摘要:

Using hour‐averaged data from the Helios and Voyager spacecraft, we have investigated the origin and evolution of low‐frequency interplanetary fluctuations from 0.3 to 20 AU. Alfvénic fluctuations in the inner solar system are found to be generally outward traveling from the Sun and at times quite pure, in general agreement with previous work. The correlation between velocity and magnetic field fluctuations can be high even on scales longer than the transit time from the Sun to the spacecraft, indicating a solar origin for the initial outward traveling waves. However, the fluctuations become substantially less Alfvénic by 1 AU, with the larger scales evolving more rapidly, and this evolution continues in the outer heliosphere. Near the Sun it is regions with small velocity gradients, rather than specifically the trailing edges of high‐speed streams, that exhibit the purest Alfvénic fluctuations. Density and magnetic field magnitude fluctuations inside 1 AU show the anticorrelation characteristic of pressure balance structures previously found in the outer heliosphere. The lower frequency positive correlation between density and field observed farther out in association with the growth of compression regions is not generally present inside 0.4 AU. Fluctuations have a somewhat higher magnetic than kinetic energy at scales of less than a day, but at lower frequencies, kinetic energy is already dominant by 0.3 AU. These results support the view that outward propagating Alfvénic fluctuations are generated near the Sun and that substantial dynamical evolution, probably involving shear‐generated nonlinear couplings, is important at all heliocentric distanc

ISSN:0148-0227    

DOI:10.1029/JA092iA11p12023

年代:1987

数据来源: WILEY

摘要:

When an MHD wave impinges on a fast shock from its upstream side, in general it may excite six diverging waves on the downstream side when the angle of incidence, the angle between the normal direction of the incident wave and the shock normal, is less than a critical angle. The boundary conditions across the shock surface determine the perturbations excited on the downstream side of the shock. We introduce a wave function approach to study the interaction of an MHD wave with an fast shock. When the angles of incidence are greater than certain critical values, less than six diverging waves can be excited; the flow becomes unstable on the downstream side of the shock. The existence of critical angles of incidence infers that the conclusion obtained on the stability of fast MHD shock with respect to normal incidence of MHD disturbances does not hold true for oblique incidence of disturbances at large angles of incidence. We investigate the critical angles of incidence, which are calculated as functions of the mode of the incident wave and three upstream conditions of the shock: the fast Mach number, the plasma β value and the shock angle. Our numerical results show that the critical angles of incidence are of the order of 60

ISSN:0148-0227    

DOI:10.1029/JA092iA11p12036

年代:1987

数据来源: WILEY

摘要:

One central problem in understanding the Martian upper atmosphere is the poor correlation between exospheric temperatures and the energy input from the Sun in the EUV and UV. Turbulence heats the atmosphere by dissipation of turbulent energy and cools it by downward heat transport. A time‐variable turbulence may introduce a stochastic component in addition to the solar‐driven, regular variation of the exospheric temperatures. To investigate the possible range of temperatures on the basis of this assumption, we developed a one‐dimensional mean‐dayside model of the energy balance of the Martian upper atmosphere. With plausible assumptions on the range of the eddy diffusion coefficient, we find a stochastic component of ±63 K for the exospheric temperatures. The comparison of observed data with the results of our model yields a best value for the efficiency of the heating by absorption of solar ultraviolet radiation of 0.14

ISSN:0148-0227    

DOI:10.1029/JA092iA11p12045

年代:1987

数据来源: WILEY

摘要:

The capacitance of a grain immersed in a steady state plasma containing a size distribution of dust particles is calculated. Assuming the equilibrium potential has been obtained by a simple balance of electron and ion collection currents, the grain charge is also obtained. It is shown that the validity of the analytical treatment given here for the linearized Poisson's equation is confined to a certain region of λD‐R(Debye length; interparticle spacing radius) space. Outside this region, a numerical solution of the full nonlinear Poisson's equation will be needed. Within the valid linear region and starting at very small λD(orR), the capacitance at first exhibits a monotonic increase with increasing λD(orR). Eventually, the capacitance reaches a maximum, and this is followed by a monotonic decrease. The charge density of the dust in the plasma, however, is found to be only a function of the Debye length; there is no significant dependence on the interparticle spacing. Finally, the results of similar calculations by Goertz and Ip (1984) and Whipple et al. (1985) for a dusty plasma with a single grain size are shown to be qualitatively different from the results given here. The reason for this is the limited λD‐Rspace investigated by these authors and the inconsistent use of a linearized calculation to interpret dust behavior in a nonlinear

ISSN:0148-0227    

DOI:10.1029/JA092iA11p12057

年代:1987

数据来源: WILEY

摘要:

During September 28 and 29, 1978, the ISEE 3 spacecraft observed several distinct types of high‐speed solar wind flows when a coronal hole‐associated high‐speed stream was followed by two interplanetary shocks, one of which was driven by flare ejecta contained in a “magnetic cloud” or interplanetary “plasmoid.” Using the University of Maryland/Max‐Planck‐Institut ultralow energy charge analyzer (ULECA) on ISEE 3, we present solar wind Fe and Si/S charge state and Fe density measurements for the different plasma regimes associated with the flare‐related shock and combine these measurements with the Los Alamos National Laboratory proton observations to obtain iron/hydrogen density ratios and iron/hydrogen velocity differences. We place special emphasis on the postshock shell of turbulent and compressed ambient solar wind and interplanetary magnetic fields constituting the “sheath region” preceding the driver plasma. It is generally expected that the abundance ratios and charge states of the solar wind ions in the driver plasma and the sheath remain distinct, reflecting their different origins in the solar corona. However, while the Fe/H abundance ratios observed in the sheath are consistent with the preshock solar wind values (and a factor of 2 to 6 times less than the values obtained in the driver plasma), the iron charge states observed in the sheath appear to indicate a transition between the lower charge states observed in the ambient (coronal hole associated) solar wind and the higher charge states observed in the driver plasma. These results may reflect X ray ionization of the solar wind plasma near the flare site, although other ex

ISSN:0148-0227    

DOI:10.1029/JA092iA11p12069

年代:1987

数据来源: WILEY

摘要:

Simultaneous observations of energetic ions (≳50 keV) and electrons (≳220 keV) by the IMP 7 and 8 spacecraft, carrying identical instruments and located within the distant (∼37 RE) magnetotail and upstream from the bow shock, have been employed to separate temporal variations from spatial variations during the upstream ion events observed on December 3, 1977 and November 2–3, 1977, in order to determine the source of these particles. The IMP data, when compared with those from ISEE 1 and 2, have also made possible the determination of field‐aligned and flux‐tube intensity gradients in the upstream region, thus enabling the test of specific predictions of the Fermi acceleration model for such events. The analysis of these three‐spacecraft observations and comparison with theory have revealed the following: (1) For each of the observed upstream enhancements, energetic ions and electrons were simultaneously present inside the plasma sheet—successive increases were in excellent time coincidence with substorm injection events seen on ground‐based magnetograms; (2) The low‐energy (≳50 keV) ion intensity profile inside the plasma sheet was relatively flat, while at higher (≳300 keV) energies there was considerable variability, with one case exhibiting an “inverse velocity dispersion” profile; (3) Relativistic electron bursts were seen inside the plasma sheet and also upstream of the shock but at substantially reduced intensities; (4) The ion energy spectrum for the December 3 event, extended to energies ∼2 MeV, was identical in form within the plasma sheet and upstream of the shock and can be described well bydj/dE∝E−5.5; (5) Ion anisotropies exhibited typically large dawn‐dusk or dusk‐dawn gradients, depending on spacecraft location, and showed large (up to 20∶1) field‐aligned streaming away from the bow shock; (6) Comparison of IMP and ISEE 2 intensities at ∼60 keV for identical magnetic connections to the bow shock during a period when the interplanetary magnetic field (IMF) was radial showed the IMP 7 intensity to be higher by a factor ∼10, even though IMP 7 was ∼20 REfurther from the bow shock, but located toward the dusk side of the earth‐sun line; and (7) Comparison of unidirectional intensities of IMP 8 and ISEE 1 along the same radial IMF line showed the field‐aligned component to be identical at both spacecraft, while the omnidirectional intensities give a gradient of ∼2.5%/RE. The observations are compared with predictions of the Fermi model, and it is found that they are inconsistent with the principal features of the model, including the energy extent, form of the spectrum, anisotropies, scale lengths of acceleration region

ISSN:0148-0227    

DOI:10.1029/JA092iA11p12083

年代:1987

数据来源: WILEY

摘要:

We elaborate upon the leakage model for the escape of energetic magnetospheric particles into the magnetosheath. Unlike the merging model, no interconnection (or merging) of magnetospheric and magnetosheath magnetic field lines is required. Because outer magnetospheric energetic particle drift paths intersect the magnetopause, the leakage model requires the continual escape of ions at postnoon local times and electrons at prenoon local times, regardless of solar wind conditions. It also predicts a division between dawnward and duskward streaming ions at the point where most magnetosheath magnetic field lines make their closest approach to the magnetopause, typically near 1500 LT. Like the merging model, the leakage model predicts equatorward streaming just inside the magnetopause. We study the motion of an escaping energetic ion at a planar magnetopause to show that, without scattering, ions must move dawnward and northward in a duskward magnetosheath magnetic field and dawnward and southward in a dawnward magnetosheath magnetic field. Scattering permits some ions to move duskward. We present new observations of streaming ions outside the dayside magnetopause made by the Charge Composition Explorer satellite, a part of the Active Magnetospheric Particle Tracer Explorers program. To place these observations in context, we have performed a statistical study of previous particle observations both inside and outside the dayside magnetopause. The ensemble of observations indicates that energetic magnetospheric ions of all species continually escape from the dayside magnetosphere and stream along magnetosheath magnetic field lines, even when no merging is expected. The magnetosheath magnetic field controls the direction in which the ions stream: they move away from the magnetosphere. The results of this work indicate that energetic particle observations at the dayside magnetopause need not be taken as evidence for merging of magnetosheath and magnetospheric magnetic field lines.

ISSN:0148-0227    

DOI:10.1029/JA092iA11p12097

年代:1987

数据来源: WILEY

摘要:

The delayed entry of solar protons (Ep>0.3 MeV) into the Earth's magnetotail has been studied by simultaneous observation with three Earth‐orbiting spacecraft of a sequence of specially favorable events during 1968. Principal findings are that (1) such protons have immediate access to the magnetosheath; (2) delay times increase discontinuously from essentially zero in the magnetosheath to tens of minutes as an observing point moves inward across the magnetopause; (3) well‐defined delay times in the range 30–160 min occur for points interior to the magnetotail; (4) the inferred entry points lie at downstream distances of 80–500RE; and (5) there is no discernible relationship between the direction and sense of the interplanetary magnetic field and the entry times in the northern and southern lobes of the magnetotail or the entry times at various distances from its central axis, theXGSMaxis. This data set is compatible with previously published work but adds significantly to observational knowledge of the subject. In broad terms, the results favor some form of an open model of the magnetotail and are inconsistent with diffusive entry of protons across a closed magnetotail field with a time constant substantially greater than a few

ISSN:0148-0227    

DOI:10.1029/JA092iA11p12115

年代:1987

数据来源: WILEY

摘要:

The detailed nature of the thermal and suprathermal ion and electron distributions within magnetic flux transfer events (FTEs) is examined. Examples of both magnetosheath FTEs and magnetospheric FTEs are discussed. The detailed distributions confirm that FTEs contain a mixture of magnetosheath and magnetospheric plasmas. To lowest order, the distributions are consistent with a simple superposition of the two interpenetrating populations, with no strong interactions between them. To first order, some interesting differences appear, especially in the electron distributions, suggesting that considerable pitch angle scattering and some electron energy diffusion are also occurring. These observations should provide a useful test of analytical and numerical studies of interpenetrating plasmas.

ISSN:0148-0227    

DOI:10.1029/JA092iA11p12127

年代:1987

数据来源: WILEY

摘要:

The possibility is examined of obtaining information about the orientation and state of motion of the magnetopause current layer from the convection electric field,Ec=−v × B, calculated from measured plasma velocities,v, and magnetic fields,B. Using AMPTE/IRM data from one particular traversal of a magnetopause rotational discontinuity as a guide, a new method has been developed for predicting the magnetopause normal vector,n. In its simplest form, the method consists of performing variance analysis onEcdata, or modifications thereof which take into account the motion of the magnetopause. The procedure is similar to the minimum variance analysis commonly performed onBdata, except that the direction of maximum variance ofEc, rather than minimum variance ofBis used as a predictor forn. In its most developed form, the method depends on the existence of a deHoffmann‐Teller (HT) frame in which the electric field vanishes or nearly vanishes. Procedures have been developed for the determination of the initial velocity,vHTo, and accelerationaHT, assumed constant over the analysis interval, of that frame. When these effects are taken into account, an accuratenvector may often be obtained: in the trial case, error analysis and consistency tests indicate uncertainties of the order of 2°–3°, even though the variance in the normal magnetic field,B·n, in this crossing was large enough to render the minimum variance direction ofBuseless as a predictor ofn. In general, it is not possible to determine the initial magnetopause normal speed,unofrom the electric field data but the analysis places certain, sometimes narrow, bounds onunoand on the normal magnetic field, B·n, the tangential electric field,Ec×n, and the distance,h, alongntraversed during the data interval, all of which are found to depend linearly onuno. For the trial case, the following results were obtained: −97

ISSN:0148-0227    

DOI:10.1029/JA092iA11p12137

年代:1987

数据来源: WILEY