Active Magnetospheric Particle Tracer Explorers Research Articles

Collisionless momentum transfer in space and astrophysical explosions

Larmor coupling is a collisionless momentum exchange mechanism believed to occur in various astrophysical and space-plasma environments. The phenomenon is now observed in a laboratory experiment. The AMPTE (Active Magnetospheric Particle Tracer Explorers) mission provided in situ measurements of collisionless momentum and energy exchange between an artificial, photo-ionized barium plasma cloud and the streaming, magnetized hydrogen plasma of the solar wind 1,2,3. One of its most significant findings was the unanticipated displacement of the barium ion ‘comet head’ (and an oppositely directed deflection of the streaming hydrogen ions) transverse to both the solar wind flow and the interplanetary magnetic field, defying the conventional expectation that the barium ions would simply move downwind4. While subsequent theoretical and computational efforts5,6,7 to understand the cause of the transverse motion reached differing conclusions, several authors5 attributed the observations to Larmor coupling8,9, a collisionless momentum exchange mechanism believed to occur in various astrophysical and space-plasma environments10,11 and to participate in cosmic magnetized collisionless shock formation12,13,14. Here we present the detection of Larmor coupling in a reproducible laboratory experiment that combines an explosive laser-produced plasma cloud with preformed, magnetized ambient plasma in a parameter regime relevant to the AMPTE barium releases. In our experiment, time-resolved Doppler spectroscopy reveals ambient ion acceleration transverse to both the laser-produced plasma flow and the background magnetic field. Utilizing a detailed numerical simulation, we demonstrate that the ambient ion velocity distribution corresponding to the measured Doppler-shifted spectrum is qualitatively and quantitatively consistent with Larmor coupling.

The inner magnetosphere ion composition and local time distribution over a solar cycle

AbstractUsing the Cluster/Composition and Distribution Function (CODIF) analyzer data set from 2001 to 2013, a full solar cycle, we determine the ion distributions for H+, He+, and O+ in the inner magnetosphere (L < 12) over the energy range 40 eV to 40 keV as a function magnetic local time, solar EUV (F10.7), and geomagnetic activity (Kp). Concentrating on L = 6–7 for comparison with previous studies at geosynchronous orbit, we determine both the average flux at 90° pitch angle and the pitch angle anisotropy as a function of energy and magnetic local time. We clearly see the minimum in the H+ spectrum that results from the competition between eastward and westward drifts. The feature is weaker in O+ and He+, leading to higher O+/H+ and He+/H+ ratios in the affected region, and also to a higher pitch angle anisotropy, both features expected from the long‐term effects of charge exchange. We also determine how the nightside L = 6–7 densities and temperatures vary with geomagnetic activity (Kp) and solar EUV (F10.7). Consistent with other studies, we find that the O+ density and relative abundance increase significantly with both Kp and F10.7. He+ density increases with F10.7, but not significantly with Kp. The temperatures of all species decrease with increasing F10.7. The O+ and He+ densities increase from L = 12 to L ~ 3–4, both absolutely and relative to H+, and then drop off sharply. The results give a comprehensive view of the inner magnetosphere using a contiguous long‐term data set that supports much of the earlier work from GEOS, ISEE, Active Magnetospheric Particle Tracer Explorers, and Polar from previous solar cycles.

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.

Electromagnetic waves downstream of quasi‐perpendicular shocks

When the incident solar wind H+ and other minor ions cross a shock, they are decelerated differentially by the electrostatic potential at the shock. Therefore ring beam distributions of the minor ions are formed downstream of a quasi‐perpendicular shock in the downstream frame. Moreover, the H+ distribution downstream of the quasi‐perpendicular shock usually has large perpendicular temperature anisotropy. The He2+ ring beam distribution and H+ temperature anisotropy can excite helium cyclotron waves and proton cyclotron waves, respectively. In this paper, we perform one‐dimensional hybrid simulations to investigate the competition between the helium cyclotron waves and proton cyclotron waves, and their influences on the evolution of He2+ and O6+ velocity distributions are also considered. When the wave spectrum is dominated by the helium cyclotron waves, He2+ and O6+ approximately move on the surface of a sphere, which results in shell‐like distributions for He2+and O6+. With the increase of the H+ temperature anisotropy T⊥p/T∥p, the amplitude of the proton cyclotron waves also increases. When it is sufficiently large and comparable with that of the helium cyclotron waves, the motions of He2+ and O6+ are irregular, which results in bi‐Maxwellian distributions of He2+and O6+. We also investigate the influence of the plasma β∥p on the excited ion cyclotron waves and the evolution of the He2+and O6+ distributions. The simultaneous observations for He2+ and O6+ in the downstream of shocks with Active Magnetospheric Particle Tracer Explorers (AMPTE)/CCE spacecraft have demonstrated their shell‐like distributions. The observed results verify the existence of the helium cyclotron waves downstream of supercritical quasi‐perpendicular shocks.

A model of the near magnetosphere with a dawn‐dusk asymmetry 2. Parameterization and fitting to observations

First results are presented of an empirical modeling of the Earth's inner and near magnetosphere (X ≥ −15 RE), using a new set of data and new methods, described in a companion paper. The modeling database included 5‐min average B field data, taken in a wide range of altitudes and latitudes by the International Solar Terrestrial Physics spacecraft Polar (1996–1999) and Geotail (1994–1999), as well as by earlier missions, ISEE 2 (1984–1987), Active Magnetospheric Particle Tracer Explorers (AMPTE)/CCE (1984–1988), AMPTE/Ion Release Module (1984–1986), CRRES (1990–1991), and DE 1 (1984–1990). To take into account the delayed response of the magnetosphere to the solar wind and interplanetary magnetic field (IMF), each data record in the data set was tagged by a “trail” of 5‐min averages of the IMF, solar wind, and Dst field data, covering the preceding 2‐hour interval. The axisymmetric ring current (SRC) and the partial one (PRC), both parameterized by the corrected Dst* index and the solar wind pressure Pd, were found to vary in strikingly different ways. While under quiet conditions the PRC is much weaker than the SRC, it dramatically grows in magnitude and rotates to the dusk sector with rising |Dst*| and Pd, significantly exceeding the SRC even during moderate storms, in excellent agreement with recent particle simulations. The innermost part of the cross‐tail current is quite sensitive to the southward IMF and yields ∼90% of the tail's contribution to the Dst index, in contrast with the more distant tail current, which responds mainly to the solar wind pressure and provides no appreciable contribution to Dst. In response to southward IMF conditions, Region 1 and 2 Birkeland currents rapidly grow in magnitude and expand to lower latitudes, while their peaks shift slightly in local time toward noon. The coefficient of the IMF penetration inside the magnetosphere was found to dramatically increase with growing IMF clock angle: while quite small (∼0.1) for northward IMF, it rises to ∼0.6 as the IMF turns southward. Priorities and challenges for future data‐based modeling studies are discussed.

Relative contributions of H+ and O+ to the ring current energy near magnetic storm maximum

Close to the peak of a magnetic storm, the ratio of the O+ to the H+ contribution to the ring current energy, U(O+)/U(H+), depends on both solar EUV flux and storm size. This ratio provides a lower limit on the importance of the ionospheric plasma source compared to the solar wind source since only the ionosphere supplies O+ while both the solar wind and the ionosphere supply H+. We have used measurements from the CHEM ion spectrometer on the Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE spacecraft to assess U(O+)/U(H+) near storm maximum. CHEM measured equatorial ion composition over the energy per charge range 1.5–300 keV/e for more than 4 years, from solar minimum through much of the rising phase of Solar Cycle 22. Thus our data set is large, including 67 magnetic storms with minimum Dst values less than −50 nT. To estimate the O+ and H+ contributions to the ring current energy, we have multiplied local measurements of each ion's energy density along the CCE trajectory by the appropriate dipole L shell volume and summed over L values from two to seven. We have used a bilinear regression to assess the dependence of U(O+)/U(H+) on solar EUV flux, parameterized by F10.7, and storm size, parameterized by minimum Dst value Dstmin. We have found that both F10.7 and Dstmin are important (and nearly independent) predictors of U(O+)/U(H+). Only four storms of 67 had U(O+)/U(H+) > 1. Two of the four were fairly small, with |Dstmin| < 100 nT but with high F10.7 values. This shows that O+ can sometimes contribute the majority of the ring current energy during small storms when the solar EUV flux is high (i.e., near solar maximum) as well as during very large storms throughout the solar cycle.

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.

Time domain analysis of plasma turbulence observed upstream of a quasi‐parallel shock

This paper presents, for the first time, an analysis of space plasma turbulence based on the NARMAX system identification approach. Fundamental nonlinear processes in the low‐frequency turbulence observed in the terrestrial foreshock by Active Magnetospheric Particle Tracer Explorers United Kingdom Satellite (AMPTE UKS) and Ion Release Module (AMPTE IRM) are studied using time domain identification methods developed for nonlinear dynamical systems. It is shown, directly from the experimental data, that the cubic nonlinearity has a significant influence on the steepening of the nonlinear low‐frequency waves and on the dependence of the phase velocity upon the wave amplitude. In comparison with a previous frequency domain approach, the present method requires only short data sets.

Ral ground station for small missions

A 12.5 m S Band ground station was installed at Chilton in 1980. It was used as the prime ground station for the Infrared Astronomy Satellite (IRAS) and later for command and control of the UK Active Magnetospheric Particle Tracer Explorer mission (AMPTE). More recently, it has been brought back into use for the low cost and more autonomous operations with the Advanced Composition Explorer (ACE) satellite and is being prepared for the UK Space Technology Research Vehicle (STRV) and two polar orbiter missions. This paper reviews, after 18 months of operations, the performance achieved in supporting ACE with the autonomous systems put in place at Chilton as well as, the cost effectiveness. Maintenance of an ageing antenna has been a challenge and is discussed. More automatic systems are planned for STRV including prototyping work on the new CCSDS standards for Space Link Extension services which should allow interoperability between the STRV Control Centre and the two UK antennas at Chilton and West Freugh. The advent of polar orbiting operations will increase the need for multi-satellite operations (i.e. ACE, STRV, Moroccosat and Badr-B will be operating together) and the need for contention resolution and rapid switching between the missions. New equipment standards and web-based systems play a key role in the achievements reported.

Kinetic study of the mirror mode

The linear Vlasov dispersion is solved to reveal the kinetic properties of the mirror mode. The existence of the torsional component of both the magnetic and velocity perturbations are kinetic features that are not explained by MHD theory. A parameter study is then employed to clarify the behavior of these particular components under different plasma conditions. The key parameters are the ion temperature anisotropy and the plasma to magnetic pressure ratio which act in similar ways to control the value of the torsional (or Alfvénic) components. Data from the Active Magnetospheric Particle Tracer Explorers mission show that under most magnetosheath plasma conditions the magnetic torsional component is negligible, whereas the velocity one is always larger or comparable to the other components. This experimental illustration also shows that there exist elliptically polarized mirror modes, contrary to what our, and all previous, analytical treatments predict.

A comparison of Pi2 pulsations in the inner magnetosphere and magnetic pulsations at geosynchronous orbit

A mechanism proposed for Pi2 pulsations in the inner (L = 2–5) magnetosphere is fast‐mode waves trapped in the plasmasphere. The presence of the trapped waves can be examined using multisatellite observations in the inner and outer magnetosphere. Until now, there were few simultaneous observations of Pi2 pulsations in the different magnetospheric regions. In this study we examine two Pi2 pulsations observed in the inner magnetosphere from the Active Magnetospheric Particle Tracer Explorer s/Charge Composition Explorer (AMPTE/CCE) and compare them to magnetic variations observed from the geostationary GOES 5 and 6 satellites. The first and second Pi2s were observed when CCE was located at L ∼ 3.9 and 5.0, respectively, and in the premidnight sector (∼2000–2100 magnetic local time). During both events, CCE observed strongly compressional oscillations. At geosynchronous orbit, pulsations were excited simultaneously at the onset times of the compressional oscillations at CCE. However, the transverse (by) component dominated, and its period differed from those of the compressional oscillations at CCE. This suggests that the oscillations at CCE and at geosynchronous orbit are excited by a common broadband source but that their generation mechanisms may be different (i.e., a trapped fast mode for the compressional oscillations at CCE and a field line resonance for the transverse oscillations at geosynchronous orbit).

A test of the Dessler‐Parker‐Sckopke relation during magnetic storms

The Dessler‐Parker‐Sckopke relation (DPS) predicts a linear dependence of the perturbation magnetic field at the surface of the Earth on the total ring current kinetic energy. In this paper, we test DPS by using measurements of the major ring current ion species made by the charge‐energy‐mass spectrometer on the Active Magnetospheric Particle Tracer Explorers CCE spacecraft. We use spectra from passes through the equatorial storm time ring current near the maximum phase of 80 magnetic storms between 1984 and 1989 to estimate the global ring current energy content ERC and compare it with the average value for Dst during each pass. Our work shows that DPS holds well on average. In particular, there is a strong linear correlation between ring current energy estimated from nightside ion measurements and the Dst index, and the slope of the least squares fit line giving Dst as a function of nightside ERC is in good agreement with the prediction of DPS. In contrast, day side measurements of ERC do not yield a robust correlation with Dst. Although we cannot rule out the possibility that currents other than the ring current (for example, tail currents and the magnetopause current) may cause large magnetic perturbations, we conclude that these perturbations, if they exist, must be largely compensating. By examining how the ratio of Dst to ERC varies with the local time sector of the in situ ion measurements, we obtain statistical information on the anisotropy of the storm time ring current. We find that the largest values of ERC/Dst result from nightside measurements and the smallest values result from measurements in the 0600 to 1200 LT region, as would be expected for an ion population injected on the nightside that must drift westward around the Earth, undergoing losses, to reach the dayside morning sector.

Multisatellite observations of large magnetic depressions in the solar wind

Two large depressions in the magnitude of the interplanetary magnetic field, lasting ∼10–20 min, have been observed in the solar wind just upstream of the Earth's bow shock by three spacecraft (Active Magnetospheric Particle Tracer Explorer UK Subsatellite (AMPTE UKS), AMPTE Ion Release Module (IRM), and ISEE 1). The multiple satellite observations show that the depressions are convecting with the ambient solar wind. Analysis of the depression boundaries shows that they are tangential discontinuities with normals aligned approximately parallel to the GSE x direction. The electron distributions measured within the structures are remarkably Isotropic when compared to the more anisotropic distributions found in the ambient solar wind. There is also a reduction in plasma wave activity during the depressions. The depressions exhibit characteristics similar to those of solar wind magnetic holes but are much larger than has been typically observed. The depressions also display similarities with encounters of the heliospheric plasma sheet and heat flux dropouts, both of which are typically observed near sector boundaries, close to the heliospheric current sheet. The nature of these depressions is discussed in the context of magnetic hole and heliospheric plasma sheet observations. A large magnetic hole structure formed from a conglomeration of small holes appears the more likely scenario for the observed depressions.

Structure of the magnetospheric magnetic field during magnetic storms

We report the results of a study of the contributions of the largescale magnetospheric currents to the observed Dst variation. Ground‐based magnetometer data during four magnetic storms (January 27–30, 1985; November 23–27, 1986; January 14–16, 1988; and May 6–8, 1988) were used to calculate Dst, and the paraboloid model of the magnetospheric magnetic field [Alexeev et al., 1996] was used to determine the contribution of each magnetospheric current system. Input data for our model were the solar wind plasma parameters, the interplanetary magnetic field (IMF) Bz, DMSP F6, F7, F8, and F9 satellite observations of precipitating auroral particles, and Active Magnetospheric Particle Tracer Explorers (AMPTE)/CCE satellite measurements of the total energy of the ring current ions with energy per charge between 1.5 and 300 keVq−1. We found good agreement between observed and modeled magnetic fields during the main phase of the magnetic storms. Using the paraboloid model, we have determined the contributions to Dst of different magnetospheric current systems including the magnetopause current BCF, the symmetric ring current BR, and the geotail current BT‐Such separation shows that values of BT and BCF are comparable with the value of BR during the main phase of the storms. During the recovery phase the effect of BR predominates.

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.

The role of nonlinear interaction in the formation of LF whistler turbulence upstream of a quasi‐perpendicular shock

The role of the nonlinear interaction in developed low‐frequency turbulence has been experimentally studied upstream of the ramp of a quasiperpendicular shock. The study has been carried out by application of the methods of bispectral analysis and wavelet decomposition to the Active Magnetospheric Particle Tracer Explorer Ion Release Module and Interball Tail Probe magnetic field data. It is shown that three‐wave processes play a key role in the formation of the spectrum of the turbulence. Experimental results presented and previous theoretical considerations lead to the conclusion that energy is transfered from a narrow maximum frequency pumped by nonlinear dynamic processes to lower and higher frequencies.

Frequency‐doubled density perturbations driven by ULF pulsations

We present Active Magnetospheric Particle Tracer Explorers Ion Release Module (AMPTE IRM) observations of a wave packet of compressional Pc5 ULF oscillations between 0915 and 1030 UT on October 28, 1984. The waves are observed in the local morning near the equatorial plane, have a dominant period of ∼ 380 s, and most probably have a fundamental field‐aligned harmonic structure. The waves have previously been interpreted as a magnetospheric waveguide mode propagating downtail [Mann et al., 1998]. At the time of the maximum amplitude of the waves, AMPTE IRM observes oscillations in the background density at a frequency twice that of the coincident ULF pulsations. We develop a theory which explains the generation of frequency‐doubled density fluctuations through large‐amplitude radial displacements of curved flux tubes in a nonadiabatic background plasma pressure distribution. The theory predicts a phase locking between the peaks and troughs of the radial velocity field and the peaks in the frequency‐doubled density oscillations which is observed by AMPTE IRM. The observations strongly support the hypothesis that the frequency doubling occurs as a result of finite amplitude effects.

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.

AMPTE observations of mirror mode waves in the magnetosheath: Wavevector determination

A unique method has been developed to completely determine the wavevector of a mirror mode wave using data from two closely spaced (separation less than wavelength/2) spacecraft. The method is based on the fact that mirror mode waves are linearly polarized and have zero frequency in the plasma rest frame. The method has been tested using Active Magnetospheric Particle Tracer Explorers United Kingdom Subsatellite (AMPTE UKS) and Ion Release Module (AMPTE IRM) high‐resolution magnetic field data from an interval of mirror mode waves observed in the middle magnetosheath on September 21, 1984. The results are also compared to those obtained using other methods of wavevector determination. The wavevectors estimated during the event are highly oblique to the ambient magnetic field direction as is expected from mirror wave theory. The application of the wavevector determination technique to further studies, such as studying the effect of finite electron temperature on mirror wave characteristics, is discussed.