摘要:

The interaction of subsonic plasma flow with a neutral particle cloud via charge exchange is considered. We derive a general expression for the velocity potential of the plasma flow induced by charge exchange interaction in the axisymmetric case. The particular case of the interaction of uniform plasma flow with a spherically symmetric distribution of neutral particles is considered in detail. It is shown that charge exchange interaction is able to stagnate the plasma flow completely at a finite distance along the stagnation line, and to redirect the plasma flow around the neutral cloud. We argue that this simple situation could represent the main features of the interaction of the interstellar wind with high‐speed solar hydrogen flowing from the inner heliosphere. Thus the heliopause could in part, if not as a whole, actually result from this particular form of charge exchange interaction. The degree of ionization of the local interstellar medium is shown to determine the extent of the role of the charge exchange interaction. For a partially ionized interstellar wind, charge exchange plays the main role in slowing down and stagnating the interstellar wind, while for a highly ionized interstellar wind, its role is secondary to the direct gasdynamic interaction of the two counter streaming plasma

ISSN:0148-0227    

DOI:10.1029/95JA03831

年代:1996

数据来源: WILEY

摘要:

Direct spectral method simulation of the three‐dimensional magnetohydrodynamics (MHD) equations is used to explore anisotropy that develops from initially isotropic fluctuations as a consequence of a uniform applied magnetic field. Spectral and variance anisotropies are investigated in both compressible and incompressible MHD. The nature of the spectral anisotropy is consistent with the model ofShebalin et al.[1983] in which the spectrum broadens in the perpendicular wavenumber direction, the anisotropy being greater for smaller wavenumbers. Here this effect is seen for both incompressible and polytropic compressible MHD. In contrast, the longitudinal (compressive) velocity fluctuations remain isotropic. Variance anisotropy is observed for low plasma beta compressible MHD but not for incompressible MHD. Solar wind observations are qualitatively consistent with both variance and spectral anisotropies of the type discussed her

ISSN:0148-0227    

DOI:10.1029/95JA03830

年代:1996

数据来源: WILEY

摘要:

Magnetic reconnection has traditionally been associated exclusively with the presence of magnetic null points or field lines tangential to a boundary. However, in many cases introducing a three‐dimensional perturbation in a two‐and‐half‐dimensional magnetic configuration implies the disappearance of separatrices. Faced with this structural instability of separatrices when going from two‐and‐half to three‐dimensional configurations, several approaches have been investigated to replace the topological ideas familiar in two‐dimensional, but no unanimity has yet emerged on the way reconnection should be defined. While it is true that the field line linkage is continous in three‐dimensional, we show here that extremely thin layers (called quasi‐separatrix layers (QSLs)) are present. In these layers the gradient of the mapping of field lines from one part of a boundary to another is very much larger than normal (by many orders of magnitude). Even for highly conductive media these extremely thin layers behave physically like separatrices. Thus reconnection without null points can occur in QSLs with a breakdown of ideal MHD and a change in connectivity of plasma elements. We have analyzed several twisted flux tube configurations, going progressively from two‐and‐half to three‐dimensional, showing that QSLs are structurally stable features (in contrast to separatrices). The relative thicknesswof QSLs depends mainly on the maximum twist; typically, with two turns,w≈ 10−6, while with four turns,w≈ 10−12. In these twisted configurations the shape of the QSLs, at the intersection with the lower planar boundary, is typical of the two ribbons observed in two‐ribbon solar flares, confirming that the accompanying prominence eruption involves the reconnection of twisted magnetic structures. We conclude that reconnection occurs in three‐dimensional in thin layers or QSLs, which generalise the traditional separatrices (related only to magnetic null points or fi

ISSN:0148-0227    

DOI:10.1029/95JA03558

年代:1996

数据来源: WILEY

摘要:

An implicit‐particle simulation of the collisionless parallel shock created at the interface between an injected beam and a stationary plasma is performed in one‐dimensional geometry. The solar wind plasma, which consists of ions and electrons, is injected into a stationary dense plasma that corresponds to the planetary ionosphere. Electromagnetic waves with right‐hand circular polarization that propagate upstream (R−waves) are generated at the interface of the two plasmas, which decelerate the solar wind to form a shock. The shock transition region is not monotonic but consists of two distinct regions, a pedestal and a shock ramp. The transition region, which contains the ionopause, is a few thousand electron skin depths long. The parallel shock varies in time and periodically collapses and re‐forms. The right‐hand circularly polarized electromagnetic waves that propagate downstream (R+waves) are excited at the shock ramp. Nonlinear wave‐particle interaction between the solar wind and the R+waves causes wave condensation and density modulation. These R+waves may be sweeping away the downstream plasma to suppress its thermal diffusion across the shock. The electrons at the shock ramp exhibit a flat‐topped velocity distribution along the magnetic field owing to the ion acoustic‐like el

ISSN:0148-0227    

DOI:10.1029/95JA03808

年代:1996

数据来源: WILEY

摘要:

During the Voyager 2 flyby of Uranus, strong electron plasma oscillations (Langmuir waves) were detected by the plasma wave instrument in the 1.78‐kHz channel on January 23–24, 1986, prior to the inbound bow shock crossing. Langmuir waves are excited by energetic electrons streaming away from the bow shock. The goal of this work is to estimate the location and motion of Uranus' bow shock using Langmuir wave data, together with the spacecraft positions and the measured interplanetary magnetic field. The following three remote sensing analyses were performed: the basic remote sensing method, the lag time method, and the trace‐back method. Because the interplanetary magnetic field was highly variable, the first analysis encountered difficulties in obtaining a realistic estimation of Uranus' bow shock motion. In the lag time method developed here, time lags due to the solar wind's finite convection speed are taken into account when calculating the shock's standoff distance. In the new trace‐back method, limits on the standoff distance are obtained as a function of time by reconstructing electron paths. Most of the results produced by the latter two analyses are consistent with predictions based on the standard theoretical model and the measured solar wind plasma parameters. Differences between our calculations and the theoretical model are di

ISSN:0148-0227    

DOI:10.1029/95JA03849

年代:1996

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/95JA03711

年代:1996

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/95JA03710

年代:1996

数据来源: WILEY

摘要:

The bulk parameters of the hot (>20 keV) plasmas of Jupiter's inner magnetosphere, including the vicinity of the Io plasma torus, are presented for the first time (L= 5 to 20RJ). The low‐energy charged particle (LECP) instrument on Voyager 1 that obtained the data presented here was severely overdriven within the inner regions of Jupiter's magnetosphere. On the basis of laboratory calibrations using a flight spare instrument, a Monte Carlo computer algorithm has been constructed that simulates the response of the LECP instrument to very high particle intensities. This algorithm has allowed for the extraction of the hot plasma parameters in the Jovian regions of interest. The hot plasma components discussed here dominate over other components with respect to such high‐order moments as the plasma pressures and energy intensities. Our findings include the following items. (1) Radial pressure gradients change from positive (antiplanetward) to negative as one moves outward past about 7.3RJ. While the observed hot plasma distributions will impede the radial transport, via centrifugal interchange, of iogenic plasmas throughout the Io plasma torus regions out to 8RJ, the plasma impoundment concept ofSiscoe et al.[1981] for explaining the so‐called “ramp” in the flux shell content profile of iogenic plasmas (7.4–7.8RJ[Bagenal, 1994]) is not supported. (2) We predict a radial ordering for the generation of the aurora, which translates into a latitudinal structure for auroral emissions. Planetward of about 12RJ, intense aurora (10 ergs/(cm2s) precipitation) can only be caused by ion precipitation, whereas outside of about 12RJsuch intense aurora can only be caused by electron precipitation. Uncertainties concerning the causes of Jovian aurora may stem in part from failures of some observations to resolve the latitudinal structure that is anticipated here and possibly from changes in the auroral configuration and/or charged particle spectral properties since the Vo

ISSN:0148-0227    

DOI:10.1029/96JA00006

年代:1996

数据来源: WILEY

摘要:

The commonly used formulaH=DCF+DR, whereDCFandDRare the effects of the magnetopause and ring current, respectively, neglects contribution of the cross‐tail current to theDstvariation. The formula allows us to explain satisfactorily the observed relation of theDstvariation to the ring current intensity but faces difficulties in explaining other experimental facts. First, the equatorward shift of the auroral oval cannot be caused by the sole enhancement of the ring current. Second, the observed relation of theDstgrowth rate to the southward IMF component [Burton et al., 1975] does not have any quantitative explanation up to now. We suggest using a different formula,H= (2μ0psw)1/2+DR‐Fout/2S. The formula is obtained from the conditions of magnetic flux conservation and pressure balance. The fluxFoutis directed mainly to the nightside of the magnetosphere. Hence the termFout/2Sdescribes the effect of the crosstail current and a part of the magnetopause currents. During quiet periods, each term in the right‐hand side of our formula is of the order of tens of nanoteslas. During storm time, each term can rise to hundreds of nanoteslas. The fluxFoutgrows after the interplanetary magnetic field (IMF) becomes southward owing to the flux transport from the dayside to the magnetotail. The growth rate is described by the formuladFout/dt=U−Fout/τF+ ηF, whereUis the electric potential difference between the dawnside and duskside of the magnetosphere and τFand ηFare constant. The voltageUdepends linearly on the IMF southward component. Combining the latter formula with the expression forHyields a relationship between theDstgrowth rate and the IMF southward component close to the observed one. Since the auroral oval is mapped predominantly to the plasma sheet of the magnetotail, the growth ofFoutduring a storm allows us to explain the equatorward shift of the auroral oval. Another prediction from our theory is the erosion of the stable trapping region in which the equatorial cross sectionSis related to the fluxFoutby the equationS1/2[S(2μ0psw)1/2+Fout] = 3π3/2(ME+MRC), whereMEandMRCare the magnetic moments of the Earth and ring current, respectively. Growth ofFoutleads to the decrease ofSand to the earthward movement of the dayside magnetopause. During storms this effect can be stronger than that of the region 1 Birkeland current, also moving the magnetop

ISSN:0148-0227    

DOI:10.1029/95JA03709

年代:1996

数据来源: WILEY

摘要:

The electrodynamics of the inner magnetosphere near times of substorm onsets have been investigated using CRRES measurements of magnetic and electric fields, energetic electron fluxes, in conjunction with ground‐based observations. Six events were studied in detail, spanning the 2100 to 0000 MLT sector andLvalues from 5 to 7. In each case the dawn‐dusk electric field was enhanced over typical background electric fields, and significant, low‐frequency pulsation activity was observed. The amplitudes of the pulsations were larger than the background electric fields. Dusk‐dawn excursions of the cross‐tail electric field often correlated with changes in currents and particle energies at CRRES and with ULF wave activity observed on the ground. Variations of the electric field and Poynting vectors with periods in the Pi 2 range are consistent with bouncing AlfVén waves that provide electromagnetic communication between the ionosphere and plasma sheet. Magnetic signatures of field‐aligned current filaments directed away from the ionosphere, presumably associated with the substorm current wedge, were observed during three orbits. In all cases, ground signatures of substorm expansion were observed at least 5 min before the injection of electrons at CRRES. Field‐aligned fluxes of counter‐streaming, low‐energy electrons were detected after three of the injections. We develop an empirical scenario for substorm onset. The process grows from ripples at the inner edge of the plasma sheet associated with dusk‐dawn excursions of the electric field, prior to the beginning of dipolarization. Energy derived from the braking of the inward plasma convection flows into the ionosphere in the form of Poynting flux. Subsequently reflected Poynting flux plays a crucial role in the magnetosphere‐ionosphere coupling. Substorms develop when significant energy (positive feedback?) flows in both directions, with the second cycle stronger than the initial. Pseudobreakups occur when energy flow in both directions is weak (negative feedback?). “Explosive‐growth‐phase” signatures occur after onset, early in the substorm expansion phase. Heated electrons arrive at the spacecraft while convection is earthward, during or at the end of electromagnetic energy

ISSN:0148-0227    

DOI:10.1029/95JA03856

年代:1996

数据来源: WILEY