R. C. Elphic

FAST satellite observations of large‐amplitude solitary structures

We report observations of “fast solitary waves” that are ubiquitous in downward current regions of the mid-altitude auroral zone. The single-period structures have large amplitudes (up to 2.5 V/m), travel much faster than the ion acoustic speed, carry substantial potentials (up to ∼100 Volts), and are associated with strong modulations of energetic electron fluxes. The amplitude and speed of the structures distinguishes them from ion-acoustic solitary waves or weak double layers. The electromagnetic signature appears to be that of an positive charge (electron hole) traveling anti-earthward. We present evidence that the structures are in or near regions of magnetic-field-aligned electric fields and propose that these nonlinear structures play a key role in supporting parallel electric fields in the downward current region of the auroral zone.

Observations of reconnection of interplanetary and lobe magnetic field lines at the high‐latitude magnetopause

Measurements made with the Fast Plasma Experiment on ISEE 2 in the vicinity of the high-latitude, dusk magnetopause near the terminator plane, at a time when the local magnetosheath and tail lobe magnetic fields were nearly oppositely directed, provide direct evidence for the reconnection of the open field lines of the tail lobes with the interplanetary magnetic field (IMF). The evidence consists primarily of observations of accelerated magnetosheath plasma flowing both tailward and sunward within the high-latitude magnetopause current layer. Observed speed changes at the magnetopause were of the order of twice the magnetosheath Alfven speed and were quantitatively consistent with the predictions of reconnection models. At times when the newly entering magnetosheath plasma observed at ISEE 2 was accelerated sunward a secondary beam of ions, presumably mirrored at low altitudes, was occasionally present. At times when the newly entering magnetosheath plasma observed by ISEE 2 was accelerated tailward a secondary beam of largely unaccelerated mantle plasma was occasionally present. Small plasma accelerations observed on reconnected field lines in the magnetosheath were associated with the presence of ions reflected at the magnetopause and moving at a speed of approximately twice the Alfven speed relative to the remainder of the magnetosheath plasma. Although previous work has anticipated that the re-reconnection of the open field lines of the tail lobes with the IMF would be associated with northward IMF in the magnetosheath, the present reconnection event was associated with a local magnetosheath IMF which had a small southward component.

FAST observations in the downward auroral current region: Energetic upgoing electron beams, parallel potential drops, and ion heating

Observations of plasma particles and fields by the FAST satellite find evidence of acceleration of intense upgoing electron beams by quasi-static parallel electric fields. The beam characteristics include a broad energy spectrum with peak energies between 100 eV and 5 keV, perpendicular temperatures less than 1 eV, and fluxes greater than 109/cm²sec. Diverging electrostatic shocks associated with the beams have integrated potentials that match the beam energy. These beams are found in regions of downward Birkeland current and account for the total field-aligned current when they are present. The most energetic ion conics in the auroral zone are found coincident with these beams, in agreement with the model for “trapped” conics. The measured particle densities of the electron beams and associated ion conics are approximately equal and typically range from 1 to 10 cm−3, with no evidence for additional cold density. The beams are seen frequently at altitudes between 2000 and 4000 km in the winter auroral zone. Their probability of occurrence has a strong dependence on season and altitude and is similar to that for upgoing ion beams in the adjacent upward current regions. This similarity suggests that the density and scale height of ionospheric ions play an important role in the formation of both types of beams.

FAST satellite observations of electric field structures in the auroral zone

Electric field and energetic particle observations by the Fast Auroral Snapshot (FAST) satellite provide convincing evidence of particle acceleration by quasi-static, magnetic-field-aligned (parallel) electric fields in both the upward and downward current regions of the auroral zone. We demonstrate this by comparing the inferred parallel potentials of electrostatic shocks with particle energies. We also report nonlinear electric field structures which may play a role in supporting parallel electric fields. These structures include large-amplitude ion cyclotron waves in the upward current region, and intense, spiky electric fields in the downward current region. The observed structures had substantial parallel components and correlative electron flux modulations. Observations of parallel electric fields in two distinct plasmas suggest that parallel electric fields may be a fundamental particle acceleration mechanism in astrophysical plasmas.

Evidence for water ice near the lunar poles

Improved versions of Lunar Prospector thermal and epithermal neutron data were studied to help discriminate between potential delivery and retention mechanisms for hydrogen on the Moon. Improved spatial resolution at both poles shows that the largest concentrations of hydrogen overlay regions in permanent shade. In the north these regions consist of a heavily cratered terrain containing many small (less than ∼10-km diameter), isolated craters. These border circular areas of hydrogen abundance ([H]) that is only modestly enhanced above the average equatorial value but that falls within large, flat-bottomed, and sunlit polar craters. Near the south pole, [H] is enhanced within several 30-km-scale craters that are in permanent shade but is only modestly enhanced within their sunlit neighbors. We show that delivery by the solar wind cannot account for these observations because the diffusivity of hydrogen at the temperatures within both sunlit and permanently shaded craters near both poles is sufficiently low that a solar wind origin cannot explain their differences. We conclude that a significant portion of the enhanced hydrogen near both poles is most likely in the form of water molecules.

Magnetospheric plasma analyzer: Initial three‐spacecraft observations from geosynchronous orbit

The first three magnetospheric plasma analyzer (MPA) instruments have been returning data from geosynchronous orbit nearly continuously since late 1989, 1990, and 1991. These identical instruments provide for the first time simultaneous plasma observations from three widely spaced geosynchronous locations. The MPA instruments measure the three-dimensional velocity space distributions of both electrons and ions with energies between ∼1 eV/q and ∼40 keV/q. MPA capabilities and observations are summarized in this paper. We use the simultaneous observations from three longitudinally separated spacecraft to synthesize a synoptic view of the morphology of the magnetosphere at geosynchronous orbit over a 6-week interval in early 1992. The MPA observations indicate that the spacecraft encountered seven regions with characteristic plasma populations during this period: (1) the cool, dense plasmasphere (13.1% of the data); (2) a warmer, less dense plasma trough (22.5%); (3) the hot plasma sheet (40.3%); (4) a combination of plasma trough and plasma sheet (18.6%); (5) an empty trough region, devoid of plasma sheet, plasmasphere, or plasma trough populations (4.3%); (6) the magnetosheath and/or low-latitude boundary layer (0.7%); and (7) the lobe (0.3%). The local time distributions of these regions are examined. For example, as suggested by previous authors, we find that at geomagnetically quiet times (Kp < 2) geosynchronous orbit can lie entirely within the plasmasphere while at more active times only the afternoon to evening portions of the orbit are typically within the plasmasphere. We also find that the plasma convection inside the plasmasphere is generally sunward in the corotating (geosynchronous spacecraft) reference frame, independent of activity level, in contrast to previous studies. In addition to such statistical results, the simultaneous data sets at different local times allow us to at least partially separate spatial from temporal variations. In particular, we use these observations to examine the instantaneous shapes of the plasmapause and magnetopause as they pass over geosynchronous orbit. As expected, the plasmapause is found to have a highly variable shape, at various times showing (1) a stable dusk side bulge, (2) a variable bulge which expands, contracts, and moves, (3) an overall expansion and contraction of the plasmasphere, and (4) even more complicated behavior which is best accounted for by large-scale structure of the plasmapause and/or disconnected plasma blobs. During the 6 weeks of data examined, the magnetosheath was encountered on several occasions at synchronous orbit, preferentially on the prenoon side of the magnetosphere. For the first time, simultaneous prenoon and postnoon observations confirm this asymmetry and demonstrate that the magnetopause shape can be highly asymmetric about the Earth-Sun line.

Iron abundances on the lunar surface as measured by the Lunar Prospector gamma‐ray and neutron spectrometers

[i] Global measurements of iron abundances on the lunar surface are presented using data from the Lunar Prospector (LP) Gamma-Ray Spectrometer (GRS) and Neutron Spectrometer (NS). In this study, we derive relative iron abundances from the low-altitude, high spatial resolution (∼(45 km) 2 ) LP data using the 7.6 MeV neutron capture gamma-ray doublet. As part of the LP-GRS analysis, we demonstrate the importance of accounting for variations in neutron number density across the lunar surface by measuring neutron fluxes using LP-NS data. In a first step of comparing the LP-GRS data with previously published iron abundances inferred from Clementine Spectral Reflectance (CSR) data, we show that the existing CSR FeO data are nonlinear with respect to the LP relative iron abundances. We use the LP data to linearize the relationship between the CSR and the relative iron values then recalibrate the CSR data to iron abundance using returned soil abundances. We then correlate the CSR data, except for major anomalies, with the LP relative iron measurements to convert the LP data to absolute iron abundances. When we compare the LP-GRS and revised CSR data sets, we find a very good correspondence. There are two locations (Mare Tranquillitatis and South Pole-Aitken (SPA) basin) that show major discrepancies, suggesting that the CSR data are locally overestimating iron abundances. In both these regions, the discrepancies identified by the LP-GRS/CSR comparison are possibly explained by mineralogical differences that are not accounted for in the CSR to FeO calibration. In regards to our understanding of the Moon, the LP data have found the following: (I) There exist large expanses of mare basalt in the western mare regions that have very high iron abundances (22-23 wt.% FeO) that are underrepresented but not absent from the returned sample collection and are highly unusual for mare soils, which typically contain a significant amount of highlands contamination. (2) The low iron abundances in the lunar highlands (∼5 FeO wt.%) are consistent with a previous analysis using thermal and epithermal neutrons and with the idea that the lunar crust formed by a relatively simple magma ocean process. (3) The comparison of LP and CSR derived iron abundances suggests that the material within SPA basin is similar to a norite-type rock without an enriched mantle FeO signature. (4) A comparison of LP and CSR data at Tycho Crater shows a large discrepancy such that the CSR data show moderate iron abundances of 8-9 wt.% FeO while the LP data show very low iron abundances of 3-4 wt.% FeO. This discrepancy cannot yet be easily explained by any known process.

FAST satellite wave observations in the AKR source region

The Fast Auroral SnapshoT (FAST) satellite has made observations in the Auroral Kilometric Radiation (AKR) source region with unprecedented frequency and time resolution. We confirm the AKR source is in a density depleted cavity and present examples in which cold electrons appeared to have been nearly evacuated (nhot> ncold). Electron distributions were depleted at low-energies and up-going ion beams were always present. Source region amplitudes were far greater than previously reported, reaching 2×10−4 (V/m)²/Hz (300 mV/m) in short bursts with bandwidths generally <1 kHz. Intense emissions were often at the edge of the density cavity. Emissions were near or below the cold plasma electron cyclotron frequency in the source region, and were almost entirely electromagnetic. The |E|/|B| ratio was constant as a function of frequency and rarely displayed any features that would identify a cold plasma cutoff or resonance.

Model of electron and ion distributions in the plasma sheet boundary layer

Electron and ion velocity space distributions in the plasma sheet boundary layer have distinct features and exhibit a characteristic evolution with depth in the boundary layer. Near the lobeward edge of the layer, enhanced earthward and tailward directed electron flux is observed. Somewhat deeper in the boundary layer, earthward and tailward directed ion beams are observed. The electron and ion beams have low-speed cutoffs, and the earthward directed beams are consistently observed at lower speeds than the simultaneously observed tailward directed beams. The ion distributions evolve from magnetic-field-aligned beams, to “kidney bean” shaped distributions, to isotropic shells with increasing equatorward penetration into the boundary layer. A two-dimensional model based on quasi-steady reconnection occurring in the distant magnetotail is able to reproduce all of these observed features in the electron and ion distribution functions. The essential features of the model are the finite time-of-flight effect (velocity filter effect), conservation of energy, and conservation of magnetic moment as the particles stream from the low magnetic field region near the central plasma sheet to the higher field region in the boundary layer. The model can be used to estimate the central plasma sheet density, temperature, and bulk flow speed as functions of position earthward of a reconnection site from observed plasma sheet boundary layer distributions. These plasma distributions obtained from the model may be useful in determining the stability of the boundary layer plasma to various electrostatic and electromagnetic modes.

FAST observations of VLF waves in the auroral zone: Evidence of very low plasma densities

The Fast Auroral SnapshoT (FAST) explorer frequently observes the auroral density cavity, which is the source region for Auroral Kilometric Radiation (AKR). An important factor in the generation of AKR is the relative abundance of hot and cold electrons within the cavity, since hot electrons introduce relativistic modifications to the wave dispersion. VLF wave-form data acquired by FAST within the auroral density cavity show clear signatures of whistler-mode waves propagating on the resonance cone. This allows us to obtain the electron plasma frequency, and the cavity often has densities <1 cm−3. Moreover, the hot electrons can be the dominant electron species, enabling AKR to be generated below the cold electron gyro-frequency.