摘要:

We have determined the spectral characteristics of central plasma sheet ions and electrons observed during 71 hours when geomagnetic activity was at moderate to high levels (AE≥ 100 nT). Particle data from the low‐energy proton and electron differential energy analyzer and the medium energy particle instrument on ISEE 1 are combined to obtain differential energy spectra (measured in units of particles/cm² s sr keV) in the kinetic energy range ∼30 eV/e to ∼1 MeV at geocentric radial distances>12Re. Nearly isotropic central plasma sheet total ion and electron populations were chosen for analysis and were measured to be continuous particle distributions from our lowest to highest energies. During these highAEperiods the>24 keV particle fluxes and the temperature of the entire particle distributionkTare significantly higher than during lowAEperiods (AE<100 nT). The temperatureskTiandkTeare highly correlated during both quiet and disturbed periods. The active period spectral shape appears softer for ions and somewhat harder for electrons than during quiet periods. We find that the observed active period spectrum typically is complex and cannot be represented in general by a single functional form, as during quiet periods when it can be represented by the kappa distribution function. Although a power‐law shape is observed at higher energies, ion and electron spectral shapes deviate from a strictly kappalike form in different ways. In a limited energy range near the knee of the ion spectra (the knee is that portion of the spectrum at energiesE≳Eowhere the flux starts to decrease swiftly with increasing energy), the spectral shape can often be fit with a Maxwellian form, thus rolling over faster than the typical quiet time spectrum. At higher energies this shape merges into a harder nonthermal power‐law tail. Electron spectra also display this spectral characteristic, although at a lower occurrence frequency than for ions. The electron spectra are predominantly kappalike at energies near and above the knee. At energies below the knee, both ions and electrons often have an excess of flux with respect to the functional form that best fits the shape for energies at or above the knee, be it a kappa distribution or a Maxwellian distribution; the electron flux excess is significantly greater than the ion flux excess. We conclude that both ions and electrons participate in at least two separate acceleration mechanisms as geomagnetic activity evolves from lowAEto highAEvalues. We suggest that both spectrum‐preserving and spectrum‐altering heating processes (possibly involving nonlocal betatron acceleration and crosstail current sheet acceleration, respectively) participate in overall particle energization during geomagnetic active periods. Observations are compared to

ISSN:0148-0227    

DOI:10.1029/90JA01633

年代:1991

数据来源: WILEY

摘要:

Using our three‐dimensional MHD code we have studied the dynamic evolution of a non‐symmetric magnetotail configuration, initiated by the sudden occurrence of (anomalous) resistivity. The initial configuration included variations in all three space dimensions, consistent with average tail observations. In addition, it was skewed due to the presence of a net cross‐tail magnetic field componentByNwith a magnitude as typically observed, so that it lacked commonly assumed mirror symmetries around the midnight meridian and the equatorial planes. The field evolution was found to be very similar to that of a symmetric configuration studied earlier (e.g., Birn and Hones, 1981), indicating plasmoid formation and ejection. The most noticeable new feature in the evolution of the individual field components is a reduction ofByon the reconnected dipole‐like field lines earthward from the reconnection region. The topological structure of the magnetic field, however, defined by the field line connections, shows remarkable differences from the symmetric case, consistent with conclusions by Hughes and Sibeck (1987) and Birn et al. (1989). The plasmoid, which is a magnetically separate entity in the symmetric case, becomes “open”, connected initially with the Earth, but getting gradually connected with the interplanetary field, as reconnection of lobe field lines proceeds from the midnight region to the flanks of the tail. The separation of the plasmoid from the Earth is thus found to take a finite amount of time. When the plasmoid begins to separate from the Earth, a filamentary structure of field connections develops, not present in the spatial variation of the fields; this confirms predictions by Birn et al. (1989). A localization of the electric field parallel to the magnetic field is found consistent with conclusions on general magnetic reconnection (Schindler et al., 1988a,b; Hesse and Schindler, 1988). The effect ofE∥, integrated along field lines, is found to be maximal on field lines near the plasma sheet/lobe interface. The “footprints” of these regions at the near‐Earth boundary show a clottiness, reflecting the filamentary structure of the field connections, but not present in the spatial structure

ISSN:0148-0227    

DOI:10.1029/90JA01356

年代:1991

数据来源: WILEY

摘要:

The direct injection of magnetosheath plasma into the cusp produces at low altitude a precipitation regime with an energy‐latitude dispersion—the more poleward portion of which we herein term the “cusp plume.” An extensive survey of the Defense Meteorological Satellite Program (DMSP) F7 and F9 32 eV to 30 keV precipitating particle data shows that similar dispersive signatures exist over much of the dayside, just poleward of the auroral oval. Away from noon (or more precisely, anywhere not immediately poleward of the cusp) the fluxes are reduced by a factor of about 10 as compared to the cusp plume, but other characteristics are quite similar. For example, the inferred temperatures and flow velocities, and the characteristic decline of energy and number flux with increasing latitude is essentially the same in a longitudinally broad ring of precipitation a few degrees thick in latitude over much of the dayside. We conclude that the field lines on which such precipitation occurs thread the magnetospheric plasma mantle over the entire longitudinally extended ring. Besides the location of occurrence (i.e., immediately poleward of the dayside oval), the identification is based especially on the associated very soft ion spectra, which have densities from a few times 10−2to a few times 10−1/cm³; on the temperature range, which is from a few tens of eV up to about 200 eV; and on the characteristic gradients with latitude. Further corroborating evidence that the precipitation is associated with field lines which thread the plasma mantle includes drift meter observations which show that regions so identified based on the particle data consistently lie on antisunward convecting field lines. Our observations indicate that some dayside high‐latitude auroral features just poleward of the auroral oval are embedded in the

ISSN:0148-0227    

DOI:10.1029/90JA01760

年代:1991

数据来源: WILEY

摘要:

Energetic ion events in the Earth's dayside subsolar magnetosheath (0900 ‐ 1300 Local Time) are surveyed using data from the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) Hot Plasma Composition Experiment. Ion species carrying the signature of their origin (O+and energetic He2+) are used to distinguish between magnetospheric and solar wind origins for the energetic ion events. The results of this survey indicate that the majority of energetic (10–17 keV/e) H+and He2+ions observed in the dayside magnetosheath are accelerated from the solar wind population. The energetic He2+to H+density ratio in the magnetosheath is consistent with that predicted from first‐order Fermi acceleration of solar wind ions in the turbulent regions upstream and downstream from the Earth's quasi‐parallel bow shock. Although the majority of the energetic ions appear to be of solar wind origin, magnetospheric O+is also occasionally present in the magnetosheath. The simultaneous occurrence of both energetic He2+and magnetospheric O+indicates that, on occasion, both Fermi acceleration of solar wind ions and leakage of magnetospheric ions occurs in the dayside magnet

ISSN:0148-0227    

DOI:10.1029/90JA01751

年代:1991

数据来源: WILEY

摘要:

Magnetic signatures associated with the time‐dependent magnetic reconnection processes at the dayside magnetopause are studied based on two‐dimensional compressible MHD simulations. In the simulations, magnetic and plasma signatures resemblant to the observed flux transfer events (FTEs) can be generated either by the magnetic bulges formed during the bursty single X line reconnection (BSXR) or by the magnetic islands (flux tubes) formed during the multiple X line reconnection (MXR). It is found that the FTE magnetic signatures are not exhibited on the magnetospheric side if the FTEs are due to the BSXR process andBm/Bs≥ 1.7, whereBmandBsare the magnetic field strength in the magnetosheath and in the magnetosphere, respectively. On the other hand, the bipolar FTE signatures can be detected on both the magnetosphere and magnetosheath sides if the FTEs are caused by the MXR process andBm/Bs≤ 2.6. WhenBm/Bs>2.6, the bipolar FTE signatures in the magnetosphere site become too small to be detected even if magnetic islands are formed during the MXR process. Furthermore, forBm/Bs>1, the region for the detection of FTE signatures in the magnetospheric side is smaller than that in the magnetosheath side. Since at the dayside magnetopause the typical value ofBm/Bsis 1–3, the simulation results indicate that more FTE signatures can be detected in the magnetosheath side than in the magnetosphere. It is also found that the MXR process often generates a clear bipolarBnsignature while the BSXR process tends to produce FTEs with a monopolarBnsignature near the reconnection region and a highly asymmetric bipolarBnsignature away from the reconnecti

ISSN:0148-0227    

DOI:10.1029/90JA01989

年代:1991

数据来源: WILEY

摘要:

Field‐aligned currents play a central role in the study of the magnetized plasmas of the solar terrestrial environment. In particular, if perturbations of flow develop on one part of a flux tube, field‐aligned currents must flow in order to communicate the changes to the entire flux tube. Field‐aligned currents cause the field to twist or shear, a feature that can be described in terms of the displacement of field lines from an unperturbed orientation. In this paper we derive expressions for the field‐aligned current density in a magnetized plasma that illuminate die relationship between twist or shear and current flow parallel to the background magnetic field. We show that the time evolution of the field displacements is closely linked to equations for Alfvén wave displacements and that they are driven by pressure gradient terms of the sort that are encountered in computations of interchange motions. We find that in the magnetosphere or in any other collisionless plasma regime, the field‐aligned current density is related closely to the time integral of the parallel vorticity. We make use of the fact that the field‐aligned current density in the ionosphere is related to the vorticity itself, not its time integral. Continuity relates the currents in the two regions and implies a decay time for the magnetospheric current inversely proportional to the ionospheric resistivity. The decay depends only on a magnetospheric scale length but on no other magnetospheric parameters and resembles the decay of current in an inductance caused by resistivity somewhere in

ISSN:0148-0227    

DOI:10.1029/90JA01806

年代:1991

数据来源: WILEY

摘要:

A gyroviscous model of the magnetopause current layer is applied to the entire magnetopause with the inner boundary condition provided by a quantitative model of the magnetospheric field (BM) and the outer boundary condition given by the interplanetary magnetic field (IMF) draped against the magnetopause surface. It is found that the magnetopause surface is divided into at least four different regions by a tangential singularity line (Bn= 0) that separates regions with different signs of the normal component and rotation singularity lines (BM∥BIMF) that separate regions with different magnetic polarizations (rotations). The tangential singularity line (TSL) has topological properties similar to the classical reconnection line and the rotation singularity lines (RSL) are similar to lines of “antiparallel merging.” The field topology in the regular regions is determined by field line tracing using the model solutions. It is argued that the whole magnetopause surface, except the singular separation regions, possesses a stable rotational structure that can be described locally by a planar model. It is proposed that the singular regions on the magnetopause surface (TSL and RSL) play the role of gates for solar wind plasma entry into the magnetosphere and that rotational singularities are observed as flux transfer events. Observations of velocity jumps at the magnetopause that were previously classified as “high‐speed reconnection flows,” “tangential discontinuity” crossings as well as “diffusive entry” crossings can all be explained as special cases of the gyroviscous merging theory. The paper offers a unified view of such disparate concepts as steady state reconnection, antiparallel merging, viscous interactions, patchy reconnection, flux transfer events, and im

ISSN:0148-0227    

DOI:10.1029/90JA02194

年代:1991

数据来源: WILEY

摘要:

We report the results of a preliminary analysis of a large‐amplitude rotational wave in the Venusian ionosheath. Our results are based on an analysis of the Pioneer Venus Orbiter plasma and magnetic field observations from six orbits in the first (1979) tail season. This wave appears to be the standing superalfvenic wing. It is located within the shocked plasma flow outside the boundary of the tail. The rotation of the magnetic field by about 90 deg across the wave occurs through several successive cycles making the wave similar to the group velocity wing consisting of phase velocity waves. The transition through the wave is accompanied by the vector change of the plasma velocity with the magnitude of the plasma velocity jump comparable to the vector jump of the Alfven velocity. The observed super‐Alfvenic wing appears to originate upstream and closer to the planet, possibly near the upper boundary of the magnetic barrier on the days

ISSN:0148-0227    

DOI:10.1029/90JA01841

年代:1991

数据来源: WILEY

摘要:

The newly introduced indexPCfor magnetic activity in the polar cap has been examined to establish to which extent it can serve as an indicator of auroral electrojet activity.PCis derived from a single nearpole station, as a 15‐min average index. We have derived it for two stations, one in the northern hemisphere (Thule) and one in the southern hemisphere (Vostok). The simplicity of thePCindex enables us to make a large data base for statistical investigations. We have thus used 7 years ofPCvalues for the two stations to analyze the relationship betweenPCand the auroral zone indicesAE,AU, andALstatistically. We find a very high correlation betweenPCandAEduring winter and equinox, the linear correlation coefficient being ≈0.8–0.9 for Thule and ≈0.7–0.8 for Vostok. During summer the correlation is less because thePCindex is then disturbed by polar cap currents controlled by the northward and east‐west components of the interplanetary magnetic field. We therefore stress the importance of havingPCavailable from both the northern and southern hemisphere. From event studies we find thatPCis sensitive both toDP2 type electrojet activity and to substorm intensifications of the westward electrojet in the midnight or postmidnight sector but less sensitive to substorm intensifications of the westward electrojet in the premidnight sector. We conclude thatPCcan serve as a fast available indicator ofDP2 andDP1 activity in the polar regions, excluding intrusions of the westward electrojet in the premidn

ISSN:0148-0227    

DOI:10.1029/90JA01975

年代:1991

数据来源: WILEY

摘要:

Thermospheric neutral wind data, obtained by the Fabry‐Perot Interferometer (FPI) and Wind and Temperature Spectrometer (WATS) on the Dynamics Explorer 2 satellite during periods of northward interplanetary magnetic field (IMFBzpositive), show a dependence of the circulation on the magnitude and sign of the east–west component (By) of the IMF. Averaged thermospheric neutral wind vector fields, for the northern hemisphere, and data from several sets of satellite orbits, that illustrate the variation in the neutral wind structure as a function of time and IMF orientation, are presented and discussed. The results show that near December solstice, at solar maximum: (1) sunward neutral winds develop inside the geomagnetic polar cap during periods of persistent positiveBzand relatively lowBy; (2) typical time delays in the establishment of sunward winds following a northward turning of the IMF are approximately 4 hours; (3) sunward winds are not observed when the magnitude of theBzcomponent of the IMF is less than +4 nT; (4) the maximum‐velocity antisunward winds, in the central geomagnetic polar cap region, are on the duskside (dawnside) in the northern hemisphere during periods of northward IMF andBynegative (positive). A similar asymmetry exists in the sunward flow in the center of the polar cap; (5) if the absolute magnitude of theBycomponent of the IMF exceeds that ofBz(i.e., if theBy/Bzratio is greater than 1.0) then sunward winds inside the geomagnetic polar cap are rarely observed; instead, one sees a pattern of neutral winds similar to that expected for the given sign ofByunder southward IMF conditions; (6) the summer hemisphere has neutral wind signatures that are similar to those seen in the winter hemisphere forBznorthward, i.e., sunward winds occur in the polar cap for persistently northwardBz, provided the magnitude ofBydoes not exceed that

ISSN:0148-0227    

DOI:10.1029/90JA01996

年代:1991

数据来源: WILEY