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Low-energy upflowing ion events at the poleward boundary of the nightside auroral oval: High-altitude Interball-Auroral probe observations
Abstract
Ion data acquired by the Interball-Auroral satellite during crossings of the poleward boundary of the auroral oval in the 2200-0300 MLT sector at altitudes of ~2.5-3 Earth's radii reveal the frequent occurrence of thermal and superthermal H+ ion outflows. These events are strongly correlated with suprathermal electron fluxes and broadband electromagnetic ULF waves. The pitch angle distributions give evidence of transverse heating occurring in a latitudinally narrow layer at the boundary between the polar cap and the plasma sheet boundary layer, over a broad altitude range extending up to the satellite altitude. The distributions evolve with latitude, exhibiting fluxes maximizing at pitch angles close to 90° at the poleward edge of the outflow structure and at pitch angles closer to the upward field-aligned direction at lower latitudes. The data analysis suggests that ion cyclotron resonance interaction with ULF electromagnetic turbulence can account for the observed heating, even if we cannot totally exclude that transverse velocity shears and nonresonant stochastic transverse acceleration sometimes contribute to the ion energization in view of the dc electric field fluctuations commonly observed at the same times. During the expansion phase of substorms the region of transverse heating at the poleward boundary of the discrete auroral oval exhibits a latitudinal structure characterized by an alternate occurrence of latitudinally narrow regions of intense and weak ion fluxes. These latitudinal variations are associated with magnetic fluctuations at a frequency of ~2×10-2Hz, interpreted in terms of hydromagnetic Alfvén waves. Equatorward of the heating region, the energy spectrograms recorded during the same events exhibit an energy-latitude dispersion signature with energy decreasing as latitude decreases. This dispersion is the result of the velocity filter effect due to the large-scale convection and of the poleward motion of the ion heating source associated with the poleward motion of the high-latitude edge of the active auroral region. The poleward edge of the low-energy ion structure marked by a sharp latitudinal gradient of the ion flux appears as a reliable midaltitude criterion for identifying the poleward boundary of the soft electron layer lying at the high-latitude edge of the plasma sheet boundary layer.
... The ionospheric upflow plays an important role in the magnetosphere-ionosphere coupling process, providing the ionospheric O + for the magnetosphere (Shelley et al., 1972). Previous works have investigated the upflow characteristics and mechanisms by using spacecraft and radar observations combined with model simulation (Yau et al., 1984;Heelis et al., 1984;Lockwood et al., 1985;Loranc et al., 1991;Rodger et al., 1992;Wu et al., 1992;Foster and Lester, 1996;Yau and Andre, 1997;Horwitz and Moore, 1997;Su et al., 1999;Malingre et al., 2000;Liu et al., 2001;Coley et al., 2006;Coley and Heelis, 2009). The ion upflow prefers to occur in the cusp and auroral region (Yau et al., 1984;Loranc et al., 1991;Wu et al., 1992) and at the auroral oval poleward boundary (Lockwood et al., 1985;Malingre et al., 2000). ...
... Previous works have investigated the upflow characteristics and mechanisms by using spacecraft and radar observations combined with model simulation (Yau et al., 1984;Heelis et al., 1984;Lockwood et al., 1985;Loranc et al., 1991;Rodger et al., 1992;Wu et al., 1992;Foster and Lester, 1996;Yau and Andre, 1997;Horwitz and Moore, 1997;Su et al., 1999;Malingre et al., 2000;Liu et al., 2001;Coley et al., 2006;Coley and Heelis, 2009). The ion upflow prefers to occur in the cusp and auroral region (Yau et al., 1984;Loranc et al., 1991;Wu et al., 1992) and at the auroral oval poleward boundary (Lockwood et al., 1985;Malingre et al., 2000). The occurrence frequency of the auroral upflow shows a dawn-dusk asymmetry, more frequent in the dusk sector (Liu et al., 2001). ...
A statistical study has been performed by using two years of DMSP (Defense
Meteorological Satellite Program)
plasma observations to investigate the seasonal effect of SAPS
(subauroral polarization stream) on the ion upflow in the duskside
ionosphere of the Northern Hemisphere. There are obvious upflows
occurring in the topside ionosphere around the SAPS region,
exceeding 200 m s−1 at winter solstice, indicating an important
relationship between SAPS and the local plasma upward motion. Both
SAPS and ion upward velocities show similar seasonal variations,
largest in winter and smallest in summer, irrespective of
geomagnetic activity. A good correlation is found and a linear
relationship is derived between SAPS and the ion upflow
velocities. During December solstice the average upflow flux can reach
about 2 × 108 cm−2 s−1 for more disturbed periods,
which is comparable to the typical upflow flux in the dayside cusp
region. The depression of the ion temperatures around the peak
SAPS region can be understood in terms of the adiabatic cooling.
The hot ion cools down when expanding into the low ion
concentration region. The electron temperature elevates around the
SAPS region because of the reduced Coulomb cooling in the low ion
density region. Both the changes of ion and electron temperatures
are larger in winter than in summer, however, for Kp < 4 the
electron temperatures are almost seasonably independent. The present work
highlights the important role of the SAPS-related frictional
heating at mid-latitudes on the local formation of the strong
upward flow, which might provide a direct ionospheric ion source
for the ring current and plasmasphere in the duskside sector.
... Ionospheric ion upflow, an important role in the magnetosphere-ionosphere coupling, can often be observed at altitudes ranging from 200 to several thousand kilometers in the cusp and auroral region [27][28][29] or at the poleward boundary of the auroral oval [30,31], providing O + for the magnetosphere. Previous simulations and observations revealed that ion upflow could also occur in the mid-latitudes SAPS region, and the upflow velocities increased with the westward velocities of SAPS [32][33][34][35][36]. Statistical studies showed that there was a linear relationship between SAPS and ion upflow velocities [37]. ...
Ionospheric ion upflow is an important process for magnetosphere-ionosphere coupling via O+ source for the magnetosphere. This process occurs frequently in the subauroral polarization stream (SAPS) region where the SAPS-enhanced ion-neutral frictional heating tends to push ions upward because of enhanced upward pressure gradient force. However, the SAPS-induced neutral wind transport by ion-neutral friction may also play an important role in triggering ion upflow, which has been rarely studied. In this work, the thermosphere-ionosphere-electrodynamics general circulation model (TIEGCM) with/without an empirical SAPS model has been employed to investigate the impacts of SAPS on ion upflow in the topside ionosphere. Our results separate different transport processes in the ion continuity equation, showing that SAPS can accelerate upward ambipolar diffusion along its channel because of ion-neutral frictional heating, but SAPS-induced horizontal neutral wind may have a comparable or even larger contribution to vertical ion drift when SAPS are fully developed. In addition, the neutral wind can induce both upward and downward ion drift in the SAPS region, depending on the direction of the neutral wind and the local geomagnetic declination and inclination.
... It is possible that the ions are perpendicularly energised by the waves driven at lower altitude by the shell distribution. Related to this, it was recently shown that low energy (<100 eV) ion outflows occur often in the PSBL (Malingre et al., 2000). Another interesting fact that we mention without showing a plot is that if one separates the counts in hydrogen and oxygen, one finds that the shell distribution is almost always purely hydrogen, whereas the upgoing beam or conic is a mixture of hydrogen and oxygen. ...
Ion shell distributions are hollow spherical shells in velocity space that can be formed by many processes and occur in several regions of geospace. They are interesting because they have free energy that can, in perincile, be transmitted to ions and electrons. Recently, a technique has been developed to estimate the riginal free energy available in shell deistributions from im-situ data, where some of the energy has already been lost (or consumed). We report a systematic survey of three years of data from the Polar satellite. We present as estimate of the free energy available from ion shell distribution on auroral field sampled by the Polar satellite below 6 RE geocentric radius. At these altitudes the type of ion shells that we are especially interested in is most commn on auroral field lines close to the polar cap (i.e. field lines mapping to the plasma sheet boundary layer, PSBL). Our analysis shows that ion shell distibutions that have lost some of their free energy are commonly found not only in the PSBL, but also on auroral field lines mapping to the boundary plasma sheet (BPS), especially in the evening sector auroral field lines. We suggest that the PSBL ion shell distributions are formed during the so-called Velocity Dispersed Ion Signatures (VDIS) events. Furthermore, we find that the partly consumed shells often occure in association with enhanced wave activity and middle-energy electron anisotropies. The maximum downward ion energy flux associated with a shell distribution is often 10mW m-2 and sometimes exceeds 40 mW m-2 when mapped to the ionosphere and thus may be enough to power many auroral processes. Earlier simulation studies have shown that ion shell distributions can excite ion Bernstein waves which, in turn, energise electrons in the parallel direction. It is possible that ion shell distributions are the link between the X-line and the auroral wave activity and electron acceleration in the energy transfer chain for stable auroral arcs.
The results of measuring the fluxes of ionospheric ions in the nightside polar cap at an altitude of about 20 000 km are presented. The data are obtained with the HYPERBOLOID instrument onboard the INTERBALL-2 satellite. The passages without intense precipitation of magnetospheric ions and electrons have been selected using the ION instrument data, so that the observation of ionospheric ion fluxes caused by heating in the auroral regions can be excluded. In addition, an attempt has been made to exclude observations of the cleft ion fountain from the analysis. The measurements in the summer and winter seasons (when the ionosphere was totally sunlit and completely shadowed, respectively) are considered separately. By analyzing the distribution functions of the fluxes measured, we have isolated six different types of ionospheric plasma flows in the polar cap. A strong distinction has been revealed between summer and winter flows. In winter, only weak flows of H+ ions were detected. In the summer period, we detected both H+ and O+ ions. The flux values of the ionosphere ions are found to be strongly dependent on the intensity of the polar rain. The measurements are compared to the existing models of the polar wind. The best models (for the description of our measurements) are indicated. After the choice of measurement periods, the resulting region coincides with the ion depletion zone (IDZ). Based on the earlier measurements by the AKEBONO satellite, nothing could be said about the fluxes of thermal ions in this zone, because of the insufficient sensitivity of the instrument aboard this satellite.
We report on a new feature of auroral substorms, namely, the sporadic though recurrent injections of magnetospheric ions throughout the auroral bulge. These injections are interpreted as time of flight dispersed ion structures (TDIS). Our analysis builds on a combination of measurements from Interball-Auroral, from UV imagery onboard Polar, from ground magnetometers, and also from observations on Geotail and from geostationary spacecraft. Backward tracing of ion trajectories from Interball-Auroral orbit using realistic three-dimensional magnetic and electric field models indicates that the injection region can extend over a wide range of radial distances, from ~7-40RE in the nearly equatorial magnetosphere. Both hydrogen and oxygen ions are shown to be injected toward the Earth's upper ionosphere. At Interball altitudes we find that ion injections are associated with two types of low-frequency torsional oscillations of the magnetic field: (1) shear Alfvén waves with a period of a few minutes with the highest amplitude near the bulge front and decreasing amplitude at lower latitudes and (2) higher-frequency shear Alfvén waves of the P1B type, strictly restricted to the poleward boundary of the surge, with a characteristic period of ~40 s. The systematic observation of sporadic TDIS during the auroral bulge expansion leads us to conclude that the same physical process is at work throughout the midtail. We also show that ion injections are detected well inside the bulge, which suggests that the injection fronts propagate from the outer to the inner magnetosphere over large distances. This topic is more extensively studied by V. Sergeev et al. (Plasma sheet ion injections into the auroral bulge: Correlative study of spacecraft and ground observations, submitted to Journal of Geophysical Research, 1999). We also show that the poleward boundary of the surge is associated with a prominent outflow of ionospheric H+ and O+. These ions in the hundred of eV to the keV range are heated perpendicularly to the local magnetic field and subsequently transported into the magnetotail. The expanding auroral bulge thus forms a significant source of ionospheric ions for the midtail magnetosphere.
We have examined characteristics of the field-aligned currents in the
poleward boundary region of the nightside auroral oval, using magnetic
field, electric field, and particle data obtained from the Akebono
satellite. We found that large-amplitude, wave-like fluctuations of
magnetic and electric fields frequently occur at mid-altitudes confined
in this latitudinally narrow region. We examined the relationship
between electric and magnetic field fluctuations, examined using the
complex impedance function defined as
Z~=μ0E~x/B~y, where E~x
and B~y are the X component of the electric field and the Y
component of the magnetic field, respectively. The most important result
obtained from this analysis is that the features of the impedance
functions in the frequency range between 2×10-3 and
8×10-2 Hz are well explained by a time-dependent
magnetosphere-ionosphere coupling model. Further, the calculation of the
field-aligned Poynting flux showed that most of the Alfven wave energy
is reflected at the ionosphere rather than dissipated in the ionosphere.
These facts suggest that the majority of the field-aligned current
fluctuations observed at mid-altitudes in the poleward boundary region
of the nightside auroral oval are due to the superposition of incident
and reflected Alfven waves. It is speculated that the breaking of
reflected Alfven waves in the PSBL may cause the heating of plasma sheet
electrons and ions.
The measurements of electric fields and the composition of upward
flowing ionospheric ions (UFIs), obtained by Viking's Ion Composition
Spectrometers are interpreted in terms of a mass-dependent plasma escape
process in the upper ionosphere. Results on the temperature and
mass-composition of UFI beams suggest that the beams can be generated by
a magnetic moment 'pumping' mechanism caused by low-frequency transverse
electric field fluctuations, in addition to a field-aligned
'quasi-electrostatic' acceleration process. The results agree with a
bimodal model of ionospheric ion acceleration, based on both
quasi-static and fluctuating electric fields as the cause for
ionospheric plasma escape.
Core (0-50 eV) ion pitch angle measurements from the retarding ion mass spectrometer on Dynamics Explorer 1 are examined with respect to magnetic disturbance, invariant latitude, magnetic local time, and altitude for ions H{sup +}, He{sup +}, O{sup +}, M/Z=2 (D{sup +} or He{sup ++}), and O{sup ++}. Included are outflow events in the auroral zone, polar cap, and cusp, separated into altitude regions below and above 3 R{sub E}. In addition to the customary division into beam, conic, and upwelling distributions, the high-latitude observations fall into three categories corresponding to ion bulk speeds that are (1) less than, (2) comparable to, or (3) faster than that of the spacecraft. This separation, along with the altitude partition, serves to identify conditions under which ionospheric source ions are gravitationally bound and when they are more energetic and able to escape to the outer magnetosphere. Features of the cleft ion fountain inferred from single event studies are clearly identifiable in the statistical results. In addition, it is found that the dayside pre-noon cleft is a consistent source of escape velocity low-energy ions regardless of species or activity level and the dayside afternoon cleft, or auroral zone, becomes an additional source for increased activity. The auroral oval as a whole appears to be a steady source of escape velocity H{sup +}, a steady source of escape velocity He{sup +} ions for the dusk sector, and a source of escape velocity heavy ions for dusk local times primarily during increased activity. The polar cap above the auroral zone is a consistent source of low-energy ions, although only the lighter mass particles appear to have sufficient velocity, on average, to escape to higher altitudes. 57 refs., 11 figs., 1 tab.
The authors used magnetic field measurements, and particle diagnostics on Akebono to study field-aligned current systems in the nightside auroral oval, and to relate the observed current systems to structures in the plasma sheet. They observe two main current systems, one quite narrow in latitude, and the other much broader latitudinally. Both seem to exist in closed field line regions. Then from interpretation of charged-particle precipitation fluxes the authors argue about the origins of these different current systems when mapped to the plasma sheet.
During the expansion phase of a substorm on October 10, 1997, the Interball-Auroral probe traveling across the auroral zone in the midnight sector detected sporadic injections of H+ and O+ ions with energies of a few keVs. We show that these injections are of ionospheric origin and are related to the large fluxes of low-energy (a few tens of eV) upflowing ions near the poleward boundary of the auroral oval. Using test particle trajectory simulations, we demonstrate that these low-energy ionospheric ions may be subjected to a rapid (on the timescale of expansion phase) circulation inside the plasma sheet. After their ejection from the poleward propagating auroral bulge, these ions travel into the midtail where they behave nonadiabatically and experience energization up to the keV range. As a result of magnetic moment damping in the current sheet, some of these ionospheric ions escape from the equatorial magnetosphere and travel back toward low altitudes. Numerical calculations reveal that upon reaching Interball-Auroral near -3RE altitude, these particles give rise to the observed keV injections, the sporadic character of which is due to the chaotic nature of ion motion in the magnetotail.
The electric potential of the spacecraft INTERBALL-2 (Auroral Probe) has been analyzed for a period of 18 months starting in October 1996. Data from the instrument IESP, as received filtered onboard the spacecraft by the instrument RON, serve as the database. The data have been organized in MLT and invariant latitude, and the dependence oa various geophysical parameters has been studied. Using relations between spacecraft potential and plasma density from the literature, the plasma density between 16000 and 19 000 km altitude is derived. The relative variations of the densities not only reflect the typical global structure, but also exhibit features of the plasmasphere and auroral region on smaller scales. The accuracy of the absolute density values obtained by this method is discussed.
The effect on cold ions of ionospheric origin of the low-frequency, large-amplitude electric field fluctuations (LEF) observed by Viking on auroral latitude magnetic field lines is calculated directly with the use of an observed time series of the electric field. The perpendicular acceleration of O+ ions is found to be more effective than that of photons. Energies in keV and tens of keV ranges are reached within a few tens of seconds by both O+ and H+ ions, but O+ ions obtain higher energies. Outflow velocities along the field line of hundreds of kilometers per second are generally acquired. Similar results can be found with a completely random variation of the electric field, but with less favoring of O+ ions over protons. The amplitude dependence of the perpendicular acceleration is roughly quadratic. The efficiency of LEF in bringing ionospheric ions of all kinds, but particularly heavier ions, into the magnetosphere is very high.