摘要:

The orientation of the heliospheric current sheet predicted from a source surface model is compared with the orientation determined from minimum‐variance analysis of ISEE 3 magnetic field data at 1 AU near solar maximum. Of the 37 cases analyzed, 28 have minimum variance normals that lie orthogonal to the predicted Parker spiral direction. For these cases, the correlation coefficient between the predicted and measured inclinations is 0.6. However, for the subset of 14 cases for which transient signatures (either interplanetary shocks or bidirectional electrons) are absent, the agreement in inclinations improves dramatically, with a correlation coefficient of 0.96. These results validate not only the use of the source surface model as a predictor but also the previously questioned usefulness of minimum variance analysis across complex sector boundaries. In addition, the results imply that interplanetary dynamics have little effect on current sheet inclination at 1 AU. The dependence of the correlation on transient occurrence suggests that the leading edge of a coronal mass ejection (CME), where transient signatures are detected, disrupts the heliospheric current sheet but that the sheet re‐forms between the trailing legs of the CME. In this way the global structure of the heliosphere, reflected both in the source surface maps and in the interplanetary sector structure, can be maintained even when the CME occurrence rate is h

ISSN:0148-0227    

DOI:10.1029/93JA02100

年代:1994

数据来源: WILEY

摘要:

In order to understand the solar cycle variation of interplanetary shocks and their driving source at 0.72 AU, a survey of Pioneer Venus Orbiter (PVO) magnetometer and plasma data from 1979‐1988 has been conducted. Known shock drivers at 1.0 AU include coronal mass ejections (CMEs) and fast/slow stream interactions. In our analysis, CMEs were identified by a decrease in plasma temperature to background or below accompanied by an increase in plasma density and dynamic pressure. It was also required that the magnetic field exhibit a coherent rotation over about a day and an increase and decline in magnitude on a timescale of hours to days. Stream interactions were identified by a characteristic increase in ion temperature and velocity coincident with a decrease in density and a coincident increase in the total magnetic field magnitude. These signatures were usually preceded within 24 hours by a change in flow angle. In all, 45 shocks were identified: 36 driven by CMEs, 6 resulting from fast/slow stream interactions, and 3 with sources that could not be defined. The shocks driven by CMEs show a solar cycle variation that roughly follows the sunspot number. These shocks all have normals consistent with radial propagation of the shock fronts from the sun. In contrast, the few stream interaction related shocks show a tendency to occur later in the solar cycle and have a broader distribution of shock normal

ISSN:0148-0227    

DOI:10.1029/93JA02666

年代:1994

数据来源: WILEY

摘要:

Recent studies of low‐frequency electromagnetic waves upstream of the Saturn bow shock have shown that these waves, in contrast to those at Earth, are observed not in one, but in at least two, distinct frequency bands. The results of wave mode identification based on the Hall‐MHD model of plasma and observed wave polarization suggested that these waves propagate in the high beta intermediate mode. However, the underlying instability was not unambiguously determined. In the present paper we use the full electromagnetic dispersion relation derived from linear Vlasov theory in order to examine which of the plasma modes, with observed properties, are unstable in an isotropic Maxwellian plasma in the presence of backstreaming proton beams consistent with Voyager 2 observations at Saturn. As a result we find that the unstable “Alfvenic” beam mode, as well as resonant and non‐resonant fast magnetosonic modes have properties consistent with the data. Moreover, we find that in contrast to the Earth' upstream waves, at Saturn no “kinetic” normal mode can account for the observed magnetic

ISSN:0148-0227    

DOI:10.1029/93JA01014

年代:1994

数据来源: WILEY

摘要:

We examine the large‐scale solar wind density and velocity characteristics which are associated with interplanetary disturbances detected by interplanetary scintillation (IPS) mapping. Our aim here is to characterize these remotely sensed events in order to provide a better foundation on which to consider both the origin of such phenomena and their influence on the near‐Earth environment. We select those events from a list published by Hewish and Bravo (1986) that appear to be Earth crossing and consider the average density and velocity in the solar wind at the time. We find density (and often velocity) enhancements in association with the majority of these IPS events, though the changes are modest. Postevent conditions commonly show a slight velocity increase and density decrease with respect to preevent conditi

ISSN:0148-0227    

DOI:10.1029/92JA00727

年代:1994

数据来源: WILEY

摘要:

Plasma dynamics and momentum transport near an X line during time‐dependent magnetic reconnection in a collisionless plasma are investigated based on two‐dimensional particle simulations. We find that a weakly skewed velocity distribution is formed near the magnetic X line, leading to the presence of off‐diagonal elements of the plasma pressure tensor. Let the reconnection electric field be in theydirection. The gradients of the off‐diagonal elements of the pressure tensor can provide a transport of theymomentum. During the normal magnetic reconnection, the momentum transport associated with the off‐diagonal terms of the pressure tensor mediates a transfer of theymomentum from the region near the X line to regions outside the X line. A period of “reverse magnetic reconnection,” during which the plasma kinetic energy is converted into magnetic energy, is also observed in the simulation. When reverse reconnection occurs, the gradients of the off‐diagonal pressure tensor elements can mediate a transfer ofymomentum into the X line. It is found that the inertial term also plays a significant role in the force balance near the magnetic X line. An explanation for the origin of the off‐diagonal pressure terms is also gi

ISSN:0148-0227    

DOI:10.1029/93JA02519

年代:1994

数据来源: WILEY

摘要:

We have analyzed the temporal evolution of the upstream particle fluxes and anisotropies in a set of particle events associated with interplanetary shocks, detected by ISEE 3 during the period 1978‐1980 in the 147‐238 and 620‐1000 keV energy ranges. In particular, we have studied those features that can perhaps be used as signatures of the history of the influence of the shock on the particle event. We have paid special attention to the evolution of the flux anisotropy that can carry relevant information on the contribution of shock accelerated particles to the total observed flux. Our analysis shows that during most events, long‐lasting (between 5 and 36 hours) large anisotropies are observed upstream of the shock, supporting the hypothesis of continuous injection of shock accelerated particles in the interplanetary medium while the shock is propagating outward from the Sun. The evolution of the anisotropy throughout each event shows a dependence on the longitude of the parent solar source. We have interpreted the observations considering that shock accelerated particles contribute significantly to the total observed flux from the time when the shock intersects the magnetic field line that connects with the observer. The heliocentric distance of this initial intersection point has been derived from the flux anisotropy observations for each event. The values obtained show a distribution with respect to the longitude of the solar source that can be reproduced with a simple model, which assumes a spherical shock with an angular extension ≈ 100° and an archimedian spiral structure for the interplanetary magnetic field. The results of this work provide a further insight into both the importance of shock acceleration, as well as the influence of the large‐scale shock structure in low‐energy p

ISSN:0148-0227    

DOI:10.1029/93JA02826

年代:1994

数据来源: WILEY

摘要:

The transport of typical ions from the surface of the Moon to the vicinity of Earth was calculated using a test particle approach. It was assumed that the ions were sputtered from the surface by the solar wind, with fluxes in the range determined experimentally by Elphic et al. (1991), and were accelerated initially to 10 eV by the potential of the Moon on its sunlit side. Si+and Ca+ions were selected for this transport analysis because their masses are within two prominent ion mass groups that have high sputtering yields. In the solar wind the ion trajectories were traced in the following superimposed fields: (1) a steady magnetic fieldB0at an angle of 45° to the solar wind velocityVsw, (2) the motional electric fieldE0= −Vsw× B0, and (3) turbulent magnetic and electric fields generated by hydromagnetic waves with ak‐space power spectrum of |k|−5/3propagating along both directions of the magnetic fieldB0. Interactions with Earth's bow shock and magnetosphere were included. Case histories of the ions were recorded in theXGSM,YGSMplane and in various planes perpendicular to theE0× B0drift direction of the ions between the Moon and Earth. The number density, energy and angular distributions, and directional and omnidirectional fluxes of the ions were constructed from the case histories. It was found that the diffusion of the ions increases rapidly as the amplitude of the turbulence δBrmsincreases beyond the value 0.04B0. The directional fluxes of Si+and Ca+in the solar wind, upstream from the bow shock, were found to be of the order of tens of ions cm−2s−1sr−1keV−1, with energies 35‐70 keV, for δBrms= 0.04B0,B0= 5 nT, andVsw= 400 km/s. The fluxes of Si+and Ca+ions that originate in the Moon's atmosphere were estimated to be smaller than the corresponding sputtered‐ion fluxes by about an order of magnitude. Recent measurements of lunar ions upstream of the bow shock by Hilchenbach et al. (1992) generally confirm the predicte

ISSN:0148-0227    

DOI:10.1029/93JA02672

年代:1994

数据来源: WILEY

摘要:

MHD fluctuations in the solar wind have been investigated previously by use of Elsässer variables. In this paper, we present a comparison of the spectra of Elsässer variables in the undisturbed solar wind at 1 AU and in the ion foreshock in front of the Earth. Both observations take place under relatively strong solar wind flow speed conditions (≈ 600 km s−1). In the undisturbed solar wind we find that outward propagating Alfvén waves dominate, as reported by other observers. In the ion foreshock the situation is more complex, with neither outward nor inward propagation dominating over the entire range investigated (1‐10 mHz). Measurements of the Poynting vectors associated with the fluctuations are consistent with the Elsässer variable analysis. These results generally support interpretations of the Elsässer variables which have been made based strictly on solar wind data and provide additional insight into the nature of the ion foreshock

ISSN:0148-0227    

DOI:10.1029/93JA01751

年代:1994

数据来源: WILEY

摘要:

Statistical estimates of the average value ofBZin the magnetotail neutral sheet between 10 and 22REinX, and −10 to 10REinY(GSM coordinates) are given for different phases of geomagnetic activity. With few exceptions the average value ofBZis found to be between 5 and 8 nT, but generally near 7 nT, irrespective of the type or phase of activity. This is considerably higher than is predicted in current magnetic field models of the magnetotail. The magnetic field within 2REof the neutral sheet is examined to determine whetherBZremains approximately constant. We do not find this to be true,BZdecreasing significantly over this distance from ΔZ= 0. The structure of the cross‐tail current and variations in current density with substorrn phase appear to be more complicated than is commonly acce

ISSN:0148-0227    

DOI:10.1029/93JA01752

年代:1994

数据来源: WILEY

摘要:

We present a statistical analysis of the ion plasma properties and magnetic field of the central plasma sheet at distances from 10 to 23 REfrom the Earth. This study incorporates ISEE 1 plasma sheet samples accumulated with the University of Iowa Lepedea plasma analyzer during 1979. Our study of the bulk properties of the ion plasma shows that there is relatively little change as a result of increasing geomagnetic activity. The main macroscopic effect is an increase in plasma temperature. When the transverse profile of the plasma sheet is examined, it is clear that this increase in temperature occurs preferentially at high latitudes. The magnetic field shows an increase in magnitude in the vicinity of the neutral sheet during periods of high geomagnetic activity. The radial profiles of the density, temperature, plasma, and magnetic pressures show a gradient in total energy density in the plasma sheet. The convective velocities are systematically directed earthward, and there is a bias toward positiveVyon the duskside of the magnetotail. We infer that particle motion can be attributed to a pressure gradient, combined with steady state convective electric fields. We compute the convective electric fields from our measured bulk velocities, arriving at a pattern similar to that observed at low altitudes. The cross‐tail electric fieldEyis dominant near the neutral sheet region, but at high latitudesEzis a major component. Our results suggest that the plasma sheet boundary layer, rather than the central plasma sheet, is a region of diverging electric fields. The picture of the plasma sheet that emerges from our study is that of a stable reservoir of hot plasma in which thermodynamic, rather than dynamic, processes is importan

ISSN:0148-0227    

DOI:10.1029/93JA01894

年代:1994

数据来源: WILEY