ISSN:0148-0227    

DOI:10.1029/91JA01737

年代:1991

数据来源: WILEY

摘要:

At about 14,800 km from the comet Halley nucleus, on the inbound leg, at least six of the sensors onboard the Giotto spacecraft observed an unusual, brief (∼30 to 500 ms) event: The ion mass spectrometer data show a brief flow of energetic (up to several hundred electron volts) plasma consisting of protons, water group, and heavier ions. The Johnstone plasma analyzer data show a short burst of plasma, while the dust impact detector system data show an impact event in four of its detectors. The magnetometer signature of the event shows two brief dips in the field. The sudden change in the spacecraft attitude and spin rate observed by the camera at that same time has been interpreted as the result of a large (≥5 mg) dust particle impact on the front bumper shield of the spacecraft. In addition, at about the same time the spacecraft star‐tracker suffered damage. Here we give the first report combining direct measurements of the composition and dynamics of a dust impact plasma cloud, the dust particle mass and the location of the impact on the spacecraft. Analysis of the data indicate that the impacting particle was water or ice‐bearing, possibly loosely compacted, and was composed of one or more of: carbon, nitrogen, and

ISSN:0148-0227    

DOI:10.1029/91JA01013

年代:1991

数据来源: WILEY

摘要:

The maximum auroral kilometric radiation (AKR) power flux observed by distant satellites has been increased by more than a factor of 10 from previously reported values. This increase has been achieved by a new data selection criterion and a new analysis of antenna spin modulated signals received by the radio astronomy instrument on ISEE 3. The method relies on selecting AKR events containing signals in the highest‐frequency channel (1980, kHz), followed by a careful analysis that effectively increased the instrumental dynamic range by more than 20 dB by making use of the spacecraft antenna gain diagram during a spacecraft rotation. This analysis has allowed the separation of real signals from those created in the receiver by overloading. Many signals having the appearance of AKR harmonic signals were shown to be of spurious origin. During one event, however, real second harmonic AKR signals were detected even though the spacecraft was at a great distance (17RE) from Earth. During another event, when the spacecraft was at the orbital distance of the Moon and on the morning side of Earth, the power flux of fundamental AKR was greater than 3×10−13W m−2Hz−1at 360 kHz normalized to a radial distancerof 25REassuming the power falls off as r−2. A comparison of these intense signal levels with the most intense source region values (obtained by ISIS 1 and Viking) suggests that multiple sources were observed

ISSN:0148-0227    

DOI:10.1029/91JA01172

年代:1991

数据来源: WILEY

摘要:

One‐dimensional hybrid numerical simulations of tangential discontinuities (TDs) are presented for parameters representative of both the solar wind and magnetopause. For parameters common at the frontside magnetopause near local noon (antiparallel magnetic field and density gradients) it is found that TDs have a number of characteristic features that could be detected at the magnetopause. There is an overshoot in the magnetic field intensity on the low‐density side of the TD. The overshoot is of the order of 20% for density ratios of 10 but can be somewhat larger for other parameters. It extends typically over a few ion Larmor radii. The various plasma quantities all vary on their own characteristic length scale, so that a given set of conditions on either side should result in a (possibly) unique structure. It is found that the key parameters in determining TD structure are the density (and ion temperature) ratios, the plasma beta on the magnetosheath side, and the value of the electric potential difference across the TD. The relative orientation of the magnetic fields on either side of the TD does not appear to be of critical importance. For typical solar wind conditions (density and field gradients parallel) the magnetic field profile does not possess an overshoot but it does extend over 2–3 ion Larmor radii. The presence of a cross‐field plasma flow is shown to slightly modify the magnetic field profile, but the velocity profile is very sensitive to the sign of the flow (and hence the motional electric field). It is also shown that the TD structure is dependent on the assumed initial conditions. When the TD is initially chosen to be wider than the width that would result from the evolution of an infinitesimally thin initial TD, there is little further evolution. The currents in the TD layer can be a considerable fraction of the ion thermal speed, so that relaxation via plasma microinstabilities could arise. A detailed comparison with observed magnetopause crossings will be presented in a future

ISSN:0148-0227    

DOI:10.1029/91JA01324

年代:1991

数据来源: WILEY

摘要:

A three‐dimensional electrostatic particle simulation model is developed for studying the injection of an electron beam from an isolated source (the “spacecraft”) in the ionosphere. The dimensionless beam velocity in the simulations is determined by matching the beam energy and injection current to representative experimental values. It is found that the beam stagnation timetsis typically nearly an order of magnitude longer than in previous two‐dimensional simulations. As a result, under normal conditionstsis more than an order of magnitude larger than the plasma response timetrp, and the beam is able to escape from the near environment of the spacecraft. The limit on the amount of current that can be injected is provided by the conditionts≳ 2trp, and this limit can substantially exceed the thermal current to the spacecraft. After escape, the beam is subject to strong space‐charge oscillations produced by the beam‐plasma interaction. For injection parallel to the magnetic field this interaction produces a series of trapping vortices in phase space in which the peak potentials reach 10–15% of the beam voltage. For injection at 45° to the magnetic field, the beam initially forms a hollow cylinder whose radius is the beam gyroradius and whose width is about the beam Debye length. After two or three gyroperiods, however, the space‐charge oscillations lead to a loss of both the parallel and transverse coherence of the beam. Inside the beam cylinder the wave spectrum is dominated by a peak just below the upper hybrid frequency; outside the cylinder the most intense modes are at lower frequencies near the io

ISSN:0148-0227    

DOI:10.1029/91JA01266

年代:1991

数据来源: WILEY

摘要:

Recent laboratory studies of ion‐molecule reactions relevant to the analysis of ion densities in the vicinity of space vehicles in low‐Earth orbit are discussed. The studies include new cross‐section measurements of the important O++ H2O charge transfer collisions and the H3O+‐producing H2O++ H2O collisions, as well as product kinetic energy analysis of the reaction products. The charge transfer reaction rate is found to be less collision energy dependent than previously assumed, whereas the H3O+‐producting reaction rate is highest at thermal energies and decreases rapidly with increasing collision energy. Implications of the new results on previous interpretations and recent space observations are

ISSN:0148-0227    

DOI:10.1029/91JA01410

年代:1991

数据来源: WILEY

摘要:

Magnetohydrodynamic (MHD) surface waves may decay via a process called “resonance absorption,” which is a candidate for solar coronal heating. Recently, Hollweg, Yang, Cadez, and Gakovic studied the effects of velocity shear on the rate of resonance absorption of incompressible MHD surface waves. We extend the theory to a compressible but cold plasma, which is the case more applicable to the solar corona. We find that the rate of resonance absorption can be either increased or decreased significantly by velocity shear. We also find that there can exist resonances which lead to instability of the surface mode at values of the velocity shear below the Kelvin‐Helmholtz threshold. However, the resonance instability usually occurs when the density ratio across the surface is less than about 0.1. The resonant instability may therefore not be important in the solar corona, though resonance absorption remains an attractive possibility for coronal he

ISSN:0148-0227    

DOI:10.1029/91JA01407

年代:1991

数据来源: WILEY

摘要:

The dynamics of ionospheric plasma irregularities produced by the release of electron attachment materials at orbital velocities across the geomagnetic field is studied. A two‐dimensional electrostatic fluid model which includes electron attachment and mutual neutralization chemistry, self‐consistent electric fields, and three‐species transport is developed. Numerical simulations are performed to study the behavior at early and at late times after the release. At early times, of the order of or less than the attachment material neutral collision time, the negative ion cloud produced by the release may structure owing to the shear in theE×Bvelocity within the cloud. At high altitudes the cloud may bifurcate and form vortices on the back. At lower altitudes where ion‐neutral collisional effects dominate, this structuring is suppressed. At late times, after a plasma depletion has formed due to neutralization chemistry, the cloud structures by theE×Binterchange instability. Depending on the release altitude, the depletion structures by the collisional or inertial limit of this in

ISSN:0148-0227    

DOI:10.1029/91JA01084

年代:1991

数据来源: WILEY

摘要:

It is shown that the characteristic peak in the auroral electron velocity distribution can be generated stochastically through resonant interactions with lower hybrid electrostatic turbulence. The peak itself is shown to be a direct consequence of restrictions imposed on reflection of electron velocities in the frame of reference of individual wave packets by the limitation in group velocity. A Monte‐Carlo model demonstrates how the various properties of the acceleration region are reflected in the resultant electron distribution. It is shown, in particular, that the width of the peak is governed by the amplitude of the turbulence, while the amplitude of the peak reflects the column density of wave energy. Electron distributions encountered within three auroral arcs are interpreted to yield order of magnitude estimates of the amplitude and rms electric field of lower hybrid wave packets. The velocities and frequencies of the resonant waves, the net electric field, the column density of wave energy, and the electric‐field energy density are also estimated. The results are found to be consistent with available electric‐field measurements. A general broadening of the electron distribution caused by less systematic interactions between electrons and wave packets is shown to have a negligible effect on the peak resulting from the reflection process; it does, though, lead to the creation of a characteristic high‐ener

ISSN:0148-0227    

DOI:10.1029/91JA01105

年代:1991

数据来源: WILEY

摘要:

Low Mach number collisionless shocks in a multiple‐ion plasma are investigated theoretically using multiple‐ion Rankine‐Hugoniot relations (MIRHR) and hybrid simulations. The derived MIRHR permit tractable solutions for perpendicular shocks. It is found that starting from a synchronous flow of a mixture of different ion species, light ions and heavy ions jump to different downstream velocities. Although the resulting differential velocities between the species are unstable, they produce coherent gyration of the ion species around each other. This gyration excites standing ion hybrid waves in the downstream region. Both of these effects (the jump of different ion species to different downstream velocities and the excitation of the ion hybrid waves) are confirmed by hybrid simula

ISSN:0148-0227    

DOI:10.1029/91JA00638

年代:1991

数据来源: WILEY