No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Polar Wind Observations on the Nightside of the Polar Cap at Altitudes of 2–3 RE: Results of the INTERBALL-2 Satellite
Abstract
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.
... Latitudes of 77° and 70° were taken in [11] and [16], respectively. However, in our publication [17] and later in [18], it was noted that in the polar cap at high altitudes, one can distinguish several types of ionospheric ions outflow and only one of them is caused by the polar wind mechanism. These flows could be cause by the following processes. ...
... In order to exclude even in some way possible confusion in interpretation of measurements, an attempt of more accurate determination of the polar cap region was undertaken in [17,18]. In [17] on the basis of measurements of energetic particles, the periods were selected which satisfied the condition of the least heating of the underlying ionosphere by energetic particles. ...
... In order to exclude even in some way possible confusion in interpretation of measurements, an attempt of more accurate determination of the polar cap region was undertaken in [17,18]. In [17] on the basis of measurements of energetic particles, the periods were selected which satisfied the condition of the least heating of the underlying ionosphere by energetic particles. In [18] the model of the polar cap boundary dependence on the ä index from [21] was taken, and the measurements which fell inside this boundary were considered. ...
Characteristics of polar wind fluxes at a height of ∼20000 km measured by the Hyperboloid mass-spectrometer installed onboard the Interball-2 satellite are presented in the paper. The characteristics are presented for the upwelling flows of ionospheric ions H+, He+, and O+ from the sunlit polar cap in the period of solar activity minimum. Orbit segments with minimal precipitation of magnetospheric ions and electrons were preliminarily selected, and the measurements where the fluxes of ions coming from the cusp/cleft were excluded as carefully as possible. Thus, the densities, field-aligned velocities, and temperatures of ions in the regions where fluxes of polar wind could be detected with the maximal probability degree are presented in the paper. It is found that cases when only H+ ions are reaching the detector are with high probability the polar wind outflows. Their characteristics agree well with the Tube-7 hydrodynamic model and are as follows: n ≈ 1.5 cm−3, V
∥ ∼ 21 km/s; T
∥ = 3500 K, and T
⊥ = 2000 K. In cases when He+ and O+ ions are also detected, the temperatures are substantially higher than the model ones, and the measured field-aligned velocities of O+ fluxes are several times higher than the model ones. Moreover, it was revealed that the polar wind outflows are predominantly observed in the polar cap regions where the polar rain fluxes are very small.
The problem of the outflow of ionospheric plasma into the magnetosphere is considered. In particular, the phenomenon of the polar wind observed in the polar cap is studied. The study of this phenomenon is complicated by the fact that the field-alined velocities of individual ions are small, and therefore, the electric field of the positively charged satellite prevents their measurement. This paper examines the measurements carried out on the Interball-2 satellite at altitudes of ~20000 km and compares them with the results of simulations within the framework of the GSM TIP model. It has been demonstrated the GSM TIP model well describes the outflow of H⁺ ions from the ionosphere to the magnetosphere in the polar cap.
1] The cross polar cap potential varies roughly linearly with the solar wind electric field for nominal conditions but asymptotes to a constant value of order 200 kV for large electric field. When the impedance of the solar wind across open polar cap field lines dominates the impedance of the ionosphere, Alfvén waves incident from the solar wind are partially reflected, reducing the signal in the polar cap. Thus, the ratio of the cross polar cap potential to the potential imposed by the solar wind is 2S A /(S P + S A), where S A is the Alfvén conductance of the solar wind (= (r sw /m o) 1/2 /B sw) to within a density-dependent factor on average of order 1, S P is the Pedersen conductance of the ionosphere, and r sw (B sw) is the density (magnetic field magnitude) of the solar wind. For small B sw , the response is proportional to B sw . For large B sw , the cross polar cap potential depends only on the solar wind dynamic pressure (with small viscous and density-dependent corrections). Quantitative estimates require knowledge of S P and the dependence of the potential imposed by the solar wind on its measured properties; standard assumptions yield saturation levels consistent with observations made during 13 storm intervals. Previous explanations of saturation have invoked changing reconnection efficiency, specific characteristics of the Region 1 current system, or the effect of the bow shock on the reconnecting plasma. Although our relation is mathematically similar to some previously proposed, our arguments place no constraints on reconnection efficiency or on magnetospheric geometry.
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.
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.
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.
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.
The measurement of the
thermal ion distributions in space is always strongly influenced by the ion
motion through the complex 3D electrostatic potential structure built around a
charged spacecraft. In this work, we study the related aberrations of the ion
distribution detected on board, with special application to the case of the
Hyperboloid instrument borne by the Interball-2 auroral satellite. Most of the
time, the Interball-2 high altitude auroral satellite is charged at some
non-negligible positive potential with respect to the ambient plasma, as shown
in part 1; in consequence, the measurement of magnetospheric low energy ions
(< 80 eV) with the Hyperboloid instrument can be disturbed by the complex
electric potential environment of the satellite. In the case of positive
charging, as in previous experiments, a negative bias is applied to the
Hyperboloid structure in order to reduce this effect and to keep as much as
possible the opportunity to detect very low energy ions. Then, the ions
reaching the Hyperboloid entrance windows would have travelled across a
continuous huge electrostatic lens involving various spatial scales from ~ 10
cm (detector radius) to ~ 10 m (satellite antennas). Neglecting space charge
effects, we have computed the ion trajectories that are able to reach the
Hyperboloid windows within their acceptance angles. There are three main
results: (i) for given values of the satellite potential, and for each
direction of arrival (each window), we deduced the related energy cutoff; (ii)
we found that all ions in the energy channel, including the cutoff, can come
from a large range of directions in the unperturbed plasma, especially when the
solar panels or antennas act as electrostatic mirrors; (iii) for higher energy
channels, the disturbances are reduced to small angular shifts. Biasing of the
aperture is not very effective with the Hyperboloid instrument (as on previous
missions with instruments installed close to the spacecraft body) because the
potential environment is driven by effects from the spacecraft. Our results are
used to explain some unexpected features of the low energy ion measurements,
especially during polar wind events recorded by Hyperboloid. In conclusion,
knowing the satellite potential from IESP measurements, we were able to reject
any low energy doubtful data and to perform angular corrections for all higher
energy ion data. Then the selected and corrected data are a reliable basis for
interpretation and estimation of the thermal ion distributions.Key words. Space plasma physics
(charged particle motion and acceleration; numerical simulation studies;
spacecraft sheaths, wakes, charging)
Hyperboloid is a multi-directional mass
spectrometer measuring ion distribution functions in the auroral and polar
magnetosphere of the Earth in the thermal and suprathermal energy range. The
instrument encompasses two analyzers containing a total of 26 entrance windows,
and viewing in two almost mutually perpendicular half-planes. The nominal
angular resolution is defined by the field of view of individual windows
≈13° × 12.5°. Energy analysis is performed using spherical
electrostatic analyzers providing differential measurements between 1 and 80 eV.
An ion beam emitter (RON experiment) and/or a potential bias applied to
Hyperboloid entrance surface are used to counteract adverse effects of
spacecraft potential and thus enable ion measurements down to very low energies.
A magnetic analyzer focuses ions on one of four micro-channel plate (MCP)
detectors, depending on their mass/charge ratio. Normal modes of operation
enable to measure H+, He+, O++, and O+
simultaneously. An automatic MCP gain control software is used to adapt the
instrument to the great flux dynamics encountered between spacecraft perigee
(700 km) and apogee (20 000 km). Distribution functions in the main analyzer
half-plane are obtained after a complete scan of windows and energies with
temporal resolution between one and a few seconds. Three-dimensional (3D)
distributions are measured in one spacecraft spin period (120 s). The secondary
analyzer has a much smaller geometrical factor, but offers partial access to the
3D dependence of the distributions with a few seconds temporal resolution.
Preliminary results are presented. Simultaneous, local heating of both H+
and O+ ions resulting in conical distributions below 80 eV is
observed up to 3 Earth's radii altitudes. The thermal ion signatures associated
with large-scale nightside magnetospheric boundaries are investigated and a new
ion outflow feature is identified associated to the polar edge of the auroral
oval. Detailed distribution functions of injected magnetosheath ions and
ouflowing cleft fountain ions are measured down to a few eVs in the dayside.Key words. Ionosphere (auroral ionosphere; particle
acceleration; ionosphere-magnetosphere interactions)
The satellite INTERBALL-2 has an orbit with
high inclination (62.8°), covering the altitude range between a few hundred and
about 20000 km. The ambient plasma conditions along this orbit are highly
variable, and the interactions of this plasma with the spacecraft body as well
as the photo-electron sheath around it are considered to be interesting topics
for detailed studies. The electric potential of the spacecraft with respect to
the ambient plasma that develops as a result of the current equilibrium reacts
sensitively to variations of the boundary conditions. The measurement and
eventual control of this potential is a prerequisite for accurate measurements
of the thermal plasma. We describe the purpose and technical implementation of
an ion emitter instrument on-board INTERBALL-2 utilising ion beams at energies
of several thousand electron volts in order to reduce and stabilise the positive
spacecraft potential. First results of the active ion beam experiments, and
other measures taken on INTERBALL-2 to reduce charging are presented.
Furthermore, the approach and initial steps of modelling efforts of the sheath
in the vicinity of the INTERBALL-2 spacecraft are described together with some
estimates on the resulting spacecraft potential, and effects on thermal ion
measurements. It is concluded that even moderate spacecraft potentials as are
commonly observed on-board INTERBALL-2 can significantly distort the
measurements of ion distribution functions, especially in the presence of
strongly anisotropic distributions.Key words. Space plasma physics (active perturbation
experiments; spacecraft sheaths · wakes · charging; instruments and
techniques).
The presence of unthermalized photoelectrons in the sunlit polar cap leads to an enhanced ambipolar potential drop and enhanced upward ion acceleration. Observations in the topside ionosphere have led to the conclusion that large-scale electrostatic potential drops exist above the spacecraft along polar magnetic field lines connected to regions of photoelectron production. A kinetic approach is used for the O(+), H(+), and photoelectron (p) distributions, while a fluid approach is used to describe the thermal electrons (e) and self-consistent electric field (E(sub II)) electrons are allowed to carry a flux that compensates for photoelectron escape, a critical assumption. Collisional processes are excluded, leading to easier escape of polar wind particles and therefore to the formation of the largest potential drop consistent with this general approach. We compute the steady state electric field enhancement and net potential drop expected in the polar wind due to the presence of photoelectrons as a function of the fractional photoelectron content and the thermal plasma characteristics. For a set of low-altitude boundary conditions typical of the polar wind ionosphere, including 0.1% photoelectron content, we found a potential drop from 500 km to 5 R(sub E) of 6.5 V and a maximum thermal electron temperature of 8800 K. The reasonable agreement of our results with the observed polar wind suggests that the assumptions of this approach are valid.
Measurements from the Retarding Ion Mass Spectrometer on the Dynamics Explorer have, for the first time, revealed a supersonic polar wind along polar cap field lines. The observations reported were obtained on the nightside (22:30 to 23:30 MLT) from 65 to 81 deg invariant latitude and at altitudes near 2 earth radii. Fitting the data using a thin-sheath model gives a range of temperatures of 0.1 to 0.2 eV with corresponding flow velocities of 25 to 16 km/s over the estimated range of spacecraft potential of +3 to +5 V. For these values the Mach number ranged from 5.1 to 2.6 (with a most likely value of 3). Characteristics of the H(+) flow are in general agreement with those predicted by 'classical' polar wind theory, but high variabilty of the He(+)/H(+) ratio was observed.
The frequency of occurrence and characteristics of upflowing ions (UFI) in the polar cap and auroral ionosphere are examined statistically using data from the Atmospheric Dynamics Explorer I. A total of 43,000 measurements of the UFI were made from 1981-83, and covered both masses and energies of the ions. The data indicated that many UFI observed at high altitudes originated as low altitude conical or perpendicular ions. The peak source of the ions was below 23,000 km and was latitude and time dependent. The frequency of O+ conics decreased with altitude relative to the H+ conics. The O+ and H+ conics distributions both, however, displayed dawn-dusk asymmetries that favored the dusk sector.
We present a statistical analysis of thermal H+ and O+ ion flux measurements in the high-altitude (6000-9000 km) polar ionosphere from the Suprathermal ion Mass Spectrometer (SMS) on Akebono. It is shown that the normalized H+ polar wind flux (to 2000 km altitude) varies from 107 to 108 cm-2s-1 at 2000 km altitudes. Surprisingly, the O+ ion flux is found to be comparable to the H+ ion flux and much higher than classical theory prediction. The magnetic local time (MLT) distribution of the upward ion flux and its geomagnetic activity (Kp) dependence are also presented. At both magnetically quiet and active times, the integrated H+ ion flux is largest in the noon sector (09-15 MLT) and smallest in the midnight sector (21-03 MLT); the flux ratio was found to be approximately one order of magnitude. The total flux of H+ ion outflow integrated over the polar ionosphere (ILAT ≥ 75°) and over all local times was found to correlate inversely with the Kp index. The integrated H+ flux (ILAT ≥ 75°) in quiet times was 0.9∼1.5 x 1025 ions s-1 while the flux in active times was a factor of 2∼3 smaller (0.4∼0.6 x 1025 ions s-1). It also exhibited a slight positive correlation with the IMF (interplanetary magnetic field) Bz component.
Experimental data on the "polarization jet", which is a narrow band of supersonic westward convection in the disturbed subauroral nightside ionosphere, are mainly based on measurements made at ionospheric altitudes. Though model calculations allow one to evaluate the expected effects of the polarization jet at the outer plasmasphere altitudes, direct comparisons of these model results with experimental data remain to be made. One of the reasons for this could possibly be the difficulty of taking into account the effects of the Langmuir sheath around a satellite in a rarefied plasma, especially when the satellite potential with respect to the plasma is comparable to or larger than kT of the ambient plasma and the Debye length reaches tens of cm or more. This paper presents some results of model calculations of the 3D distribution of the electric field in vacuum around a realistic model of the INTERBALL Auroral Probe satellite. This model includes the solar panels, antennas of the POLRAD experiment, and rods with electric and magnetic field sensors on them (the surfaces of the satellite and solar panels were equipotentialized). The trajectories of thermal and suprathermal ions H+ and O+ with an initial bi-Maxwellian distribution function were calculated in this electric field for a given satellite potential with respect to plasma. Examples of resulting model ion distribution functions, "measured" by the on-board HYPERBOLOID mass-spectrometer, are presented. The distortions in these functions, introduced by the complicated electric field structure, may be rather high. The conclusion is drawn that a reliable interpretation of anisotropic distribution functions of ions in a rarefied plasma is possible only if the values of satellite's potential with respect to the plasma are low.
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.
An analysis of the wave data obtained onboard the Interball-2 (Auroral Probe) satellite in the polar cap region at altitudes from 2 to 3 radii of the Earth is performed. The analysis shows that, in the conditions of a disturbed magnetosphere, when electron fluxes intruding from other magnetospheric regions are recorded, small-scale bursts of the Langmuir oscillations are observed in the region in question. The burst-like character, irregular structure, amplitude variations, and low-frequency modulations are typical features of these oscillations. Generation of the oscillations with such characteristic properties may be explained in the framework of the theory of stochastic wave growth. The comparison of the probability distributions for the measured values of the electric fields with the distributions predicted by the theory demonstrate a good agreement in the low-energy region when nonlinear wave processes are not important. As soon as the energy sufficient for nonlinear process development in the system is reached, the experimental curves decrease more steeply than the decrease predicted by the theory, which is true only in the linear approximation.
The geomagnetic field is deformed by the solar wind so that field lines from the polar caps extend away from the earth in the antisolar direction to form a magnetic tail. The ionosphere over the polar caps is sufficiently hot that plasma (mainly ionized hydrogen) can escape from the two polar ionospheres and flow along the magnetic field lines into the tail. We argue that the resulting flow of protons and electrons out of the ionosphere acts to limit the thermal plasma number density to the order of 102/cm3 immediately above the polar ionosphere, decreasing ultimately to very small values (10−1 cm3) at large distances down the tail (30 RE). The plasma that flows into the tail must be replaced, presumably by ionization of the neutral atmosphere over the polar caps. It is proposed that this ionization is caused by solar wind particles diffusing into the geomagnetic tail and thence down the field lines to the polar caps. Recent measurements of total cislunar electron content have been interpreted as indicating an electron number density of about 2×102/cm3 in the quadrant opposite the sun. It seems difficult to account for such high densities either by transporting ionospheric plasma into the tail or by compressing solar plasma. We conclude that either the tail is filled principally with solar plasma with a number density no greater than about 10/cm3 or else we have overlooked some important physical process that is capable of condensing plasma in the tail to a high density.
A mathematical one-dimensional model is developed for calculating the concentration, temperature, and velocity of ions along a magnetic field tube from a height of 125 km to a height of several Earth's radii. The model allows the solution of a complete set of hydrodynamic equations for seven ion species (including molecular): O+, H+, He+, N+, O+2, NO+, and N+2, and electrons. Numerical methods and schemes for solving the equations are presented. It is shown that inclusion of the longitudinal motions of molecular ions in the nonstationary processes occurring in the field tube and associated with ion and electron heating strongly modifies the height distributions of major ion components.
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 authors present results from the suprathermal ion mass spectrometer on the Akebono satellite of what they refer to as ion depletion zones, areas in the nightside magnetosphere where the ion densities are below the detection limit of the instrument, typically 10â»Â² cmâ»Â³. These regions were observed typically at altitudes above 8000 km. Outside these zones they typically saw upwelling ion flows of hydrogen, helium and oxygen ions. At altitudes below these depletion zones the authors saw ion distributions consistent with ions stationary in the earth`s rotating frame.
In this study we investigate how the condition of zero current on open flux tubes with polar wind outflow, subjected to large photoelectron fluxes, can be achieved. We employ a steady state collisionless semikinetic model to determine the density profiles of O(+), H(+), thermal electrons and photoelectrons coming from the ionosphere along with H(+), ions and electrons coming from the magnetosphere. The model solution attains a potential distribution which both satisfies the condition of charge neutrality and zero current. For the range of parameters considered in this study we find that a 45-60 volt discontinuous potential drop may develop to reflect most of the photoelectrons back toward the ionosphere. This develops because the downward flux of electrons from the magnetosphere to the ionosphere on typical open flux tubes (e.g. the polar rain) appears to be insufficient to balance the photoelectron flux from the ionosphere.
In the present polar magnetosphere model, which includes the effects of convection electric fields and gravitation, the trajectories of mainly low energy ionospheric ions injected near the polar cusp into the polar magnetosphere display the ion mass and energy differentiation seen in recent satellite observations of low energy ionospheric ions. Two interesting trajectory classes are noted for low energy heavy ions: parabolic trajectories, in which ions injected into the polar cusp at small pitch angles rise, and then fall, into the polar cap atmosphere, and 'hopping' trajectories, in which heavy ions injected at large pitch angles at the polar cusp will mirror as they convect at low to medium altitudes across the polar cap.
We correlate solar wind and IMF properties with the properties of O(+) and H(+) in the polar cap in early 1996 during solar minimum conditions at altitudes between 5.5 and 8.9 Re geocentric using the Thermal Ion Dynamics Experiment (TIDE) on the POLAR satellite. Throughout the high altitude polar cap, we observe H(+) to be more abundant than O(+). H(+) is a significant fraction of both the ionosphere and the solar wind, and O(+) is not a significant species in the solar wind. O(+) is the major species in the ionosphere so the faction of O(+) present in the magnetosphere is commonly used as a measure of the ionospheric contribution to the magnetosphere. For these reasons, 0+ is of primary interest in this study. We observe O(+) to be most abundant at lower latitudes when the solar wind speed is low (and low Kp), and at higher solar wind speeds (and high Kp) O(+) is observed across most of the polar cap. We also find that O(+) density and parallel flux are well organized by solar wind dynamic pressure; they both increase with solar wind dynamic pressure. H(+) is not as highly correlated with solar wind and IMF parameters, but H(+) density and parallel flux have some negative correlation with IMF By, and some positive correlation with VswBIMF. In this solar minimum data set, H(+) is dominant so that contributions of this plasma to the plasma sheet would have a very low O(+) to H(+) ratio.
High altitude observations made by DE-1 in the polar regions indicate that the polar wind consists of both a heated and an unheated component. The unheated component constitutes what may be called the 'classical polar wind' while the heated component is thought to be the result of the interaction of the primal polar wind with regions of ion perpendicular heating. The altitude at which the heating occurs is estimated to lie between 8000 and 12000 km.
Obser-vations of the Polar Wind
- M O Chandler
- J H Waite
- Jr
- T E Moore
Chandler, M.O., Waite, J.H., Jr., and Moore, T.E., Obser-vations of the Polar Wind, J. Geophys. Res., 1991, vol. 96, pp. 1421–1428.