperature, and spacecraft potential in the polar ionosphere from data acquired with the suprathermal mass spectrometer (SMS) on the Akebono satellite. The method is based on fitting the spin angle distributions of the observed ion flux for a number of retarding potential analyzer (RPA) settings to a drifting Maxwellian distribution. A nonlinear fitting procedure is used to relate the observed fluxes to the plasma parameters. The spacecraft potential is taken into account by means of the thin-sheath approximation. The analysis is applicable to the ``thermal'' (total energy few eV) polar wind ion population. A study of a number of representative passes at various altitudes, latitudes, and local times indicates that all major ions (H+, He+, and O+) have low temperatures, in the range of 0.05-0.35 eV, with little temperature dependence on altitude, longitude, or latitude. The velocity estimates confirm the previous analysis using the moment method: all ions have a significant upward velocity component (antiparallel to the magnetic field, in the northern hemisphere) which increases with altitude. The velocity estimates from data at the higher RPA settings are sometimes higher than those at low RPA settings. This indicates that the actual ion distributions are not Maxwellian, perhaps due to a higher-energy tail component drifting at higher velocity. All ions are in general supersonic at high altitudes.
The ionospheric convection electric fields that occur at high latitudes cause plasma to drift across the cusp region and the polar cap. Since the magnetic field at high latitudes is close to vertical, pointing downward (upward) in the northern (southern) hemisphere, the convecting plasma experiences a centrifugal acceleration as it crosses the polar region because of the diverging magnetic field geometry. The centrifugal force is directly proportional to the mass of the plasma particles, and it is reasonable to ask whether this force has an effect on polar plasma outflow, par- ticularly for the more massive ion O +. To date, a number of studies have addressed this question, but the theoretical models used in these studies were either overly simplified (i.e., neglected pro- cesses known to be important in the polar ionosphere) or else did not use appropriate boundary conditions or take account of the time variability of the problem. The results of these prior inves- tigations were often contradictory. In order to overcome the limitations of these earlier studies, we have used a macroscopic particle-in-cell (PIC) code, which is sophisticated in the sense that a broad range of physical processes are incorporated in its description, in conjunction with time-varying boundary conditions obtained from a time-dependent, three-dimensional, hydrodynamic model of the polar ionosphere. This enables us to properly account for the variation of boundary conditions along a flux tube trajectory. Initially, our macroscopic PIC model was solved for steady state con- ditions. This allowed us to compare results from our code with those of a prior study of centrifu- gal acceleration that uses a PIC formulation. Also, by obtaining steady state solutions for both low and high electron temperatures, we have been able to directly compare the effects of electron temperature and centrifugal force on the polar plasma outflow, a comparison that a time-dependent simulation might obscure. Then time-dependent PIC solutions were obtained for the plasma in a convecting flux tube, using solutions to a time-dependent, three-dimensional, hydrodynamic model to provide realistic boundary values for the electron and ion temperatures and the H + and O + densi- ties and drift velocities along a flux tube trajectory. Both steady state and time-dependent solutions indicate that centrifugal acceleration does not significantly contribute to the loss of plasma from the polar ionosphere.
In the classical picture of the polar wind, the H/sup +/ flow becomes supersonic and collisionless, and the H/sup +/ velocity distribution becomes anisotropic and asymmetric at altitudes above about 3000 km. Previously, the stability of the classical polar wind was studied and found to be stable for a wide range of electron temperatures. However, this classical picture of the polar wind results from steady state models that neglect horizontal plasma convection. The plasma convection at high latitudes can significantly attect the characteristics of the polar wind. For example, ions can be energized in the cusp region and then convected into the polar cap, which subsequently results in energetic ion beams and/or conics passing through the classical polar wind. The effect of energetic H/sup +/ beams on the stability of the polar wind was studied with regard to the excitation of electrostatic waves. The cases considered covered a wide range of electron-to-background ion temperature ratios (T/sub e//T/sub i/ = 0.1, 1, 10) and beam-to-background ion density ratios (n/sub b//(n/sub i/+n/sub b/) = 0.1, 0.5, 0.9). A relatively cold beam was assumed (T/sub b/ = 0.1T/sub i/). A combination of the Nyquist technique and a direct solution of the plasma dispersion equation was used to find the minimum beam drift velocity required to destabilize the plasma.
The effects of flux transfer events (FTE) on the dayside auroral ionosphere are studied, using a simple twin-vortex model of induced ionospheric plasma flow. It is shown that the predicted and observed velocities of these flows are sufficient to drive nonthermal plasma in the F region, not only within the newly opened flux tube of the FTE, but also on the closed, or ''old'' open, field lines around it. In fact, with the expected poleward neutral wind, the plasma is more highly nonthermal on the flanks of, but outside, the open flux tube: EISCAT observations indicate that plasma is indeed driven into nonthermal distributions in the regions. The nonthermal plasma is thereby subject to additional upforce due to the resulting ion temperature anisotropy and transient expansion due to Joule heating and also to ion accelerations associated with the FTE field aligned current system. Any upflows produced on closed field lines in the vicinity of the FTE are effectively bunched-up in the ''wake'' of the FTE. Observations from the AMPTE-UKS satellite at the magnetopause reveal ion upflows of energy approx.100 eV flowing out from the ionosphere on closed field lines which are only found in the wake of the FTE. Such flows are also only found shortly after two, out of all the FTEs observed by AMPTE-UKS. The outflow from the ionosphere is two orders of magnitude greater than predicted for the ''classical'' polar wind. It is shown that such ionospheric ion flows are only expected in association with FTEs on the magnetopause which are well removed from the sub-solar point: either towards dusk or, as in the UKS example discussed here, towards dawn. It is suggested that such ionospheric ions will only be observed if the center of the FTE open flux tube passes very close to the satellite.
The authors address the problem of modeling the polar wind. As their starting point they take the experimental data from DE1/RIMS and DE1/PWI as boundary conditions which they want their model to fit. This data covers areas out to 8R[sub E]. They then use a semikinetic polar wind model, constrained to the above boundary conditions, to derive solutions which provide agreement with the polar cap electron density profiles of Peterson, et al. They had to invoke a multiple component solution to get agreement. The agreement supports the argument for an oxygen ion dominated polar wind at very high altitudes.
Observations of the H(+), He(+), and O(+) polar wind ions in the polar cap above the collision-dominated altitudes (greater than 2000 km), from the suprathermal mass spectrometer (SMS) on EXOS D (Akebono) are reported. A statistical study of the altitude, invariant latitude, and magnetic local time distributions of the parallel velocities of the respective ion species is described, and preliminary estimates of ion temperatures and densities, uncorrected for perpendicular drifts and spacecraft potential effects, are also presented. For all three ion species, the parallel ion velocity increased with altitude. In the high-latitude polar cap, the average H(+) velocity reached 1 km/s near 2000 km, as did the He(+) velocity near 3000 km and the O(+) velocity near 6000 km.
A semikinetic model was used to study the steady state, collisionless, polar wind outflow from the Jovian polar caps. H(+)-escape fluxes and energies were calculated for a range of conditions, including several values of the ambient electron temperature, different hot electron populations, and both with and without the effects of the centrifugal force. The calculations indicate that if hot electron populations exist over the Jovian polar caps, as they do on earth, polar wind escape fluxes of the order of 10 to the 8th per sq cm s are possible. When integrated over the polar cap area, escape fluxes of this order of magnitude imply an ionospheric source strength of 2 x 10 to the 28th ions/s, which is comparable to the present estimate of the total magnetospheric plasma source population. Therefore, the ionosphere may play an important role in populating the Jovian magnetosphere, specifically the hidden, low energy, light ion component of the population.
The overall goal of our NASA Theory Program was to study the coupling, time delays, and feedback mechanisms between the various regions of the solar-terrestrial system in a self-consistent, quantitative manner. To accomplish this goal, it will eventually be necessary to have time-dependent macroscopic models of the different regions of the solar-terrestrial system and we are continually working toward this goal. However, with the funding from this NASA program, we concentrated on the near-earth plasma environment, including the ionosphere, the plasmasphere, and the polar wind. In this area, we developed unique global models that allowed us to study the coupling between the different regions. These results are highlighted in the next section. Another important aspect of our NASA Theory Program concerned the effect that localized 'structure' had on the macroscopic flow in the ionosphere, plasmasphere, thermosphere, and polar wind. The localized structure can be created by structured magnetospheric inputs (i.e., structured plasma convection, particle precipitation or Birkland current patterns) or time variations in these input due to storms and substorms. Also, some of the plasma flows that we predicted with our macroscopic models could be unstable, and another one of our goals was to examine the stability of our predicted flows. Because time-dependent, three-dimensional numerical models of the solar-terrestrial environment generally require extensive computer resources, they are usually based on relatively simple mathematical formulations (i.e., simple MHD or hydrodynamic formulations). Therefore, another goal of our NASA Theory Program was to study the conditions under which various mathematical formulations can be applied to specific solar-terrestrial regions.
Preface.- Introduction.- Kinetic Equations and Particle Collisions.- General Description of Wave Particle Interaction Phenomena.- Hydrodynamic Description of Space Plasma.- Analysis of Superthermal Electron Transport.- Analysis of Cold Plasma Transport.- Kinetic Theory of Superthermal Electron Transport.- Kinetic Superthermal Electron Instabilities in the Ionosphere.- Kinetic Theory of Ring Current and Electromagnetic Ion Cyclotron Waves: Fundamentals.- Kinetic Theory of Ring Current and Electromagnetic Ion Cyclotron Waves: Applications.- Concluding Remarks.
In this paper the application of the kinetic theory to the collisionless regions of the polar and solar winds is discussed. A brief historical review is given to illustrate the evolution of the theoretical models proposed to explain the main phenomenon and observations. The parallelism between the development of the solar wind models and the evolution of the polar wind theory is stressed especially. The kinetic approaches were in both cases preceded by the hydrodynamic models, and their publication gave rise to animated controversies; later on, semikinetic and hydromagnetic approximations were introduced. A kinetic method, based on the quasi neutrality and the zero current condition in a stationary plasma with open magnetic field lines, is described. The applicability of this approach on the solar and polar winds is illustrated by comparison of the predicted results with the observations. The kinetic models are also compared with hydrodynamic ones. The validity of the criticism and remarks uttered during the Chamberlain-Parker controversy (for the solar wind), and the dispute between Banks and Holzer on the one hand, and Dessler and Cloutier on the other (for the polar wind), are carefully analyzed. The main result of this study is that both approaches are in fact not contradictory but complementary. The classical hydrodynamic descriptions are only appropriate in the collision-dominated region, whereas the kinetic theory can be applied only in the collision-free domain.
Intense fluxes of keV electrons were observed over large regions of the north polar cap during two periods of magnetospheric quieting following large storms. These fluxes were distinctly field aligned, had energy spectra peaked at both approx. 100 eV and approx. 2 keV, and were not observed in the conjugate hemisphere. A search for similar fluxes in the interplanetary medium or the dayside magnetosheath yielded negative results. In one instance the polar cap fluxes were observed for approx. 40 hours during a period of large-scale expansion of the bow shock. The flux intensity and spectrum varied with observed changes in the bow shock configuration. This and other evidence leads us to suggest that these fluxes were accelerated by a potential barrier located either at the downstream bow shock or within the magnetospheric cavity on field lines intersecting the distended bow shock. The flux intensity and energy spectrum above 1 keV were similar over the polar cap and in the morningside plasma sheet, a finding which suggests that electrons in these regions had a similar source during these events. From the data we infer that the source of the polar cap fluxes was, in general, spatially and temporally uniform and that the structure within the region of their observation was related to processes and field line configurations that are internal to the magnetosphere. Details of the structure of the high-latitude magnetosphere are examined by using the polar cap fluxes as tracer particles. It is found that keV electrons have attenuated access to the postnoon region of closed field lines from high latitudes but are excluded from lower latitudes on the morningside by the prenoon cleft. (AIP)
Although solar wind and solar flare electrons can have direct access to low altitudes on open field lines, low-energy electron fluxes over the polar caps may at times be accelerated and trapped by a large-scale potential barrier in the magnetotail. If the effective potential difference at the barrier fluctuates randomly about an average value, a fraction of the beam flux accelerated toward the earth from beyond the barrier can be trapped between the barrier and the earth. Additional low-energy electrons diffusing onto field lines closed to particle losses by the potential barrier would also be quasi-trapped. The effects of such a fluctuating magnetotail barrier potential are described with a simple model. When the observed tail lobe spectrum is taken to be the electron source distribution beyond the barrier, the model adequately simulates the electron intensity, energy spectrum, and pitch angle distribution observed at low altitudes over the polar cap.
A semikinetic model was used to describe the steady state collisionless
flow of the polar wind along diverging geomagnetic field lines at high
latitues. Emphasis was given to studying the behavior of O(+) ions for a
wide range of boundary conditions at the baropause (4500 km). The main
result obtained was that for high electron temperatures Te = about
10,000 K) O(+) is not gravitationally bound and significant escape
fluxes (about 10 to the 7th/sq cm sec) of suprathermal (2eV) O(+) ions
occur. The O(+) flow is supersonic over most of the altitude range
considered, and O(+) Mach numbers as large as 20 are predicted at 10
earth radii. Also, depending on the boundary conditions, the O(+)
density can be comparable to or greater than the H(+) density at high
altitudes above the baropause when the electron temperature is high.
The density and bulk velocity distributions of warm magnetosheath particles and cold ionospheric O(plus) and H(plus) ions are calculated along a polar cusp (cleft) magnetic field line. The warm collisionless plasma in injected at 12 earth radii, and interacts with the cold polar wind electrons and ions of ionospheric origin. After magnetic reflections in the exosphere (above 1400 km altitude) most of the ions move upwards along tail magnetic field lines and feed the plasma mantle flow. The bulk velocity of these warm protons is approximately 200 km/sec and happens to be proportional to the average thermal speed of the magnetosheath ions at their injection point in the magnetosphere. The density in the plasma mantle is found to be about half that in the entry layer. The electric potential distribution is deduced from the quasi-neutrality condition. A large parallel electric field corresponding to an electrostatic double layer is obtained for the assumed boundary conditions.
The effect of a photoelectron escape flux on a polar wind model is
discussed. It is shown that the additional electric drag force due to the
escaping photoelectrons can accelerate the ions to higher velocities if the
photoelectron flux is larger than the escape flux of the thermal electrons.
Furthermore, it is shown that, even if the number density of the photoelectrons
remains small compared to the number density of the thermal electrons, their
kinetic pressure becomes impontant at very high altitudes. As a consequence a
positive gradient in the electron temperature can be expected in the sunlit polar-
cap upper region. (STAR)
The field aligned current density (J), a nonlinear function of the
applied potential difference (phi) between the ionosphere and the
magnetosphere, was calculated for plasma boundary conditions typical in
a dayside cusp magnetic flux tube. The J-phi characteristic of such a
flux tube changes when the temperatures of the warm magnetospheric
electrons and of the cold ionospheric electrons are modified. It also
changes when the relative density of the warm plasma is modified. The
presence of trapped secondary electrons influences the J-phi
characteristic. The partial currents contributed by the warm and cold
electrons, and by warm and cold ions are illustrated. The resistive
ionosphere, the return current region, and the region of particle
precipitation are discussed.
Ionospheric and plasma sheet particle densities, fluxes and bulk velocities along an auroral magnetic field line have been calculated for an ion-exosphere model. It is shown that such a collisionless model accounts for many features observed above the auroral regions. Except for very strong plasma sheet electron precipitation, no large potential difference is needed along the magnetic field lines to account for the usual proton and electron fluxes, their pitch angle distributions, and auroral field aligned currents.
In this paper it is shown how the hydrodynamic solutions of the polar wind can be fitted to the kinetic solutions applicable to the upper ionospheric polar regions where the Coulomb collisions are negligible. The bulk velocity and density distribution of the O+, H+ and He+ are determined for boundary conditions deduced from the OGO 2 observations by Tayloret al. (1968), and for unequal ion and electron temperatures. The results are compared to the observations of Hoffman (1971) (Explorer 31) near 3000km altitude.RésuméDans ce travail on montre comment on peut raccorder les solutions hydrodynamiques du Vent Polaire aux solutions cinétiques applicables aux régions supérieures de l'ionosphère polaire où les collisions Coulombiennes sont négligeables. Les distributions de vitesse moyenne et de densité des ions O+, H+ et He+ sont données pour des conditions de densité à 950 km déduites des observations de Tayloret al. (1968) (OGO 2) et pour des températures ioniques et électroniques inégales. Les rÉsultats sont comparés aux observations de Hoffman (1971) (Explorer 31) au voisinage de 3000 km d'altitude.
Three types of auroral particle precipitation have been observed over the polar caps, well inside the auroral oval, by means of the soft particle spectrometer on the Isis 1 satellite. The first type is a uniform, very soft (about 100 eV) electron 'polar rain' over the entire polar cap; this may well be present with very weak intensity at all times, but it is markedly enhanced during worldwide geomagnetic storms. A second type of precipitation is a structured flux of electrons with energies near 1 keV, suggestive of localized 'polar showers'; it seems likely that these are the cause of the sun-aligned auroral arcs that have been observed during moderately quiet conditions. During periods of intense magnetic disturbance this precipitation can become very intense and exhibit a characteristic pattern that we have come to call a 'polar squall'.
Collisionless ion exosphere kinetic and hydrodynamic models comparison for polar wind supersonic flow characteristics
Plasma transport models of polar ionosphere, discussing physical processes involved and effects on electron concentration, ion composition and speeds
The steady state flow of a fully ionized H(+)-O(+) electron plasma along geomagnetic field lines in the high-latitude topside ionosphere is studied. The theoretical formulation is based on a 13-moment system of transport equations, and allows for different species temperatures parallel and perpendicular to the geomagnetic field and nonclassical heat flows. For subsonic and supersonic flows, an appreciable H(+) temperature anisotropy occurs at all altitudes above 1500 km, and tends to be regulated at high altitudes. The direction of the temperature anisotropy is related to the direction of the H(+) heat flow to some extent; for supersonic flow an upward flow of heat from the lower ionosphere is required, while for subsonic flow solutions can be obtained with a downward H(+) heat flow. For subsonic flow, H(+)-H(+) collisions have an important effect on the H(+) stress and heat flow balance at all altitudes between 1500 and 12,000 km.
Winningham and Heikkila (1974) presented observations of various polar cap particle morphologies. They interpreted observations of 'anomalous' photoelectron angular distribution over the polar caps to be indicative of a large scale, outwardly directed, parallel electric field over the polar cap. The parallel field was observed to be spatially and/or temporarily variable. However, results obtained by Winningham and Heikkila have one weakness, which is related to the lack of simultaneous observations at many pitch angles. The present investigation is, therefore, concerned with the presentation of results from Dynamics Explorer 2 (DE-2) which confirm the experimental results of Winningham and Heikkila. It is concluded that the earth's polar caps act much like any conductor immersed in a plasma and illuminated by sunlight. DE-1 and DE-2 would then represent tiny point probes examining the internal details in the sheath region of the polar cap.