RE Geocentric Distance Research Articles

Signatures of Auroral Potential Structure Extending Through the Near‐Equatorial Inner Magnetosphere

AbstractThe auroral acceleration region plays an important role in the magnetosphere‐ionosphere coupling system. In this study, signatures of an auroral U‐shaped potential structure were found for the first time in the near‐equatorial inner magnetosphere by the Arase satellite at ∼6.0 RE geocentric distance and 11° magnetic latitude. The observed magnetic and electric field variations corresponded to the equatorward motion of the upward field‐aligned current and converging perpendicular electric field. Examining the three‐dimensional velocity distribution function of H+ and O+ ions, we demonstrate that upflowing ion beams were significantly deflected in an east‐west direction with a perpendicular velocity up to ∼80 km/s, which is consistent with the ExB drift velocity. A simple particle drift model with the inferred auroral perpendicular potential presents a new kink‐like drift path of ions from the magnetotail, implying that the auroral potential structure has a great impact on particle dynamics in the near‐earth plasma sheet.

Can Earth's Magnetotail Plasma Sheet Produce a Source of Relativistic Electrons for the Radiation Belts?

AbstractSimultaneous observations from Van Allen Probes in Earth's outer radiation belt (∼4–6 RE) and Magnetospheric Multiscale (MMS) in the magnetotail plasma sheet at >20 RE geocentric distance are used to compare relative levels of relativistic electron phase space density (PSD) for constant values of the first adiabatic invariant, M. We present new evidence from two events showing: (a) at times, there is sufficient PSD in the central plasma sheet to provide a source of >1 MeV electrons into the outer belt; (b) the most intense levels of relativistic electrons are not accelerated in the solar wind or transported from the inner magnetosphere and thus must be accelerated rapidly (within ∼minutes or less) and efficiently across a broad region of the magnetotail itself; and (c) the highest intensity relativistic electrons observed by MMS were confined within only the central plasma sheet. The answer to the title question here is: yes, it can, however whether Earth's plasma sheet actually does provide a source of several 100s keV to >1 MeV electrons to the outer belt and how often it does so remain important outstanding questions.

Position of projections of the nightside auroral oval equatorward and poleward edges in the magnetosphere equatorial plane

The position of the auroral oval poleward and equatorward boundary projections on the equatorial plane in the nightside MLT sector during magnetically quiet periods (|AL| < 200 nT, |Dst| < 10 nT) has been determined. The oval boundary positions were determined according to the precipitation model developed at Polar Geophysical Institute (http://apm.pgia.ru/). The isotropy of the averaged plasma pressure and the experimentally confirmed balance of pressures during the nighttime have been taken into account. The morphological mapping method has been used to map the oval poleward and equatorward edges without the use of any magnetic field model on the assumption that the condition of magnetostatic equilibrium is valid. Ion pressures at ionospheric altitudes and in the equatorial plane have been compared. It has been shown that the auroral oval equatorward boundary in the midnight sector is localized at geocentric distances of ~7 RE, which is in good agreement with the position of the energetic particle injection boundary in the equatorial plane. The oval poleward edge is localized at the ~10 RE geocentric distance, which is in good agreement with the position of the equatorward boundary of the region with a high turbulence level in the Earth’s magnetosphere plasma sheet.

“Snowplow” injection front effects

AbstractAs the Polar spacecraft apogee precessed through the magnetic equator in 2001, Polar encountered numerous substorm events in the region between geosynchronous orbit and 10 RE geocentric distance; most of them in the plasma sheet boundary layers. Of these, a small number was recorded near the neutral sheet in the evening sector. Polar/Thermal Ion Dynamics Experiment provides a unique perspective on the lowest‐energy ion plasma, showing that these events exhibited a damped wavelike character, initiated by a burst of radially outward flow transverse to the local magnetic field at ~80 km/s. They then exhibit strongly damped cycles of inward/outward flow with a period of several minutes. After one or two cycles, they culminated in a hot plasma electron and ion injection, quite similar to those observed at geosynchronous orbit. Cold plasmaspheric plasmas comprise the outward flow cycles, while the inward flow cycles contain counterstreaming field‐parallel polar wind‐like flows. The observed wavelike structure, preceding the arrival of an earthward moving substorm injection front, suggests an outward displacement driven by the inward motion at local times closer to midnight, that is, a “snowplow” effect. The damped in/out flows are consistent with interchange oscillations driven by the arrival at the observed local time by an injection originating at greater radius and local time.

Centrifugal acceleration in the magnetotail lobes

Abstract. Combined Cluster EFW and EDI measurements have shown that cold ion outflow in the magnetospheric lobes dominates the hydrogen ion outflow from the Earth's atmosphere. The ions have too low kinetic energy to be measurable with particle instruments, at least for the typical spacecraft potential of a sunlit spacecraft in the tenuous lobe plasmas outside a few RE. The measurement technique yields both density and bulk velocity, which can be combined with magnetic field measurements to estimate the centrifugal acceleration experienced by these particles. We present a quantitative estimate of the centrifugal acceleration, and the velocity change with distance which we would expect due to centrifugal acceleration. It is found that the centrifugal acceleration is on average outward with an average value of about of 5 m s−2. This is small, but acting during long transport times and over long distances the cumulative effect is significant, while still consistent with the relatively low velocities estimated using the combination of EFW and EDI data. The centrifugal acceleration should accelerate any oxygen ions in the lobes to energies observable by particle spectrometers. The data set also put constraints on the effectiveness of any other acceleration mechanisms acting in the lobes, where the total velocity increase between 5 and 19 RE geocentric distance is less than 5 km s−1.

THEMIS ground-space observations during the development of auroral spirals

Abstract. A simultaneous observation of an auroral spiral and its generator region in the near-Earth plasma sheet is rather unlikely. Here we present such observations using the THEMIS spacecraft as well as the THEMIS ground network of all-sky imagers and magnetometers. Two consecutive auroral spirals separated by approximately 14 min occurred during a substorm on 19 February 2008. The spirals formed during the expansion phase and a subsequent intensification, and were among the brightest features in the aurora with diameters of 200–300 km. The duration for the formation and decay of each spiral was less than 60 s. Both spirals occurred shortly after the formation of two oppositely rotating plasma flow vortices in space, which were also accompanied by dipolarizations and ion injections, at ~11 RE geocentric distance. Observations and model calculations also give evidence for a magnetic-field-aligned current generation of approximately 0.1 MA via the flow vortices, connecting the generator region of the spirals with the ionosphere, during the formation of both spirals. In the ionosphere, a pair of equivalent ionospheric current (EIC) vortices with opposite rotations (corresponding to upward and downward currents) was present during both auroral spirals with enhanced EICs and ionospheric flows at the locations of the auroral spirals and along the auroral arcs. The combined ground and space observations suggest that each auroral spiral was powered by two oppositely rotating plasma flow vortices that caused a current enhancement in the substorm current wedge.

Magnetospheric solitary structure maintained by 3000 km/s ions as a cause of westward moving auroral bulge at 19 MLT

Abstract. In the evening equatorial magnetosphere at about 4 RE geocentric distance and 19 MLT, the four Cluster spacecraft observed a solitary structure with a width of about 1000~2000 km in the propagation direction. The solitary structure propagates sunward with about 5~10 km/s carrying sunward electric field (in the propagation direction) of up to about 10 mV/m (total potential drop of about 5~10 kV), depletion of magnetic field of about 25%, and a duskward E×B convection up to 50 km/s of He+ rich cold plasma without O+. At the same time, auroral images from the IMAGE satellite together with ground based geomagnetic field data showed a westward (sunward at this location) propagating auroral bulge at the magnetically conjugate ionosphere with the solitary structure. The solitary structure is maintained by flux enhancement of selectively 3000 km/s ions (about 50 keV for H+, 200 keV for He+, and 750 keV for O+). These ions are the main carrier of the diamagnetic current causing the magnetic depletion, whereas the polarization is maintained by different behavior of energetic ions and electrons. Corresponding to aurora, field-aligned accelerated ionospheric plasma of several keV appeared at Cluster from both hemispheres simultaneously. Together with good correspondence in location and propagation velocity between the auroral bulge and the solitary structure, this indicates that the sunward moving auroral bulge is caused by the sunward propagation of the solitary structure which is maintained by energetic ions. The solitary structure might also be the cause of Pi2-like magnetic variation that started simultaneously at Cluster location.

Substorm current wedge driven by plasma flow vortices: THEMIS observations

A multipoint analysis of conjugate magnetospheric and ionospheric flow vortices during the formation of the substorm current wedge (SCW) on 19 February 2008 is presented. During the substorm, four Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft were located close to the neutral sheet in the premidnight region between 9 and 12 RE geocentric distance, of which three closely (∼1–2 RE) clustered at ∼23 MLT and one was farther west at ∼21 MLT. The closely clustered spacecraft were engulfed by a counterclockwise plasma flow vortex, while the single spacecraft recorded a clockwise plasma flow vortex. Simultaneously, a pair of conjugate flow vortices with clockwise and counterclockwise rotation appeared in the ionosphere, as inferred from equivalent ionospheric currents. The counterclockwise space vortex, which corresponded to a downward field‐aligned current, was at least 1–2 RE in diameter and had rotational flow speeds of up to 900 km/s. Current density estimates associated with the formation of the space vortex in the first 30 s yielded 2.8 nA/m2 (14 μA/m2 mapped to the ionosphere), or a total current of 1.1 × 105 A. Model calculations based on midlatitude ground magnetometer data show a gradual increase of the field‐aligned current, with 1–2 × 105 A within the first minute and a peak value of 7 × 105 A after 10 min, associated with the SCW, and a matching meridian of the downward current of the SCW and the downward current (counterclockwise) space vortex. The combined ground and space observations, together with the model results, present a scenario in which the space vortices generated the field‐aligned current of the SCW at the beginning of the substorm expansion phase and coupled to the ionosphere, causing the ionospheric vortices.

Plasma transport along discrete auroral arcs and its contribution to the ionospheric plasma convection

Abstract. The role of intense high-altitude electric field (E-field) peaks for large-scale plasma convection is investigated with the help of Cluster E-field, B-field and density data. The study covers 32 E-field events between 4 and 7 RE geocentric distance, with E-field magnitudes in the range 500–1000 mV/m when mapped to ionospheric altitude. We focus on E-field structures above the ionosphere that are typically coupled to discrete auroral arcs and their return current region. Connected to such E-field peaks are rapid plasma flows directed along the discrete arcs in opposite directions on each side of the arc. Nearly all the E-field events occur during active times. A strong dependence on different substorm phases is found: a majority of intense E-field events appearing during substorm expansion or maximum phase are located on the nightside oval, while most recovery events occur on the dusk-to-dayside part of the oval. For most expansion and maximum phase cases, the average background plasma flow is in the sunward direction. For a majority of recovery events, the flow is in the anti-sunward direction. The net plasma flux connected to a strong E-field peak is in two thirds of the cases in the same direction as the background plasma flow. However, in only one third of the cases the strong flux caused by an E-field peak makes an important contribution to the plasma transport within the boundary plasma sheet. For a majority of events, the area covered by rapid plasma flows above discrete arcs is too small to have an effect on the global convection. This questions the role of discrete auroral arcs as major driver of plasma convection.

Correlation of substorm injections, auroral modulations, and ground Pi2

In this case study we report a substorm, 23 March 2007, which exhibited oscillations with a period of ∼135 s in three substorm phenomena all of which were one‐to‐one correlated. The in‐situ observations are from one THEMIS spacecraft (8.3 RE geocentric distance) and the geosynchronous LANL‐97A spacecraft. The focus here is on the intensification phase during which THEMIS was conjugate to the region of auroral brightening and its foot point was near the high‐latitude ground station Kiana. The following results will be demonstrated: (1) THEMIS and LANL‐97A (time‐delayed) recorded periodic ion injections (&gt;100 keV). (2) Near‐conjugate high‐latitude ground magnetometer data show very large Pi2 (δH∼150 nT) with a 6‐s time delay compared to the THEMIS ion injections. (3) Low‐latitude ground magnetometer data also show Pi2 with the same waveform as the high‐latitude Pi2 but with longer time delays (20–31 s). (4) Auroral luminosity was periodically modulated during the intensification phase. (5) All three signatures (ion injections, ground Pi2, optical modulation) had the same periodicity of ∼135 s but with various time delays with respect to the THEMIS ion injections. These observations demonstrate that the three substorm phenomena had a common source which controlled the periodicity.

Scale sizes of intense auroral electric fields observed by Cluster

Abstract. The scale sizes of intense (&gt;0.15 V/m, mapped to the ionosphere), high-altitude (4–7 RE geocentric distance) auroral electric fields (measured by the Cluster EFW instrument) have been determined in a statistical study. Monopolar and bipolar electric fields, and converging and diverging events, are separated. The relations between the scale size, the intensity and the potential variation are investigated. The electric field scale sizes are further compared with the scale sizes and widths of the associated field-aligned currents (FACs). The influence of, or relation between, other parameters (proton gyroradius, plasma density gradients, and geomagnetic activity), and the electric field scale sizes are considered. The median scale sizes of these auroral electric field structures are found to be similar to the median scale sizes of the associated FACs and the density gradients (all in the range 4.2–4.9 km) but not to the median proton gyroradius or the proton inertial scale length at these times and locations (22–30 km). (The scales are mapped to the ionospheric altitude for reference.) The electric field scale sizes during summer months and high geomagnetic activity (Kp&gt;3) are typically 2–3 km, smaller than the typical 4–5 km scale sizes during winter months and low geomagnetic activity (Kp≤3), indicating a dependence on ionospheric conductivity.

Characteristics of high altitude oxygen ion energization and outflow as observed by Cluster: a statistical study

Abstract. The results of a statistical study of oxygen ion outflow using Cluster data obtained at high altitude above the polar cap is reported. Moment data for both hydrogen ions (H+) and oxygen ions (O+) from 3 years (2001-2003) of spring orbits (January to May) have been used. The altitudes covered were mainly in the range 5–12 RE geocentric distance. It was found that O+ is significantly transversely energized at high altitudes, indicated both by high perpendicular temperatures for low magnetic field values as well as by a tendency towards higher perpendicular than parallel temperature distributions for the highest observed temperatures. The O+ parallel bulk velocity increases with altitude in particular for the lowest observed altitude intervals. O+ parallel bulk velocities in excess of 60 km s-1 were found mainly at higher altitudes corresponding to magnetic field strengths of less than 100 nT. For the highest observed parallel bulk velocities of O+ the thermal velocity exceeds the bulk velocity, indicating that the beam-like character of the distribution is lost. The parallel bulk velocity of the H+ and O+ was found to typically be close to the same throughout the observation interval when the H+ bulk velocity was calculated for all pitch-angles. When the H+ bulk velocity was calculated for upward moving particles only the H+ parallel bulk velocity was typically higher than that of O+. The parallel bulk velocity is close to the same for a wide range of relative abundance of the two ion species, including when the O+ ions dominates. The thermal velocity of O+ was always well below that of H+. Thus perpendicular energization that is more effective for O+ takes place, but this is not enough to explain the close to similar parallel velocities. Further parallel acceleration must occur. The results presented constrain the models of perpendicular heating and parallel acceleration. In particular centrifugal acceleration of the outflowing ions, which may provide the same parallel velocity increase to the two ion species and a two-stream interaction are discussed in the context of the measurements.

A statistical study of intense electric fields at 4−7 R&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;E&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; geocentric distance using Cluster

Abstract. Intense high-latitude electric fields (&gt;150 mV/m mapped to ionospheric altitude) at 4–7 RE geocentric distance have been investigated in a statistical study, using data from the Cluster satellites. The orbit of the Cluster satellites limits the data collection at these altitudes to high latitudes, including the poleward part of the auroral oval. The occurrence and distribution of the selected events have been used to characterize the intense electric fields and to investigate their dependance on parameters such as MLT, CGLat, altitude, and also Kp. Peaks in the local time distribution are found in the evening to morning sectors but also in the noon sector, corresponding to cusp events. The electric field intensities decrease with increasing latitude in the region investigated (above 60 CGLat). A dependence on geomagnetic activity is indicated since the probability of finding an event increases up to Kp=5–6. The scales sizes are in the range up to 10 km (mapped to ionospheric altitude) with a maximum around 4–5km, consistent with earlier findings at lower altitudes and Cluster event studies. The magnitudes of the electric fields are inversely proportional to the scale sizes. The type of electric field structure (convergent or divergent) is consistent with the FAC direction for a subset of events with electric field intensities in the range 500–1000 mV/m and with clear bipolar signatures. The FAC directions are also consistent with the Region 1 and NBZ current systems, the latter of which prevail only during northward IMF conditions. For scale sizes less than 2 km the majority of the events were divergent electric field structures. Both converging and diverging electric fields were found throughout the investigated altitude range (4–7 RE geocentric distance). Keywords. Magnetospheric physics (Electric fields; Auroral phenomena; Magnetosphere-ionosphere interactions)

Statistics of high-altitude and high-latitude O&amp;lt;sup&amp;gt;+&amp;lt;/sup&amp;gt; ion outflows observed by Cluster/CIS

Abstract. The persistent outflows of O+ ions observed by the Cluster CIS/CODIF instrument were studied statistically in the high-altitude (from 3 up to 11 RE) and high-latitude (from 70 to ~90 deg invariant latitude, ILAT) polar region. The principal results are: (1) Outflowing O+ ions with more than 1keV are observed above 10 RE geocentric distance and above 85deg ILAT location; (2) at 6-8 RE geocentric distance, the latitudinal distribution of O+ ion outflow is consistent with velocity filter dispersion from a source equatorward and below the spacecraft (e.g. the cusp/cleft); (3) however, at 8-12 RE geocentric distance the distribution of O+ outflows cannot be explained by velocity filter only. The results suggest that additional energization or acceleration processes for outflowing O+ ions occur at high altitudes and high latitudes in the dayside polar region. Keywords. Magnetospheric physics (Magnetospheric configuration and dynamics, Solar wind-magnetosphere interactions)

Neutral hydrogen density profiles derived from geocoronal imaging

Measurements of the Lyman α column brightness by the Geocoronal Imager (GEO), part of the FUV imaging system on board the IMAGE satellite, have been used to derive an empirical model of the neutral hydrogen density distribution at high altitudes (&gt;3.5 RE geocentric distance) on the night‐side of the Earth. The model presented is an effort to provide the density profiles needed to analyze the energetic neutral atom imaging data at ring current altitudes and above. The variable solar Lyman α flux is obtained from the UARS/SOLSTICE measurements and the scattered solar Lyman α emissions from interplanetary hydrogen are obtained from a model. Assuming that the exosphere at high altitudes (&gt;3.5 RE geocentric distance) can be considered as an optical thin medium and that the hydrogen density profile can be expressed as a double exponential we show that the Lyman α column brightness can be converted to hydrogen density profiles. The hydrogen density above 5 RE is found to be slightly higher for large solar zenith angles than for 90° solar zenith angle. The hydrogen density shows temporal variations which are not controlled by any solar quantity or geomagnetic parameter alone. Our Lyman α profiles and derived hydrogen density profiles are close to what was observed by Dynamics Explorer 1 [Rairden et al., 1986]. Above 8 RE we find higher densities than they did for all solar zenith angles &gt;90°. We do not find any evidence of depletion due to charge exchange with solar wind protons outside the magnetopause. Our results are only valid above 3.5 RE.

Telescopic and Microscopic views of the magnetosphere: Multispacecraft observations

The magnetospheric research community has long sought the capability to view the Sun-Earth system in a global way and to probe concurrently the microphysical details of key physical regions. This objective has now been substantially realized with the combination of the CLUSTER and IMAGE missions. With the additional use of SOHO, ACE, FAST, SAMPEX, POLAR, and geostationary orbit spacecraft, there is a remarkable ability to apply both telescopic and microscopic principles. As an example, a bright active region on the Sun gave rise on 29 March 2001 to a fast halo coronal mass ejection (CME) event observed by SOHO instruments. Subsequently on 31 March, a strong interplanetary shock wave ahead of a magnetic cloud (probably arising from the earlier CME) passed the ACE spacecraft and hit the Earth’s magnetosphere. This driver compressed the subsolar magnetopause to ≤ 4 RE geocentric distance and initiated a powerful geomagnetic storm (minimum Dst ~ −360 nT). The CLUSTER set of four spacecraft were located in the midnight sector of the magnetosphere near perigee (r~4 RE) at ~0635 UT and observed a dispersionless injection of energetic (E ≥ 20 keV) electrons in association with a large magnetospheric substorm expansion phase onset (AE ~1200 nT). Concurrent to these in situ observations, the IMAGE spacecraft was returning a sequence of global Energetic Neutral Atom (ENA) images from the medium-energy (MENA) and high-energy (HENA) sensor systems. These data showed a very prominent injection of substorm ions in the premidnight (and postdusk) sector of the inner magnetosphere. This event is consistent with a substorm onset that pushed the substorm ‘injection boundary’ far inside of geostationary orbit and far toward the dusk sector. In another event on August 27, 2001, the IMAGE-CLUSTER combination provided evidence that magnetic reconnection began in the mid-tail plasma sheet some 7 min prior to auroral onset and brightening. Alternative interpretations may also be possible even with the multis-pacecraft data available. Notwithstanding, these data gave an unprecedented view both telescopically and microscopically of a magnetospheric substorm onset and help establish key process timing in the magnetosphere. Such available events reveal the power of multispacecraft observations.

Small‐scale field‐aligned plasmaspheric density structures inferred from the Radio Plasma Imager on IMAGE

Among the objectives of the Radio Plasma Imager (RPI) on IMAGE is the observation of the Earth's plasmasphere from the satellite's polar orbit, with apogee ≈8 RE geocentric distance and perigee near 1200 km altitude. This objective is here pursued by (1) remote sounding from high‐altitude regions outside the main plasmasphere, (2) sounding within the plasmasphere, and (3) in situ passive measurements of natural wave activity. During sounding of the plasmasphere from both outside and inside the plasmapause, RPI echoes that follow non‐field‐aligned ray paths are usually not the discrete traces on range‐versus‐frequency records (plasmagrams) that are predicted by ray‐tracing simulations in smooth magnetospheric density models. Instead, such RPI echoes exhibit various amounts of spreading, from ≈0.5 RE to ≈2 RE in virtual range (range assuming free‐space speed of light propagation). The range spreading is attributed to scattering from, partial reflection from, and propagation along geomagnetic field‐aligned electron density irregularities. There exists a substantial body of theoretical work on mechanisms that might explain the appearance of such irregularities both within and beyond the plasmasphere. That the spread‐producing irregularities are field‐aligned is suggested by the efficiency with which RPI excites discrete echoes that propagate along the geomagnetic field, sometimes into both hemispheres. The spatial distributions and scale sizes of the spread‐producing irregularities remain to be investigated. The RPI echo data, however, coupled with earlier evidence from topside sounders and whistler mode instruments, suggest that they can have cross‐field scale sizes within a range from ≈200 m to over 10 km and electron densities within ≈10% of background. RPI is found capable of detecting plasmapause locations from distances of ≈3 RE or more. When minimal signal integration is used, the location and range of density values of a steep plasmapause can be determined from distances of order 1 RE, and echoes can at times be returned from points extending inward from the plasmapause to locations where the electron density reaches ≈3000 cm−3, which is usually at L &lt; 3.

A telescopic and microscopic view of a magnetospheric substorm on 31 March 2001

On March 31, 2001 at ∼0635 UT when the CLUSTER constellation was near local midnight and at ∼4 RE geocentric distance, sensors observed an energetic electron injection event associated with a strong (AE ∼ 1200 nT) magnetospheric substorm. Geostationary spacecraft 1991‐080 located at ∼20 LT also saw an abrupt electron injection event at ∼0630 UT and FAST spacecraft instruments (∼19 LT) detected a powerful set of magnetic field, electric field, and energetic plasma signatures at ∼0637 UT. The energetic neutral atom imaging experiments onboard the IMAGE spacecraft detected an injection of substorm‐produced ions in the pre‐midnight sector commencing at ∼0630 UT. Electron injection signatures at the four separate CLUSTER locations allow us to infer the location, speed, and direction of the substorm injection boundary. Hence, the CLUSTER (and IMAGE) telescope‐microscope combination is a long‐sought realization of a major magnetospheric research objective and shows the power of localized multi‐point measurements from CLUSTER.

Polar wind survey with the Thermal Ion Dynamics Experiment/Plasma Source Instrument suite aboard POLAR

In February 1996, the POLAR spacecraft was placed in an elliptical orbit with a 9 RE geocentric distance apogee in the northern hemisphere and 1.8 RE 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 ∼8 RE 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 8 RE 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 V0+ ∶ VHe+ ∶ VH+ ∼ 2 ∶ 3 ∶ 5, which may suggest a tendency toward comparable energy gains, such as via an electric potential layer.

Small‐scale, dispersive field line resonances in the hot magnetospheric plasma

The formation, temporal behavior, and spatial structure of field line resonance (FLR) layers formed by shear Alfvén waves standing along auroral magnetic field lines between the ionospheres are investigated when the layer develops transverse structure on the scale of the ion Larmour radius. Using a new numerical model including full ion Larmour radius correction in dipole magnetic geometry with realistic distributions of background plasma temperature and density, the following is shown: (1) Hot magnetospheric ions significantly retard the development of a parallel electric field in ion gyroscale dispersive Alfvén waves. (2) A fundamental FLR forming near L = 7.5 can contract to a transverse scale size of several hundred of meters in the direction perpendicular to the geomagnetic field at ionospheric altitudes, with a parallel electric field sufficient to produce a k V potential drop along the resonance field line from the ionosphere up to ∼ 4 RE altitude, in the region where the wave dispersion is due to the finite electron inertia. (3) A plasma density depletion in the lower auroral magnetosphere (≈ 2–5 RE geocentric distance) enables the formation of a nonradiative fundamental FLR. (4) Dispersive FLRs for the higher harmonics are more radiative at the equatorial magnetosphere than the fundamental mode.