摘要:

We have used a time‐dependent magnetohydrodynamic (MHD) simulation to investigate the magnetic topology of an idealized streamerlike configuration in three dimensions. Starting from an initially current‐free multipole field and a transonic flow, a quasi‐steady, streamerlike configuration forms. The simulated streamer exhibits a closed magnetic field region bounded by a current layer and surrounded by open fields. This helmet region is surmounted by a current layer, with an enhanced density in the closed field and current sheet regions. The simulated streamer has a finite longitudinal extent, with the closed field region primarily confined to the center of the stru

ISSN:0094-8276    

DOI:10.1029/GL017i013p02281

年代:1990

数据来源: WILEY

摘要:

Voyager's post‐encounter trajectory at Neptune allows the directions of the magnetic field in the magnetosheath produced by the draping of the typical interplanetary field and by alignment with the magnetotail to be distinguished. Changes of the field from the draped to the magnetotail‐aligned direction accompanied by decreases of plasma velocity, density, and temperature, observed at intervals of approximately a Neptunian day, are consistent with the assumption that Voyager repeatedly encountered a plasma mantle region extending well into the magnetosheath and modulated by the rotation of the planet's magnetic dipole. Other interpretations are either implausible or inconsistent with the observations. Previously reported periodic velocity decreases in the magnetosheath of Uranus can be interpreted in the same way. Extended plasma mantles may thus be a general property of planetary magnetospheres interacting with the magnetized solar wind. Analogous effects at Earth cannot yet be observed because of lack of suitable spacecraft missions, but their existence is suggested by theoretical argume

ISSN:0094-8276    

DOI:10.1029/GL017i013p02285

年代:1990

数据来源: WILEY

摘要:

The production of fast oxygen atoms in the Martian exosphere as a result of the electron dissociative recombination ofions is re‐evaluated for the time period appropriate to the Phobos 2 mission. Because of the prevalent condition of solar maximum activities in 1989, both the upper atmosphere and ionsphere should have more extended structures than those encountered during the Viking Orbiter (VO) mission in 1976 which was near solar minimum. We compare the distributions of the hot oxygen atoms for these different levels of solar activities and find that the number density of the hot oxygen atoms during the Phobos 2 mission should be a factor of two larger than estimated for the time period of the VO mission. The non‐thermal escape rate of the exospheric oxygen atoms is thus comparable to the ionospheric escape rate measured in the Martian magnetotail. Furthermore, the flux of the oxygen ions picked up in the hot atomic corona should be more intense than previously thou

ISSN:0094-8276    

DOI:10.1029/GL017i013p02289

年代:1990

数据来源: WILEY

摘要:

Previous studies have shown that the Venus foreshock region contains low‐frequency upstream waves similar to those in the terrestrial foreshock, but perhaps with different amplitudes than at Earth. Herein, we compare the properties of a second class of upstream waves, analogous to the so‐called 1 Hz waves at Earth. The waves observed at Mercury, Venus, and Earth have very similar properties, i.e., propagation angles less than 55 degrees to the magnetic field and less than 35 degrees to the solar wind flow direction. The waves occur exclusively on the field lines connected to the bow shock. They are most commonly left‐hand elliptically polarized with similar fractional amplitudes, approximately 0.1 of the background field strength. Their amplitudes decrease with increasing distance from the shock. The observed frequencies are similar for Mercury, Venus and Earth when scaled by the interplanetary magnetic field. If, as generally assumed at Earth, these waves arise in regions of backstreaming electrons, these results imply that similar electron foreshocks occur at Earth, Venus and Mercury despite differences in bow shock size and the nature of the obstacle to the solar

ISSN:0094-8276    

DOI:10.1029/GL017i013p02293

年代:1990

数据来源: WILEY

摘要:

The linear and nonlinear properties of the obliquely propagating electromagnetic ion/ion cyclotron instability are investigated. The instability is driven by the relative, field‐aligned streaming of two ion beams and can exist at a lower velocity threshold than the more commonly studied, parallel propagating electromagnetic ion beam instabilities. It is shown that the instability plays an important role in the formation of, dissipation at, and waves upstream of slow mode shocks. Possible application of this instability to ion beams in the plasma sheet boundary layer is also briefly discusse

ISSN:0094-8276    

DOI:10.1029/GL017i013p02297

年代:1990

数据来源: WILEY

摘要:

Shortly after the AMPTE/IRM satellite had released Lithium ions into the solar wind, magnetometers on the ground and in the frontside magnetosphere observed deflections of a type similar to those associated with solar wind pressure pulses. Since the natural solar wind was very stable during this event and did not exhibit pressure changes, the magnetic deflections have likely been caused by the artificially injected Lithium cloud when it hit the magnetosphere. Order of magnitude estimates show that 300 g of Lithium vapor released in the solar wind are indeed able to produce a 300 nT magnetic deflection on the ground under favourable conditions.

ISSN:0094-8276    

DOI:10.1029/GL017i013p02301

年代:1990

数据来源: WILEY

摘要:

Impulsive reconnection near the subsolar magnetopause is a fundamental process of interaction between the magnetosphere and the solar wind flow. Flux Transfer Events (FTEs) appear to be a manifestation of this process. It is therefore important to fully characterize the FTE so as to better understand the physics of the interaction process. We use magnetic field signatures from the AMPTE/CCE spacecraft to identify two magnetospheric FTEs. Electron distributions measured in the plane of the magnetopause inside these FTEs complement previously reported electron measurements. We report, for the first time, ion composition measurements in the energy range ∼0 to 20 keV/e within an FTE. We find that the ion distributions, the ion composition, and the flow velocities are unique to the FTE and unlike either the adjacent magnetosphere, the nearby boundary layer, or the nearby magnetosheath. The H+, He+, and He2+distribution functions in the FTEs have reversed temperature anisotropies and the relative He2+abundance is depressed with respect to either the magneosheath or the low latitude boundary laye

ISSN:0094-8276    

DOI:10.1029/GL017i013p02305

年代:1990

数据来源: WILEY

摘要:

Tail‐like reconfiguration observed in the near‐earth plasma sheet during the substorm growth phase is surrounded by enhanced convection except inside and tailward of the reconfiguration region itself. The required distribution of enhanced convection can be driven by an enhanced dayside reconnection. The enhanced dayside convection can be transmitted across open field lines by the tangential discontinuity to reach the last open field line in the tail lobe in about 20 minutes. This can cause Bxto increase in the plasma sheet. On the other hand, the enhanced dayside convection can also be transmitted across closed field lines by the fast rarefaction wave to arrive at the near‐earth plasma sheet (x ≤ −10 RE) in about 15 minutes, causing Bzto decrease in the plasma sheet. Thus, a tail‐like reconfiguration can start within about 15 to 20 minutes after a southward turning of the IMF (interplanetary magnetic field). The duration of the growth phase can be identified with the M‐I coupling time scale of ∼30 to ∼40 minutes. The region‐2 field‐aligned current is shown to increase to 0.2 μA/m2at the ionospheric level, resulting from the tai

ISSN:0094-8276    

DOI:10.1029/GL017i013p02309

年代:1990

数据来源: WILEY

摘要:

Observations are presented which show that plasma parameters vary on a scale length smaller than the ion gyroradius at the interface between the plasma sheet boundary layer and the lobe. The Vlasov equation is used to investigate the properties of such a boundary layer. The existence, at the interface, of a density gradient whose scale length is smaller than the ion gyroradius implies that an electrostatic potential (e ф ∼ 2 κTe, whereeandTeare the charge and electron temperature, respectively) is established in order to maintain quasineutrality. Strongly sheared (scale lengths smaller than the ion gyroradius) perpendicular and parallel (to the ambient magnetic field) electron flows develop whose peak velocities are on the order of the electron thermal speed and which carry a net current. The free energy of the sheared flows can give rise to a broadband spectrum of electrostatic instabilities starting near the electron plasma frequency and extending below the lower hybrid freque

ISSN:0094-8276    

DOI:10.1029/GL017i013p02313

年代:1990

数据来源: WILEY

摘要:

In this paper the steady‐state proton distribution in the Earth's magnetotail is derived both in configuration and velocity space, taking into account the proton's energization in the dawn‐dusk electric field and its motion due to deterministic chaos. This stochastic motion effectively leads to pitch angle scattering and a redistribution of protons between the central plasma sheet and its boundary, the plasma sheet boundary layer. Knowledge of these effects allows us to explain the previously observed features of the ion distribution and to predict others for future experimental w

ISSN:0094-8276    

DOI:10.1029/GL017i013p02317

年代:1990

数据来源: WILEY