Charge Composition Explorer Research Articles

Quiet time equatorial mass density distribution derived from AMPTE/CCE and GOES using the magnetoseismology technique

An inversion technique for estimating the properties of the magnetospheric plasma from the harmonic frequencies of the toroidal standing Alfvén waves has been used to derive the global equatorial mass density covering radial distances from 4 to 9 Earth radii (RE), within the local time sector spanning from 0300 to 1900 h. This broad range of L shell extending to the outer magnetosphere allows us to examine the local time and radial dependence of the quiet time equatorial mass density during solar minimum and thereby construct a global distribution of the equatorial mass density. The toroidal Alfvén waves were detected with magnetometers on the Active Magnetospheric Particle Tracer Explorers (AMPTE)/Charge Composition Explorer (CCE) during the nearly 5 year interval from August 1984 to January 1989 and on the Geostationary Operational Environmental Satellites (GOES) (10, 11, and 12) for 2 years from 2007 to 2008, both of which were operating during solar minimum years. The derived equatorial mass density, ρeq, at geosynchronous orbit (GEO) monotonically increases with increasing magnetic local time (MLT) from the nightside toward the dusk sector. At other radial distances, ρeq has the same MLT variation as that of GEO, while the magnitude logarithmically decreases with increasing L value. An investigation of the Dst and Kp dependence shows that the median value of ρeq varies little in the daytime sector during moderately disturbed times, which agrees with previous studies. ρeq calculated from the F10.7 dependent empirical model shows good agreement with that of CCE but overestimates that of GOES probably due to the extreme solar cycle minimum in years 2007–2008.

Pi2 pulsations in the inner magnetosphere simultaneously observed by the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer and Dynamics Explorer 1 satellites

[1] We statistically studied the spatial characteristics of Pi2 pulsations using magnetic field observations in the magnetosphere at the equatorial-orbiting Active Magnetospheric Particle Tracer Explorers (AMPTE)/Charge Composition Explorer (CCE) and the polar-orbiting Dynamics Explorer 1 (DE 1) satellites and on the ground at the low-latitude station Kakioka (KAK, L = 1.23). We defined Pi2 pulsations from wavelet analysis of KAK data covering August 1984 to January 1989 and obtained 849 nightside (20:00–04:00 LT) events. For each KAK Pi2 event, we evaluated the coherence between the ground H component and the magnetospheric radial (B⊥R), azimuthal (B⊥A), and compressional (B//) components at the frequency of the ground Pi2. High-coherence ground-satellite Pi2 events were found most often in B//, with many of them evident at both spacecraft when CCE was located near the equator (magnetic latitude 30°) and outside of the estimated plasmapause. In these simultaneous CCE and DE 1 events, the B// amplitude was larger at CCE; in addition, the H-B// cross phase was ∼0° at CCE but was ∼180° at DE 1. We show detailed analysis of one such event as well as two other events that occurred when CCE was outside of the estimated plasmapause and exhibited different coherence and cross-phase properties at this spacecraft. Overall, the two-satellite observations provide additional evidence of plasmaspheric virtual resonance, which was previously suggested from studies using single satellites.

Toroidal wave frequency at L = 6–10: Active Magnetospheric Particle Tracer Explorers/CCE observations and comparison with theoretical model

Magnetospheric standing Alfvén waves are guided along the ambient magnetic field, and their frequency depends on the mass density of the plasma distributed along the field lines. These properties allow us to use Alfvén waves to map time‐dependent phenomena between space and ground and to estimate the mass density. In this paper we present a statistical study of the spatial variation of the fundamental frequency fT1 of toroidal‐mode standing Alfvén waves in the L range from 6 to 10, where L indicates the maximum geocentric distance on the field line. The data used for this analysis are energetic particle flux anisotropy (proxy of transverse electric field) and magnetic field measurements from the Active Magnetospheric Particle Tracer Explorers /Charge Composition Explorer (CCE) spacecraft. Using CCE data covering 4 years, we obtained ∼5000 20‐min intervals containing a clear signature of toroidal waves. The median fT1 is 6–10 mHz at L = 7 and decreases to 4–8 mHz at L = 9. The frequency tends to be lower at noon than at midnight. The observed frequencies are compared with numerically derived frequencies using an empirical mass density model [Gallagher et al., 2000] and a magnetic field model [Tsyganenko, 1989]. We found a good agreement for 1200–2400 magnetic local time (MLT) but a large discrepancy near 0300 MLT. This may indicate that the flux tubes at this local time are more heavily loaded than specified in Gallagher et al.'s [2000] model.

Upper Atmosphere Research Satellite observation of a Pi2 pulsation

Satellites in low‐altitude orbits provide unique opportunities to study the ionospheric screening effects on ULF waves propagating from the magnetosphere. This paper presents the first report of observation of a Pi2 pulsation from the Upper Atmosphere Research Satellite (UARS) at an altitude of ∼580 km. The Pi2 pulsation occurred on November 18, 1993, when the satellite was over the Pacific near the three ground magnetometer sites, Chichijima (L = 1.14, MLT = 23), Ewa Beach (L = 1.17, MLT = 03), and Guam (L = 1.07, MLT = 23). On the ground the Pi2 was visible in the H (northward) component from 1337 to 1343 UT at all locations with the same period of 43 s but with an amplitude ranging from 2.1 nT (Guam) to 0.6 nT (Ewa Beach). At the satellite the same pulsation was detected in the H component with an amplitude of 1.7 nT. The transverse radial component and the parallel component at the satellite oscillated in phase, consistent with previous Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer observations at L > 2. When the ground signals are linearly interpolated to the geographic longitude and latitude of UARS, the orientation of the major axis of polarization agreed well with that observed by UARS, as is expected for a fast‐mode wave propagating through the ionosphere.

Statistical analysis of compressional Pc3–4 pulsations observed by AMPTE CCE at L = 2–3 in the dayside magnetosphere

Few spacecraft observations of Pc3–4 (7–100 mHz) pulsations at L < 3 have been reported, although ground observations of the pulsations in the same L range are routine. In particular, there are no systematic spacecraft observations of compressional pulsations expected at the time of low‐latitude ground Pc3–4 pulsations that exhibit a constant frequency over a range of L. These pulsations have been attributed to fast mode waves trapped in the plasmasphere (referred to as plasmaspheric cavity mode waves) or to evanescent waves that exist inward of the turning point for fast mode waves propagating from the outer magnetosphere. We studied the properties of compressional Pc3–4 pulsations at L < 3 using magnetic field data simultaneously acquired by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) and at the Kakioka ground station (L = 1.25) over a 2‐year period from 1985 to 1986. From time intervals when the satellite was on the dayside and between L = 2 and L = 3, we identified 63 cases of magnetospheric compressional (bz) Pc3–4 pulsations with or without an accompanying perturbation in the radial (bx) component, depending on the latitude of the satellite. The pulsations were observed primarily when AMPTE CCE was at prenoon hours (magnetic local time (MLT) = 0800–1200), and they were often accompanied by nearly identical magnetic pulsations at Kakioka. In an attempt to construct the spatial mode structure of the compressional pulsations, we have examined the relationship between the bx and bz components and also between the bz (or bx) component and the horizontal H (north–south) component at Kakioka. We find that the phase and amplitude relationship between bx and bz is consistent with waves standing in the meridian plane. The ground‐satellite cross phase is clustered near the values expected for radially standing or evanescent waves but also exhibits considerable deviations from the expected values. Although we have demonstrated that compressional Pc3–4 pulsations exist in the low‐L magnetosphere and that they give rise to pulsations on the ground, we conclude that further study is required to distinguish between the cavity and evanescent modes.

Pi2 pulsations observed from the Akebono satellite in the plasmasphere

Magnetic field, electric field, and electron density measurements from the Akebono satellite are used to study the properties of two Pi2 pulsations that occurred in succession on February 13, 1990, when the satellite was in the plasmasphere at L = 2.4–3.8, 24°–40° magnetic latitude, and 22.5 hours magnetic local time. Magnetic pulsations with a nearly identical waveform were observed in the same time interval at three ground stations (Aedey, L ≈ 6.84; York, L ≈ 2.55; and Hermanus, L ≈ 1.83), which were located near midnight, confirming that the pulsations propagated to the ground. At the satellite the pulsations had comparable radial and azimuthal components in both the magnetic and electric fields. In contrast to the observations near the magnetic equator by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer spacecraft [Takahashi et al., 1995], no compressional component was detected in the magnetic field. The orthogonal components of the electric and magnetic fields oscillated either in phase or 180° out of phase, a property of a propagating (rather than standing) wave. The Poynting flux of the Pi2 pulsations was parallel to the ambient magnetic field and directed toward the nearer ionosphere, with little indication of ionospheric reflection. This unidirectional flow of electromagnetic energy is consistent with the strong ionospheric damping of Alfvén waves estimated from a numerical calculation. It is significant that the measured Poynting flux could damp a cavity mode oscillation in ∼10 s, assuming that the previously reported equatorial compressional Pi2 pulsations represent the cavity mode. Since the transverse Pi2 pulsations at Akebono lasted ∼400 s, they cannot be due to a gradual energy leakage from a simple cavity mode oscillation. Consequently, if the observed energy flow is a general property of plasmaspheric Pi2 pulsations, a simple cavity mode oscillation excited by an impulsive source is not an appropriate model for Pi2 pulsations.

Ground‐satellite coherence analysis of Pc3 pulsations

Although Pc3 magnetic pulsations are commonly observed in the dayside magnetosphere and are believed to propagate into the inner magnetosphere and to the ground, how and where they establish themselves as a regular oscillation are not completely understood. In particular, it is not clear whether the cavity mode plays a significant role in determining the spectral properties of Pc3 pulsations. The mode, in principle, can be identified by multipoint observations of the spatial variation of the amplitude and phase of magnetic field perturbations, with at least one satellite providing evidence of compressional magnetic field oscillation in the magnetosphere. Motivated by these requirements for detection of the cavity mode, we surveyed combined magnetic field data from the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) satellite and the Kakioka ground station. We identified an interval on November 4–5, 1984, during which both the satellite (dipole L = 3–6) and ground station (L = 1.25) were on the morningside (near 8‐hour local time) with a small local time separation (within 0.9 hour) and observed Pc3 magnetic pulsations exhibiting a compressional component in space. Unlike previous studies, which used satellites located at L > 6, we observed high coherence in the Pc3 band between the compressional component (bz)in space and the horizontal component (H) on the ground. A cross‐phase analysis of the merged bz and H time series indicates that these components oscillated in antiphase over a range of satellite radial distance. We discuss whether the observations can be taken as evidence of a cavity mode oscillation in the magnetosphere.

Multisatellite and ground‐based observations of a tailward propagating Pc5 magnetospheric waveguide mode

Using data from the Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer (CCE) satellite near local noon, and the AMPTE Ion Release Module (IRM) and United Kingdom Subsatellite (UKS) on the morning flank, we investigate the tailward propagation of a compressional Pc5 wave (380 s period) on October 28, 1984, and suggest that the observation represents a magnetospheric waveguide mode. The observed wave is a transient oscillation lasting about five wave cycles with an observed time of flight from CCE to IRM/UKS of 180 s. The first three or four cycles at the leading edge of the event display a remarkable wave packet coherence between CCE and IRM/UKS despite their large separation in the magnetosphere (∼ 6.2 RE). A time of flight analysis suggests a waveguide mode group speed ∼ 220 km s−1, being much less than the local Alfvén speed and comparable to the expected sub‐Alfvénic propagation speed of a waveguide mode. The ground‐based signature of the event is also observed near local noon by the European Incoherent Scatter magnetometer cross, displaying the same period and lasting for the same number of cycles. The ground‐based data show no evidence of a field line resonance; the event is monochromatic, and there is little variation of amplitude and polarization with latitude. An increase in solar wind ram pressure of ∼ 30% is observed by IMP 8 upstream in the solar wind just prior to the event, and this may have provided an impulsive energy source for the waveguide mode. This is the first time the signature of a particular waveguide mode harmonic has been observed by satellites which are widely spaced in the magnetosphere. Moreover, we present the first unambiguous observation of the downtail propagation of a waveguide mode. The observation also provides possible evidence of dispersion in the Earth's outer magnetospheric waveguide.

Drift‐shell splitting of energetic ions injected at pseudo‐substorm onsets

One feature of a magnetospheric substorm is the injection of energetic particles into closed drift orbits. Injections are routinely observed by geosynchronous satellites and have been used to identify the occurrence of substorms and the local time of particle energization. In this study we examine pitch angle distributions of ion injections in the 50‐ to 300‐keV energy range observed by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE/CCE) satellite, hereinafter CCE. In a dipole field, all pitch angles follow the same drift shell, but the day‐night asymmetry of the magnetospheric magnetic field introduces a pitch angle dependence in particle drift orbits, so that particles with different pitch angles disperse radially as they drift. The effect is known as drift‐shell splitting. For satellite observations near noon at a fixed geocentric distance, the guiding center orbits of ions detected at small pitch angles intersect the midnight meridian at larger geocentric distances than do ions with near‐90° pitch angles. The ion pitch angle distributions detected on the dayside therefore provide information about the radial distance of the nightside acceleration region. We apply this principle to study ion injection events observed on September 17–18, 1984, in association with pseudo‐substorm onsets. CCE was at 13 hours local time near its apogee (8.8 RE) and observed a series of ion flux enhancements. Energy dispersion of the timing of the flux increases assures that they are due to injections on the nightside. The flux increases were observed only at pitch angles from 0° to 60°. We calculate drift orbits of protons using the Tsyganenko 89c magnetic field model and find that the drift orbits for 60° pitch angle protons observed at the satellite pass through midnight at 9 RE, well outside of geostationary orbit, indicating that the ion injections occurred tailward of 9 RE. Energetic ion data from geostationary satellites for the same time interval show no evidence of injections at 6.6 RE, consistent with the calculated inner boundary of the injections. Model calculations are presented demonstrating that dispersive injections observed near noon outside geosynchronous orbit provide the greatest sensitivity to drift‐shell splitting effects and are therefore most suitable for remote sensing the radial boundaries of substorm injections.

Spacecraft transits across simulated field line resonance regions

The field line resonance (FLR) mechanism has become a cornerstone of magnetohydrodynamic wave theory in the magnetosphere and the solar atmosphere. Evidence for the mechanism comes mainly from auroral radar and geosynchronous spacecraft observations. Spacecraft in elliptical near‐equatorial orbits (particularly Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE)) have seen frequency‐varying oscillations of magnetic field lines but have not seen much evidence of the large‐amplitude, narrowband FLRs seen by auroral radars. We simulate the resonance region of a large event observed at GOES 7 and also of a smaller‐amplitude event we term a “typical Scandinavian Twin Auroral Radar Experiment (STARE) event” with reference to the STARE auroral radar results. One set of simulated narrowband FLRs is driven by a magnetopause Kelvin‐Helmholtz wave. We add a fundamental continuum oscillation and analyze the resulting time series using the AMPTE CCE time‐frequency analysis. We find that at the observed milliHertz frequencies the narrowband FLRs appear as large or small enhancements in the continuum of the azimuthal magnetic field but are not obviously “monochromatic.” For large events, enhancements in the spectrum of the compressional magnetic component may suggest a narrowband driver. We also analyze FLRs driven by waveguide modes and find no monochromatic signatures in the compressional magnetic field.

Magnetopause crossings without a boundary layer

The microstructure of pristine magnetopause crossings has been analyzed by using high‐resolution particle and field data obtained by the Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer (CCE) and ISEE 2 spacecraft. These crossings are pristine in the sense that they exhibit no adjoining magnetospheric boundary layer, or, at most, a low‐density plateau. The CCE crossings include the low‐latitude near‐noon region not typically sampled by ISEE 2, which covers all other local time sectors in a complementary way. Magnetopause crossings without a boundary layer are found to occur to all local times, and such crossings constitute about 10% of all magnetopause crossings. Total pressure balance across the magnetopause is observed to within experimental errors; however, electron data, full‐energy composition measurements, and occasionally field stress are needed to fully evaluate pressure balance. The microstructure of the magnetopause current layer is also found to depend on local time. Crossings within about 1 hour local time of the noon meridian often exhibit very sharp density gradients on scale lengths down to a few plasma skin depths. These gradients are reduced for crossings farther from local noon such that, for cases near the dawn‐dusk meridian, the scale length for density gradients and the magnetopause current are roughly comparable. Magnetopause crossings without a boundary layer impose severe constraints on various theories of boundary layer formation. Pristine magnetopause crossings may be direct cuts through the diffusion region for reconnection. With this interpretation our results are in qualitative agreement with recent simulations of the diffusion region and associated turbulence by Drake et al. [1994], who propose the current convective instability as the dominant process for current transport at the magnetopause.

Statistical analysis of Pi 2 pulsations observed by the AMPTE CCE Spacecraft in the inner magnetosphere

The spatial variation of the properties of magnetospheric Pi 2 pulsations is studied using magnetic field records acquired simultaneously by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) satellite at radial distances less than 6.6 Earth radii and at geomagnetic latitudes from −16° to 16° and at the Kakioka ground station located at magnetic shell of L = 1.23. Pi 2 magnetic pulsations are identified from the Kakioka data acquired within 3 hours of midnight, but no restriction is imposed on the local time of CCE. An automated Pi 2 selection procedure resulted in 249 events from the Kakioka data. We have characterized magnetic field variations in the radial Bx, azimuthal By, and compressional Bz components at CCE in terms of their spectral density, coherence, and phase relative to those of the Pi 2 pulsation in the horizontal H component of the Kakioka data and then examined how these parameters depend on the location of CCE. It is found that high‐coherence events (coherence between CCE and Kakioka > 0.6) are observed primarily when CCE is on the nightside and at L < 4. For these events the magnetic field perturbations at CCE are dominated by the poloidal components Bx and Bz, and these components exhibit a ground‐to‐satellite cross phase of either ∼0 or ∼180°, depending on the location of the satellite. The spatial phase structure is consistent with the eigenmode structure of a compressional cavity‐mode‐type resonance excited between two reflecting boundaries. We find no evidence supporting the view that ground Pi 2 are midlatitude toroidal field line resonances excited in response to source waves on auroral zone field lines. Rather, the results imply that midlatitude (2 <L< 5) Pi 2 pulsations observed on the ground originate from a cavity‐mode‐type resonance excited in the inner magnetosphere bounded below by the ionosphere and at high altitudes by an Alfvén velocity gradient. The cavity resonance is probably excited by earthward propagating fast mode waves launched at substorm onset by the large‐scale magnetic reconfiguration associated with cross‐tail current disruption.

Magnetic fluctuations associated with tail current disruption: Fractal analysis

The objective of the present study is to assess the mechanism of substorm‐associated tail current disruption on the basis of magnetic field observations in the near‐Earth tail. We examined 15 events observed by the Charge Composition Explorer (CCE) of the Active Magnetospheric Particle Tracer Explorers (AMPTE), with an emphasis on the August 28, 1986, event. In these events the satellite observed magnetic fluctuations to start almost simultaneously with ground substorm onsets, strongly suggesting that these fluctuations are related to the trigger of substorms. In this study we applied the new method, fractal analysis, to these fluctuations. This method enables us to examine fluctuations quantitatively and to pick up characteristic timescale(s) of fluctuations, even if fluctuations are far from sinusoidal. The results are summarized as follows: (1) Whereas before the onset of tail current disruption, magnetic fluctuations are suppressed in each of the magnetic components, after the onset, the magnitude of the H (north‐south) component fluctuations is about 30% larger than the magnitudes of the fluctuations of the other components. (2) The magnetic fluctuations have a characteristic timescale, which is several times the proton gyroperiod. The first result suggests that observed magnetic fluctuations are actually related to changes in the tail current intensity, that is, tail current disruption. This result also indicates that the microprocess of tail current disruption should be described in terms of turbulent perturbation electric currents, although away from the onset region the effects of tail current disruption may be approximated by those of an orderly decrease in the tail current intensity. The second result strongly suggests that tail current disruption is driven by a certain instability, which grows most rapidly around that characteristic time scale, and in which ions should play an important role.

Observation and modeling of compressional Pi 3 magnetic pulsations

Compressional magnetic pulsations with irregular waveforms and periods longer than 150 s (here termed Pi 3) have been studied by using data from Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE/CCE) and GOES 5 and 6 in the dayside magnetosphere and compared with signatures on the ground at low latitudes by using data from Kakioka station (L = 1.25). On the ground, the pulsations appear in the horizontal component. A study of 17 such concurrent events during a 2‐month period in 1986 reveals the following pulsation characteristics. (1) The peak‐to‐peak amplitudes in space (δBT) and on the ground (δH) are comparable and are in the range of 0.5–7 nT. (2) On the ground the pulsations can be seen at all local times, even at midnight, while at geostationary orbit they are observed only on the dayside with a clear amplitude maximum at noon. (3) The pulsations on the ground lag those observed by CCE near local noon, and the lag increases as the local time separation between CCE and the ground station increases. The time lag is 1–2 min longer when the ground station is on the nightside than when it is on the dayside. (4) The time lag between pulsations observed at geostationary orbit and near noon by CCE varies systematically with local time and is about 2 min per 6 hours of local time separation. These observations indicate that some nightside pulsations in the Pi 3 band have dayside origins. The position dependence of the pulsation amplitude can be explained well by changes in the magnetopause current, which are in turn presumably caused by changes in the solar wind dynamic pressure. The time lags observed in space are consistent with signal propagation in the MHD fast mode, but the variation in space‐ground time lags with ground station local time must be attributed to another mechanism.

Studies of Magnetospheric ULF Waves Using Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer.

Recent studies of ULF pulsations in the inner magnetosphere (L < 6.6) with the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer are reviewed. Pulsations in the Pc 3-4 and Pi 2 bands with distinctive waveform, polarization, and spatial amplitude distribution are discussed separately. Specific pulsation types include toroidal-mode Pc 3-4 oscillations, equatorial compressional Pc 3 pulsations, Pi 2 pulsations, and giant pulsations. In addition to the highlights of published results, unresolved problems for each pulsation type are described.

Pseudobreakup and substorm growth phase in the ionosphere and magnetosphere

We present observations made in space and on the ground during the growth phase and the onset of a substorm on August 31, 1986. About 20 min after the ε parameter at the magnetopause had exceeded 1011 W, magnetic field dipolarization with an increase of energetic particle fluxes was observed by the AMPTE Charge Composition Explorer (CCE) spacecraft at the geocentric distance of 8.7 RE close to magnetic midnight. The event exhibited local signatures of a substorm onset at AMPTE CCE and a weak wedgelike current system in the midnight sector ionosphere. However, it did not lead to a full‐scale substorm expansion, as determined by several ground‐based instruments, nor did it produce large particle injections at geostationary orbit. Only after another 20 min of continued growth phase the entire magnetosphere‐ionosphere system could apparently allow the onset of a regular substorm expansion. The initial activation is interpreted in the present paper as a “pseudobreakup.” We examine the physical conditions in the near‐Earth plasma sheet using spacecraft observations and analyze the development in the ionosphere using ground‐based magnetometers and electric field observations from the STARE radar. We find that the main observable differences between pseudobreakups and ordinary breakups are the strength and consequences. Furthermore, it is shown that ionospheric activity at the time of a pseudobreakup is not necessarily as localized in longitude as generally believed.

Remote sensing of two‐dimensional magnetopause structures

A technique is developed for analyzing remote measurements made by a single spacecraft of two dimensional disturbances in the ambient magnetosheath or magnetospheric magnetic field, caused, for example, by flux transfer events or pressure pulses. The methodology is based on a recent linear theory for isentropic field‐aligned MHD flow over gently sloping two‐dimensional obstacles (Sonnerup et al., 1992). Using only magnetic field measurements, the analysis technique can provide information about the orientation and actual cross‐sectional shape of the event, as well as information about the spacecraft trajectory relative to the bulge. If three‐dimensional plasma velocity measurements are also available, the technique provides the velocity and size of the event as well, and it allows one to determine whether the current sources causing the disturbance were in fact located on the side of the spacecraft trajectory facing the magnetopause. Analysis of two sample events, one recorded by the spacecraft AMPTE/IRM (Active Magnetospheric Particle Tracer Explorer/Ion Release Module) in the magnetosheath and the other by AMPTE/CCE (Charge Composition Explorer) in the magnetosphere, indicates that the bulges on the magnetopause surface causing the magnetic field and flow perturbations for these events did not have the semicircular cross section suggested in previous work; instead they had a more elongated shape, the dimension tangential to the magnetopause being substantially larger than that normal to it. The calculated invariant axes of the two events were found to differ substantially from the corresponding minimum variance directions of the measured magnetic field. The IRM event was found to move at a speed of 227 km/s away from the subsolar region. For the CCE event, plasma flow data were not available, but it was deduced indirectly that the event moved at a speed of about 90 km/s, presumably away from the subsolar region.

Radial transport of storm time ring current ions

Radial transport of energetic ions for the development of the main phase of geomagnetic storms is investigated with data from the medium energy particle analyzer (MEPA) on the Charge Composition Explorer (CCE) spacecraft, which monitored protons (Ep &gt; 56 keV), helium ions (EHe &gt; 72 keV), and the carbon‐nitrogen‐oxygen group, which is mostly dominated by oxygen ions (E0 &gt; 137 keV). From a study of four geomagnetic storms, we show that the flux increase of these ions in the inner ring current region (L ≲ 5) can be accounted for by an inward displacement of the ring current population by ∼0.5 to 3.5 RE. There is a general trend that a larger inward displacement occurs at higher L shells than at lower ones. These results are in agreement with previous findings. The radially injected population consists of the prestorm population modified by substorm injections which occur on a much shorter time scale than that for a storm main phase. It is also found that the inward displacement is relatively independent of ion mass and energy, suggesting that the radial transport of these energetic ions is effected primarily by convective motion from a large electric field or by diffusion resulting from magnetic field fluctuations.

Radial profiles of quiet time magnetospheric parameters

Magnetospheric plasma parameters during geomagnetically quiet conditions are studied with ion measurements covering energies from ∼1 keV to ∼4 MeV from the Charge Composition Explorer. It is found that the observed quiet time plasma pressure in the midnight sector is comparable (within a factor of 2) to that deduced by Spence et al. (1989) and Kan et al. (1992) from inverting the magnetic field models of Tsyganenko and Usmanov (1982) and Tsyganenko (1987). The radial profile of the total plasma pressure shows a peak generally at L ≈ 3 to 4 and decreases from L ≈ 4 to L ≈ 9 rather monotonically. No large‐scale earthward decrease in plasma pressure occurs in these outer L shells, contrary to some theoretical expectation for the magnetospheric closure of the region 2 field‐aligned current system. The plasma pressure within L ∼ 9 is generally anisotropic with a larger pressure component perpendicular to the magnetic field than parallel to the field. This anisotropy tends to increase with smaller L. While the anisotropy is always below the fire hose instability threshold during all quiet time passes studied, it is near the mirror instability threshold in the midnight sector at large L shells due mostly to the large value of plasma beta (&gt;1) there. The radial profile of the azimuthal volume current density is also computed, with the result suggesting that the current density associated with the quiet time ring current population is distributed broadly over the L shells between L ≈ 3.5 and L ≈ 7 with average values in the range of ∼1 to 4 nA/m² and peak values in the range of ∼4 to 8 nA/m². Another current region situated beyond L ≈ 7, which may be related to the inner portion of the quiet time cross‐tail current, is also apparent and has average and peak current density values similar to those of the ring current.

AMPTE CCE observations of Pi 2 pulsations in the inner magnetosphere

Magnetic field data acquired with the AMPTE Charge Composition Explorer satellite in the inner magnetosphere (L = 2–5) often show Pi 2 pulsations whose waveforms match Pi 2 pulsations simultaneously observed on the ground at Kakioka (L = 1.2). From a study of 25 such events, we find that the magnetic pulsations in the equatorial magnetosphere are dominated by poloidal‐mode oscillations. The relative phase between the compressional component at CCE and the horizontal component at Kakioka is either near zero or near 180°, with the ∼180° lag observed only when the satellite is at L &gt; 3. This observation implies that there is a node of a radial standing wave at L &gt; 3. We argue that the nodal structure arises from reflection of MHD fast‐mode waves at some inner boundary of the magnetosphere and discuss the relevance of the nodal structure to cavity‐mode resonances and oscillations in the inner magnetosphere forced by a source wave external to the inner magnetosphere.