ISSN:0148-0227    

DOI:10.1029/94JA02360

年代:1994

数据来源: WILEY

摘要:

A model for the elliptical polarization of Jovian decametric radiation is presented, based on the electron cyclotron mechanism. The aim is to determine whether the observed elliptical polarization is consistent with the radiation being generated by mildly relativistic electrons streaming along converging magnetic field lines. The growth rate for electron cyclotron maser emission is considered, assuming a drifting DGH distribution function for the streaming electrons. Constraints on the allowable parameters of this distribution function are made by the observed polarization, timescale, bandwidth, and angular range of the radiation.

ISSN:0148-0227    

DOI:10.1029/94JA01843

年代:1994

数据来源: WILEY

摘要:

Observations of auroral hiss obtained from the Voyager 1 encounter with Jupiter have been reanalyzed. The Jovian auroral hiss was observed near the inner boundary of the warm Io torus and has a low‐frequency cutoff caused by propagation near the resonance cone. A simple ray tracing procedure using an offset tilted dipole of the Jovian magnetic field is used to determine possible source locations. The results obtained are consistent with two sources located symmetrically with respect to the centrifugal equator along anLshell (L≃ 5.59) that is coincident with the boundary between the hot and cold regions of the Io torus and is located just inward of the ribbon feature observed from Earth. The distance of the sources from the centrifugal equator is approximately 0.58 ± 0.01RJ. Based on the similarity to terrestrial auroral hiss, the Jovian auroral hiss is believed to be generated by beams of low energy (∼tens to thousands of eV) electrons. The low‐frequency cutoff of the auroral hiss suggests that the electrons are accelerated near the inferred source region, possibly by parallel electric fields similar to those existing in the terrestrial auroral regions. A field‐aligned current is inferred to exist atLshells just inward of the plasma ribbon. A possible mechanism for driving this current is

ISSN:0148-0227    

DOI:10.1029/94JA01904

年代:1994

数据来源: WILEY

摘要:

An induced magnetosphere of a comet rotates around the Sun‐comet line along with the interplanetary magnetic field (IMF) vector. During the Giotto flyby near comet Halley the direction of the IMF changed several times. For this reason, the trajectory of the Giotto spacecraft, being represented in the IMF‐related coordinate system, covered rather well the transverse cross section of the comet Halley magnetosphere. As a result, two‐dimensional distributions of both the magnetic field and electric current density have been calculated in the transverse cross section of the induced magnetosphere of comet Halley. The distributions reveal the following facts. The magnetic barrier is axially symmetric (within the accuracy of the grid ∼ 10³ km). Such a behavior is associated with the fact that the magnetic field strength in the barrier is governed by the dynamic pressure of the solar wind and does not depend on the ion‐neutral friction (which affects only the location of the magnetic field maximum). Along with draping about the dayside of a comet, magnetic field lines also drape about its flanks (where the magnetic field turns out to be enhanced). As a result, the Lorentz electric field inside the cometary ionosphere decreases. Lines of the electric current drape about the cometary ionosphere in a manner resembling the magnetic field lines. The region of the magnetic barrier in front of the contact surface is shown to be an electric load for the MHD generator arising as a result of the solar wind interaction wi

ISSN:0148-0227    

DOI:10.1029/94JA01628

年代:1994

数据来源: WILEY

摘要:

Simultaneous measurements of electrons and magnetic field are used to study the topology of the Martian magnetosphere. Halo solar wind electrons streaming along magnetic field lines replicate variations of theBxcomponent, thus tracing the interplanetary magnetic field draping in most parts of the Martian tail. This suggests that the magnetic field observed near the equatorial plane of Mars is mainly induced. However, there are regions where this perfect tracing is violated. This could be considered as evidence for an intrinsic planetary field. An alternative interpretation involves the relationship between these regions and ionospheric holes.

ISSN:0148-0227    

DOI:10.1029/94JA01271

年代:1994

数据来源: WILEY

摘要:

We present a numerical study of the formation of current sheets in ideal plasmas. First we confirm the development of singular current sheets in a one‐dimensional model. In a second step we extend the analysis to two‐dimensional equilibria. Here it is found that the resulting structures are quite insensitive to the boundary conditions. For the special case of a magnetotaillike equilibrium it will be shown that the resulting current distribution provides a possibility to understand the onset of a localized anomalous resistivity from a macroscopic point of view. Furthermore, the resulting structures provide an explanation for the dramatic decrease of the thickness of the current sheet in the magnetotail prior to the onset of geomagnetic substo

ISSN:0148-0227    

DOI:10.1029/93JA02074

年代:1994

数据来源: WILEY

摘要:

The entire three‐spacecraft magnetospheric plasma analyzer (MPA) data set has been examined for intervals of magnetosheath plasma at geosynchronous orbit. Over the 6 1/2 spacecraft years of data reviewed, 916 5‐min intervals of magnetosheath plasma observations were identified and cataloged. Unlike previous studies that suggested that magnetopause encounters were far more likely on the prenoon than postnoon side at geosynchronous orbit, this study examines three independent lines of evidence and finds that only a small asymmetry (in this same sense) exists. On a statistical basis, a local time of ∼1130 is both the median and mean for our magnetosheath observations; this local time is simply consistent with the effects of an aberrated solar wind direction due to the motion of the Earth around the Sun. Simultaneous multipoint observations across both sides of local noon are also consistent with a small offset due to aberration of the magnetopause. Finally, we examined the locations of the dawn‐dusk flow reversals for a number of cases where one of the geosynchronous spacecraft was outside the magnetopause in the magnetosheath flow. These flow reversals tended to occur near local noon, again indicating that in contrast to previous findings, no large asymmetry

ISSN:0148-0227    

DOI:10.1029/94JA01094

年代:1994

数据来源: WILEY

摘要:

Using a common methodology to analyze data from the AMPTE/IRM and ISEE 2 satellites we report on the statistical properties of bursty bulk flow events (BBFs) in the inner plasma sheet (IPS). A positive correlation between BBFs and the AE index suggests that BBFs are predominantly geomagnetically active time phenomena. Earthward BBFs are more frequent close to midnight and away from Earth, up to a distance of ∼19 RE. Tailward BBFs are very infrequent in the IRM data set and somewhat less infrequent in the ISEE 2 data set in the region of the satellites' spatial overlap, possibly due to the more active conditions prevailing during the ISEE 2 mission in that region. However, in both data sets the ratio of tailward to earthward BBFs increases with distance from Earth; more than 20% of all BBFs are anti‐sunward tailward of X = −19 REin the ISEE 2 data set. BBFs are responsible for 60‐100% of the measured earthward transport of mass, energy and magnetic flux past the satellite in the regions of maximum occurrence rate, even though they last approximately 10‐15% of the IPS observation time there. Thus BBFs represent the primary transport mechanism at those regions. The one‐to‐one correspondence between BBFs and substorm phase, as well as the relative contribution of BBFs to the total transport observed during substorms are questions that await further investigation based on multi instrument studies of indi

ISSN:0148-0227    

DOI:10.1029/94JA01263

年代:1994

数据来源: WILEY

摘要:

The time‐dependent second‐order force generated by standing Alfvén waves in a dipolar magnetospheric geometry is used to derive the time‐dependent specific parallel energy (parallel energy per proton mass) of magnetospheric plasma particles in the wave fields. Time‐dependent energy signatures are evaluated at the dipole equatorial plane (EP), in order to provide a means of identifying ponderomotive energization of plasma in spacecraft data sets. Starting magnetic latitudes for particles arriving at the EP are also derived. In general, specific energies increase rapidly at the EP soon after the driving wave is switched on, reaching a plateau as particles arrive from increasingly higher latitudes. Oscillations in specific energy occur at twice the driving wave frequency. At geosynchronous orbit these oscillations are unlikely to be detectable, but at highLvalues (whereLis the McIlwain parameter) the oscillation amplitudes can be significant. For the fundamental mode with reasonable amplitude, ions from the ionosphere can reach the EP in a few wave cycles at highLvalues. Higher harmonic standing waves include negative acceleration regions (or potential barriers) on a magnetic field line which create “stop bands” of magnetic latitudes from which particles cannot reach the EP. (In some cases ionospheric ions are “shielded” from reaching the EP.) The stop bands in turn create characteristic gaps in the time series of specific energy at the EP. At highLvalues, O+ions from the ionosphere can just overcome the third harmonic potential barrier and can be accelerated to the EP with energies ∼2 keV for reasonable wave amplitudes of the

ISSN:0148-0227    

DOI:10.1029/94JA01828

年代:1994

数据来源: WILEY

摘要:

We present theory and numerical simulations of strong nonlinear effects in standing shear Alfvén waves (SAWs) in the Earth's magnetosphere, which is modeled as a finite size box with straight magnetic lines and (partially) reflecting boundaries. In a low β plasma it is shown that the ponderomotive force can lead to large‐amplitude SAW spatial harmonic generation due to nonlinear coupling between the SAW and a slow magnetosonic wave. The nonlinear coupling leads to secularly growing frequency shifts, and in the case of driven systems, nonlinear dephasing can lead to saturation of the driven wave fields. The results are discussed in the context of their possible relevance to the theory of standing ionospheric cavity wave modes and field line resonances in the high‐latitude magnetos

ISSN:0148-0227    

DOI:10.1029/94JA01629

年代:1994

数据来源: WILEY