摘要:

The charge on grains of simulated lunar regoliths (MLS‐1 and JSC‐1) has been measured in a plasma containing thermal electrons with a few electron volts temperature and a second population of nearly monoenergetic fast electrons with energy variable from 15 to 100 eV. The measured charges are similar to that on glass microspheres of the same diameter. Both the glass and the simulated lunar samples showed multiple charge states in a finite electron energy range, that might help the theoretical models suggesting active electrostatic dust transport on the Moon. The measurements can be well described by a charging model which includes currents due the background thermal plasma, monoenergetic fast electrons and secondary electr

ISSN:0094-8276    

DOI:10.1029/95GL02287

年代:1995

数据来源: WILEY

摘要:

We used 5 years of GOES‐6 and 6 years of GOES‐7 data to correlate the y‐component of the geosynchronous magnetic field with the y‐component of IMF. One motivation is that the subsolar merging model and the antiparallel model predict distinctly different patterns forByin the magnetosphere as a function of IMFBy. Although both models predict that nightside magnetospheric magnetic field will tilt in the direction of the IMFBy, the antiparallel merging model predicts that magnetospheric magnetic field lines in the vicinity of local noon will tilt in the direction of the IMF, where as the subsolar model does not. The correlation coefficients between the geosynchronous data and the IMFBypeak at both noon (0.61) and midnight (0.50), favoring the antiparallel model. These results were obtained with 1789 (noon) and 1312 (midnight) data points. The slopes of the regression lines indicate 29% IMFByentry at noon and 79% at midnight. The local‐time distribution of the slopes varies smoothly from midnight peak to a broad minimum

ISSN:0094-8276    

DOI:10.1029/95GL02261

年代:1995

数据来源: WILEY

摘要:

On December 8, 1990, during its first flyby of the Earth, Galileo recorded signatures of several flux rope type structures in the Earth's magnetotail. One of the prominent structures had a negative/positive polarity in theBzcomponent and was moving earthward, whereas another of the prominent structures had a positive/negative polarity inBzand was observed to be traveling away from the Earth. Both of these flux ropes had strong core fields (≥10 nT) aligned in the positiveYGSMdirection. We have performed an analysis of combined field and particle data obtained within these flux ropes. We show that the length scale of a flux rope can be determined quite accurately from magnetic data if an independent measure of either the current density or the convection velocity is available. A surprising result that emerges from this analysis is that even though temporal durations (5–10 min) and magnetic field perturbations of the two flux ropes are similar, the scale sizes of the two structures are very different. The earthward moving flux rope with a radius ≤1000 km was traveling very slowly (υx<10 km/s). The tailward moving flux rope with a radius of ∼8 REhad a speed of ∼150 km/s. Even though the earthward moving flux rope was small in size, the current density in its core was well above that of the tail current sheet and was detected directly by the plasma instrument. We believe that this is the first time that measurements of field aligned current in a flux rope core have been made directly by a plasma instrument. We discuss the details of the distribution functions of the current‐carrying particles. We also speculate on the conditions that led to the vast differences in the scale sizes of the tw

ISSN:0094-8276    

DOI:10.1029/95GL01518

年代:1995

数据来源: WILEY

摘要:

The collisionless, supercritical, quasi‐perpendicular fast shock is investigated on sub‐ion scales using an implicit, two‐dimensional (2‐D) full particle code. For the first time, simulations are carried out with realistic characteristic frequencies and sufficiently high mass ratio between the protons and electrons. As a result, there is relatively little scattering of the electrons, i.e., they behave largely adiabatically as previously suggested based on spacecraft observations at the Earth's bow shock. The large mass ratio also allows for a realistic description of the whistler mode dispersion. Phase‐standing whistlers with propagation along the shock normal appear as transients. The dominant whistlers found at late times in the simulations have upstream directed group velocity but propagate at oblique direction between the shock normal and the ambient magnetic field. Their properties match those of the ubiquitous observed upstream whistlers (“one‐

ISSN:0094-8276    

DOI:10.1029/95GL01782

年代:1995

数据来源: WILEY

摘要:

Impulsive VLF wave packets and electron beams may be formed in the magnetosphere from the superposition of Gendrin mode components defined by the conditioncosθ=2ffHwhere θ=wave normal angle measured from the Earth's static magnetic fieldB¯o,f=frequency of the wave component andfH= electron gyrofrequency. The frequency range of the Gendrin mode spectrum is defined byfL

ISSN:0094-8276    

DOI:10.1029/95GL02003

年代:1995

数据来源: WILEY

摘要:

Localized packets of electrostatic whistler emissions near the lower hybrid frequency have been observed in the low‐attitude auroral region by sounding rockets and the Freja satellite. We present a model for wave localization that requires two ion species, O+and H+, whereby the minority H+ions control the localization process. The theoretical basis is that the plasma dispersion is dramatically affected by small changes in the H+density and that the observed emissions would selectively heat H+. We show observational support for the proposed mode

ISSN:0094-8276    

DOI:10.1029/95GL02004

年代:1995

数据来源: WILEY

摘要:

The Freja TESP electron spectrometer has repeatedly observed ∼100 eV ‐ 1 keV upward‐directed, anti‐field‐aligned electron beams near 1700 km altitude in the auroral zone. A particularly intense event, at energies up to 2 keV, is described. The beam perpendicular temperatureT⟂ewas as low as 0.1–0.2 eV at 100–200 eV parallel energy. The 10–15 s period of upward fluxes was coincident with a low density (∼10 cm−3) period and a ∼5 keV ion conic. Strong low frequency waves and the lack of any downward motion in the simultaneously observed ion conic suggest a strong element of wave acceleration, while electric field and ion loss cone measurements provide limited evidence of potential acceleration to a fraction o

ISSN:0094-8276    

DOI:10.1029/95GL01777

年代:1995

数据来源: WILEY

摘要:

Anomalous electron heat fluxes and recent observations of day‐night asymmetries in polar wind features indicate that photoelectrons may affect polar wind dynamics. These anomalous fluxes require a global kinetic description (i.e.,mesoscale particle phase space evolution involving microscale interactions); their impact on the polar wind itself requires a self‐consistent description. In this Letter, we discuss results of a self‐consistent hybrid model that explains the dayside observations. This model represents the first global kinetic collisional description for photoelectrons in a self‐consistent classical polar wind picture. In this model, photoelectrons are treated as test particles, ion properties are based on global kinetic collisional calculations, thermal electron features and the ambipolar field are determined by fluid calculations. The model provides the first global steady‐state polar wind solution that is continuous from the subsonic collisional regime at low altitude to the supersonic collisionless regime at high altitude. Also, the results are consistent with experiments in several aspects, such as order of magnitude of the ambipolar electric potential, qualitative features of the ion outflow characteristics, electron anisotropy and upwardly directed electron heat flux on th

ISSN:0094-8276    

DOI:10.1029/95GL01846

年代:1995

数据来源: WILEY

摘要:

The power spectra of low latitude (L<3), ground‐based recordings of dayside geomagnetic pulsations in the 10‐100 mHz band indicate that these pulsations come from discrete, driven field line resonances which are coupled to compressional, magnetohydrodynamic, cavity or waveguide modes. The spectral bands of individual harmonics of the field line resonances seen by the magnetometers have envelopes which are typically 10's of mHz wide but also show a fine scale structure of almost equispaced peaks with separations of about 3 to 5 mHz. A WKB analysis of a cavity or waveguide model shows that the fine structure is likely due to field line resonances driven by waveguide modes with turning points within the plasmasphere and reflection at the dayside magnetopa

ISSN:0094-8276    

DOI:10.1029/95GL01770

年代:1995

数据来源: WILEY

摘要:

Mass resolved fluxes of Ba+and Sr+ions were measured with the Quadrupole Ion Mass Spectrometer during the CRRES G‐11b chemical release experiment in darkness. The flux of Sr+relative to Ba+at a distance of 4 km from the release point was more than ten times larger than expected based on estimated charge transfer cross sections of the ground state metals. In each metal, we have identified at least one low‐lying metastable state that is closer to charge‐transfer resonance with O+than the ground state. These metastable states are 5s5p ³P2in Sr at 14, 899 cm−1above the ground state and 6s5d ³D3in Ba at 9,597 cm−1. The Sr 5s5p ³P2state is nearly exactly resonant with O+(9 meV energy defect) and may have a significantly enhanced charge transfer cross section. In a proposed mechanism that may explain the measured ion fluxes, neutral metal atoms are excited to these metastable states by electron‐atom collisions early in the expansion of the neutral cloud. High energy electrons could be produced by a plasma process such as CIV. This energy is stored in the neutral cloud. The metastable atoms are ionized by near‐resonant charge exchange with ambient O+and are detected after the cloud expands to reac

ISSN:0094-8276    

DOI:10.1029/95GL02063

年代:1995

数据来源: WILEY