摘要:

Waves in the foreshock of Saturn have been studied using data from the plasma and magnetic field experiments onboard Voyager 1. The appearance of the waves, in the plasma as well as in the magnetic field data, is related to magnetic connection of the spacecraft to the bow shock. Their relative amplitude ΔB/B lies between 0.3 and 0.8 and their period in the spacecraft frame is ∼500 s. The waves are elliptically polarized and propagate at ∼30° to the average magnetic field dire

ISSN:0094-8276    

DOI:10.1029/91GL00858

年代:1991

数据来源: WILEY

摘要:

A two‐dimensional, time‐dependent, shock capturing single‐species (O+) hydrodynamic model of the Venus ionosphere, which solves the coupled continuity, momentum and energy equations for the altitude range of 150–500 km and the solar zenith angle range of 0°–180° has been developed and is presented in this paper. It was again demonstrated that the introduction of topside heat inflows leads to calculated dayside electron and ion temperatures, which are consistent with the measured values. In order to reproduce the measured electron temperatures, which are roughly constant over all SZA's, the heat inflows had to be reduced significantly over the nightside compared to the dayside values. The calculated transterminator ion flows are supersonic and relatively close to the observed average values. The model predicts a deceleration shock at a SZA of about 135°, consistent with the ion temperature and velocity

ISSN:0094-8276    

DOI:10.1029/91GL00362

年代:1991

数据来源: WILEY

摘要:

Combined latitude profiles of optical aurora, magnetic disturbances, particle precipitation and field‐aligned currents are used to investigate the electrodynamics of the cusp/cleft ionosphere during northward interplanetary magnetic field (Bz = 12 nT; By= −17 nT). An active zone of strong azimuthal plasma convection, manifested by the Svalgaard‐Mansurov effect in ground magnetic deflections (due to Hall current between pairs of Birkeland currents) and auroral structure at the poleward boundary of the cusp/cleft is observed within∼1030–1230 MLT. Contrary to the typical cusp/cleft auroral activity during negative IMF Bz the discrete auroral forms in the present case move into the cusp/cleft arc, from the poleward edge. Both the optical observations and the inferred field‐aligned current/plasma flow pattern indicate a generator mechanism poleward of the cusp/cleft (i.e. associated with the plasma mantle), giving rise to strong momentum transfer and discrete electron precipitation within a∼200 km wide latitudinal zone at the cusp/cleft poleward boundary, at the time of a poleward moving cusp/cleft arc. From this and other similar studies of the cusp/cleft ionosphere, we conclude that a qualitatively similar mode of solar wind‐magnetosphere interaction is operating both during northward an

ISSN:0094-8276    

DOI:10.1029/91GL00849

年代:1991

数据来源: WILEY

摘要:

Nonlinear Alfvén waves, in inhomogeneous plasmas, are shown to be governed by the MDNLS (Modified Derivative Nonlinear Schrödinger) equation. The inhomogeneity leads to acceleration (deceleration) of the Alfvén solitons depending upon plasma β (kinetic pressure/magnetic pressure) and on density gradients vis‐a‐vis direction of wave propagation. Potential applications, of the model, to space and astrophysical plasmas are poin

ISSN:0094-8276    

DOI:10.1029/91GL00854

年代:1991

数据来源: WILEY

摘要:

Diurnal variations on a plasmaspheric flux tube were modelled using a time‐dependent, multi‐species, one‐stream, interhemispheric model for plasmaspheric flows. In the model the coupled time‐dependent hydrodynamic equations (continuity, momentum, and energy) of a two ion (H+and O+) quasineutral, currentless plasma are solved for a closed geomagnetic field line; no diffusive equilibrium assumptions are made. For this work an L=2 field line with conjugate points displaced in latitude and longitude was used in order to model the tilt of the geomagnetic field. The simulation was done for June solstice conditions during solar minimum. In general the results reproduce those found by Richards and Torr [1986]. However, a striking new result is the presence of large downward H+velocities (∼2.2 km/s) at about 320 km altitude in the winter (southern) hemisphere, in early morning when the summer hemisphere is already sunlit but the winter hemisphere is still in darkness. These are accompanied by large H+temperature enhancements, reaching 5600 K, at about 280 km altitude. The large velocities are a resilt of an increase in the H+pressure gradient coupled with a decrease in the friction encountered by the H+ions. These flows are stopped at lower altitudes where the neutral densities are higher; the bulk kinetic energy is converted to thermal energy, resulting in the temperature enh

ISSN:0094-8276    

DOI:10.1029/91GL00139

年代:1991

数据来源: WILEY

摘要:

Plasmaspheric refilling after geomagnetic storms is studied using a model in which the ion temperatureanisotropy (TAN) is allowed to evolve self‐consistently with the interhemispheric plasma flow. The refilling processes predicted from this TAN model and those from a model based on the assumption oftemperatureisotropy (TISO) are compared. TISO model predicts that a flux tube with L=4 undergoes a substantial refilling in about 12 hours after a severe depletion and during this time the flows from the conjugate ionospheres remain supersonic with a flux ∼2 × 108cm−2s−1. The above refilling time is too short compared to observed refilling times. The TAN model predicts much longer refilling time divided in two stages; during the early stage, the flows from the conjugate ionospheres are supersonic, and it is characterized by interhemispheric plasma exchange, shock formations and their propagations like in the TISO model. But in the TAN model, the supersonic flows last only over a few hours (T⟂. This anisotropy causes a downward force, which balances the upward electric and pressure forces on ions keeping the flux tube depleted. The ensuing second stage starts when the temperature anisotropy begins to relax due to Coulomb collisions at high latitudes. In this stage the flow is generally subsonic and in the late stage of the refilling the equatorial density is seen to increase at a rate<1

ISSN:0094-8276    

DOI:10.1029/91GL00026

年代:1991

数据来源: WILEY

摘要:

To study the nonlinear plasma dynamics associated with electrodynamic tether system, we performed computer experiments using a 2D electromagnetic particle code. In the computer experiments, two conducting bodies, corresponding to the satellite and the orbiter, are placed in thex‐yplane in a vehicle frame. The electrons collected by the tether system are emitted from the orbiter. We focus on the plasma response to the tether system, the electron beam dynamics, and the wave emission. At the satellite region, plasma depletion owing to the electron collection alongB0by the high potential satellite is found as well as a wake structure in the downstream region of the plasma flow. At the orbiter region, an electron beam cloud is clearly created in the downstream region. The local electric field induced by the cloud modifies the beam pitch angle and accelerates some of the beam electrons alongB0. Moreover, due to the cross‐field current by the electronE×Bdrift motion around the cloud, LHR mode waves are enhanced locally. The local LHR wave excitation is quantitatively interpreted by the theory of the lower‐hybrid‐drift ins

ISSN:0094-8276    

DOI:10.1029/91GL00126

年代:1991

数据来源: WILEY

摘要:

The first observations of mesospheric winds made between January–August 1990 with an MF partial reflection radar located on Christmas Island (2°N, 157°W) in the central Pacific are described. The mean zonal winds are in general westward, but show clear evidence for a wave‐driven circulation. Power spectral studies indicate that waves are present over a wide range of periods. Ultra‐fast Kelvin waves are especially evident in January–March, with peak amplitudes∼20 ms−1, and intrinsic phase speeds of∼150 ms−1indicated. The Kelvin waves are estimated to contribute an eastward acceleration of up to 10 ms−1day−1. Gravity wave amplitudes are also found to be almost as large as those observed at mid‐latitude sites, which suggests that convection is a major source o

ISSN:0094-8276    

DOI:10.1029/91GL00768

年代:1991

数据来源: WILEY

摘要:

The high energy solar proton event of 19 October 1989 had an atypical particle anisotropy during the initial phase with the Deep River, Ottawa and Goose Bay neutron monitors in the Eastern Canadian region recording the intensity increase about 25 minutes before comparable detectors in Europe and Western Canada. Using the asymptotic cones of acceptance for these neutron monitors, we show that this early onset in the particle intensity increase may be the detection of relativistic protons that are the decay product of solar flare generated relativistic neutrons.

ISSN:0094-8276    

DOI:10.1029/90GL02668

年代:1991

数据来源: WILEY

摘要:

The effects of sulfate aerosols on stratospheric ozone have been studied with a 2D model. The model includes a comprehensive chemical code for homogeneous and heterogeneous reactions; both background and volcanic aerosols have been taken into account according to different scenarios. The basic temperature field is prescribed, while the perturbation introduced by the presence of volcanic aerosols is predicted. Increase of total chlorine in the stratosphere causes a well known ozone depletion by itself, but. the effects could be highly enhanced in presence of a large amount of volcanic aerosols that affect the balance of nitrogen and chlorine reservoirs through heterogeneous chemical reactions. For a volcanic eruption similar to El Chichon we show that the maximumO3depletion (7% for a 2.5ppbv amount of stratospheric chlorine and 12% by doubling Cl) is to be expected at high latitudes in spring, because of a large chemical destruction acting in situ.

ISSN:0094-8276    

DOI:10.1029/90GL02785

年代:1991

数据来源: WILEY