ISSN:0148-0227    

DOI:10.1029/JA095iA02p00915

年代:1990

数据来源: WILEY

摘要:

We describe a set of solar coronal mass ejection (CME) events where coincident data sets from both X ray and white light instruments have been made available through deliberate planning. Using these we have been able to put tight limits on possible descriptions of the typical sequence of events, and these we relate to interpretations of models involving flares and CMEs. Our findings confirm recent suggestions that CME onsets precede any related flare activity and that the associated flaring commonly lies to one side of the CME span. The CME launch appears to be associated with minor X ray (flare precursor) activity. Although this scenario has been previously discussed (seeHarrison, 1986, and references therein), the abundance of flare and CME models which are not compatible with this picture demands that confirmation be sought using programs such as this.

ISSN:0148-0227    

DOI:10.1029/JA095iA02p00917

年代:1990

数据来源: WILEY

摘要:

Because Mars has a weak intrinsic magnetic field and a substantial atmosphere, instruments on orbiting spacecraft should detect a population of energetic heavy planetary ions which result from comet‐like ion pickup in the solar wind and magnetosheath convection electric fields, in addition to those that might result from processes internal to a Martian “magnetosphere.” Although this ion exosphere has been previously discussed in the literature, detailed predictions that might be directly applied to the interpretation of data are not available. Here a test particle model is used to construct a global picture of Martian pickup ions in the Mars environment. The model makes use of the recent Nagy and Cravens [1988] model of the Martian exosphere and Spreiter and Stahara's [1980]gas dynamic model of the magnetosheath. The pickup of ions originating at Phobos is also considered. Notable properties of the resulting ion distributions include their near‐monoenergetic spectra, pancake pitch angle distributions, and large gyroradii compared to the planetar

ISSN:0148-0227    

DOI:10.1029/JA095iA02p00939

年代:1990

数据来源: WILEY

摘要:

In this paper we investigate the extent to which the properties of the waves observed at comet Halley during the Giotto encounter can be understood within the context of linearized Vlasov theory. In the region that is magnetically connected to the comet, fluctuations in the plasma frame of reference are detected near 4 and 10 mHz, close to the water cyclotron frequency, as well as at 20–60 mHz, which is well above the water cyclotron frequency. Using a variety of approximations for the ion distribution function, we show that waves having properties similar to those observed can then be generated with appropriate choices of plasma parameters. In the region that is magnetically disconnected from the comet, distinct peaks are observed in the magnetic power spectrum at 7, 21, 29, and 35 mHz, with a hint of a peak at 14 mHz. We show that an anisotropic distribution of pickup water group ions is able to generate unstable waves at as many as 6 harmonics in wave number in the observed range of frequencies. When Doppler shifted into the frame of the Giotto spacecraft, these harmonics in wave number will be observed as harmonics of the water ion cyclotron frequenc

ISSN:0148-0227    

DOI:10.1029/JA095iA02p00947

年代:1990

数据来源: WILEY

摘要:

The plasma and field properties of large‐amplitude magnetic field pulsations upstream from the quasi‐parallel region of the Earth's bow shock are examined in high time resolution using data from ISEE 1 and 2. The relative timing of the magnetic field profiles observed at the two spacecraft shows that some of the pulsations are convecting antisunward across the spacecraft while others are brief out/in motions of the bow shock across the spacecraft. Pulsations with both timing signatures are the site of slowing and heating of the solar wind plasma. The ions tend to be only weakly heated in the convecting pulsations, while within the out/in pulsations the ion heating can be quite substantial but variable. This variation occurs not only from pulsation to pulsation but also from point to point within a given pulsation. In general, the hottest distributions within the out/in pulsations tend to occur in regions of lower density and field strength. Magnetic pulsations bear a number of similarities to previously identified hot diamagnetic cavity events as well as to more durable crossings of the quasi‐parallel shock itself. These various phenomena may be different manifestations of the same basic physical processes, in particular the coupling of coherently reflected ions to the solar wind

ISSN:0148-0227    

DOI:10.1029/JA095iA02p00957

年代:1990

数据来源: WILEY

摘要:

A long‐lasting Pc 5 pulsation at the dawn flank of the magnetosphere is studied using particle and field instrumentation from the ISEE 1 and 2 satellites. Electric field and particle modulation signatures were clearer than magnetic field variations, consistent with the satellites' position in latitude near the equatorial node of a fundamental resonance. Pulsation flow velocities along the ISEE 1 trajectory were calculated from particle characteristics using data from several instruments and from electric and magnetic field data. These flow velocities were all consistent with each other, but the velocities derived from plasma and energetic particle observations were a factor of 2.5 larger than velocities derived from the fields data. We have not been able to find the source of this discrepancy; one possibility is that the field near the spacecraft differs from the large‐scale field. In contrast to observations of pulsations during magnetic storms, which often involve resonant or gyrating particle behavior, particles at all energies sampled (10 eV to 200 keV) appeared to respond passively to the pulsation throughout most of the period of interest. Comparison of data from the two spacecraft, which traveled from ∼15REto ∼7REwith a time separation of ∼1 hour, suggests the propagation of relatively broadband pulsation energy from the magnetopause/low latitude boundary layer and subsequent resonance of independentLshells in the fundamental toroidal mode after the cessation of po

ISSN:0148-0227    

DOI:10.1029/JA095iA02p00967

年代:1990

数据来源: WILEY

摘要:

The properties of 23 magnetic pulsation events observed by the AMPTE CCE spacecraft are studied. These events are selected on the basis of the field magnitude which oscillated at the second harmonic of a simultaneously present transverse oscillation. The events have a second harmonic period of 80–600 s (roughly the Pc 5 range), are observed in cluster in the dawn (0300–0800 magnetic local time, MLT) and dusk (1600–2100 MLT) sectors, and are localized near the magnetic equator. Although the azimuthal wave number,m, estimated from an ion finite Larmor radius effect, is generally large (|m| ∼ 50), there is a marked difference between the events observed in the dawn and dusk sectors. In the dawn sector the waves have low frequencies (1–5 mHz), indicate left‐hand polarization with respect to the ambient magnetic field, and propagate eastward with respect to the spacecraft. In the dusk sector the waves have high frequencies (5–15 mHz), indicate right‐hand polarization, and propagate westward. We suggest that the waves are all westward propagating in the plasma rest frame and that local‐time‐dependent Doppler shift is the reason for the local time dependence of the wave properties. The drift mirror instability is considered to be the mechanism for exciting the westward propagating waves. An analytical formula for the ion flux oscillations is derived on the basis of the nonlinear gyrokinetic theory. The observed correlation between the ion flux and the parallel magnetic field perturbation δB∥can be adequately explained with t

ISSN:0148-0227    

DOI:10.1029/JA095iA02p00977

年代:1990

数据来源: WILEY

摘要:

We have analyzed the vector magnetic field data acquired with the AMPTE CCE satellite for prolonged periods of disturbed conditions (2 ≤Kp≤ 6, and −70 ≤Dst≤ −20 nT). A total of 300 orbits for 180 days during 1985 and 1986 were subjected to the present analysis. We have determined the characteristics of the intensities and flow directions of currents flowing parallel to and near the geomagnetic equatorial plane in the region betweenL= 4.0 and 8.8RE. This analysis was done in a cylindrical magnetic coordinate system and by assuming north–south symmetry with respect to the geomagnetic equatorial plane. The characteristics determined from this study include the following: (1) The azimuthal currents flow westward and exhibit pronounced midnight‐noon asymmetries, with the nightside intensities greater than dayside intensities by a factor of 2 to 3. The enhanced nightside current intensities have localized relative minima between 2200 and 0200 MLT andL= 5.6 and 8.8RE, resulting from the integrated effect of a number of substorm‐associated geomagnetic dipolarizations. (2) The inferred equatorial radial currents flow toward the Earth in the evening sector and away from the Earth in the morning sector throughout the region betweenL= 4.0 and 8.8RE. (3) A net positive divergence (source) of equatorial current in the evening sector and a net negative divergence (sink) in the morning sector is inferred from the distribution of azimuthal and radial currents. The location, flow direction, and intensities of these divergent currents are almost identical with those of the region 2 Birkeland system. We conclude that the equatorial currents, primarily the westward ring current, and the region 2 Birkeland current system are

ISSN:0148-0227    

DOI:10.1029/JA095iA02p00991

年代:1990

数据来源: WILEY

摘要:

Heating of heavy ions has been observed in the equatorial magnetosphere in GEOS 1 and 2 and ATS 6 data due to ion cyclotron waves generated by anisotropic hot ring current ions. A one‐dimensional hybrid‐Darwin code has been developed to study ion heating in the ring current. The mechanism for the heating of thermal hydrogen and helium by ion cyclotron waves has been studied previously in particle simulations of a bi‐Maxwellian plasma. Here, a strong instability and heating of thermal ions is investigated in a plasma with a loss cone distribution of hot ions. The linear growth rate calculation and particle simulations are conducted for cases with different loss cones and relative ion densities. The linear instability of the waves, the quasi‐linear heating of cold ions and dependence on the thermal H+/He+density ratio are analyzed, as well as nonlinear parallel heating of thermal ions. Effects of thermal oxygen and hot oxygen are also

ISSN:0148-0227    

DOI:10.1029/JA095iA02p01001

年代:1990

数据来源: WILEY

摘要:

Recent satellite observations at high altitudes (>5000 km) in the auroral zone have shown the existence of hybrid or bimodal ion beam distributions that are evidence of both parallel and perpendicular ion acceleration. Acceleration parallel to the magnetic field is most likely due to quasi‐static electric fields (double layers) which can create outflowing ion beams; since ions of different mass will have different drift speeds due to this acceleration, a plasma configuration unstable to the ion‐ion two‐stream acoustic mode develops. When the net drift velocity (U) between the two ion species is greater than the sound speed (Cs), the ion‐ion instability has maximum growth at oblique wave propagation. To study the nonlinear effects of the ion‐ion instability in terms of plasma heating, a numerical simulation parametric study has been performed. It was found that the parallel acceleration that forms the ion beams occurs on a time scale faster than ion‐ion wave growth at low drifts; thus ion‐ion wave growth is expected to occur primarily for higher drift speeds (U>Cs), which results in strong oblique heating of the ions (both hydrogen and oxygen) forming elevated ion conics (sometimes called “bowl” distributions). Also, strong parallel electron heating in the direction of the ion beams can occur, and electrons near the top of the acceleration region may attain a net upward drift along with the elevated ion conics. Variation of the oxygen density greatly affects the ion heating due to the ion‐ion instability; as the oxygen density decreases, oxygen heating increases, in agreement with observations (Collin et al., 1987). Ion‐ion electrostatic wave properties and the plasma heating that results over a wide range of auroral zone pa

ISSN:0148-0227    

DOI:10.1029/JA095iA02p01015

年代:1990

数据来源: WILEY