摘要:

A coronal mass ejection with the appearance of two sets of overlapping loops occurred at about 0600 UT on September 1, 1980, over the northwest limb of the sun. It was one of the fastest events observed by the Solar Maximum Mission coronagraph during the 1980 epoch, with apparent radial velocity components of several features approaching 1000 km s−1. A study of the slow evolution of Hαprominence filaments and coronal structures in the northwest solar sector suggests that the mass ejection resulted from the disruption of a helmet streamer in association with, possibly, two filaments to give rise to the double‐loop structure. This event is well covered by 10 coronagraph images of good quality so that the complex velocity field, defined by the apparent motions of many different parts of the mass ejection, can be mapped out as a function of space and time. The results of such an analysis are presented and related to current concerns in the theoretical understanding of mass ejections. In particular, it is concluded that a self‐similar description of the velocity field is a gross oversimplification and that although some evidence of wave propagation can be found, the bright features in this mass ejection are plasma structures moving (presumably) with frozen‐in magnetic fields, rather than waves propagating through plasmas and magneti

ISSN:0148-0227    

DOI:10.1029/JA092iA03p02221

年代:1987

数据来源: WILEY

摘要:

Time profiles and histograms of plasma data from Pioneers 10 and 11 are examined for the period between 1975 and 1983. During this time, Pioneer 10 traveled between a heliocentric distance of 8.7 and 30.4 AU. The velocity structure of the solar wind at these heliocentric distances is found to have one of two distinct forms: Approximately 70% of the time the solar wind has a nearly flat velocity profile. Occasionally, this flat velocity profile is accompanied by quasi‐periodic variations in density and in thermal speed consistent with the concept that the “corotating interaction regions” which are produced by the interaction of high‐ and low‐speed streams at intermediate heliocentric distances are replaced by “pressure regions” in the outer heliosphere. The remaining 30% of the time the solar wind is marked by large (50–200 km/s) long‐term (30–120 day) shifts in the average

ISSN:0148-0227    

DOI:10.1029/JA092iA03p02231

年代:1987

数据来源: WILEY

摘要:

Applying power, coherence and magnetic and cross helicity spectral analysis of the solar wind plasma and magnetic field obtained by Helios‐1 and ‐2, we investigate the nature of MHD fluctuations occuring upstream and downstream of a parallel, supercritical, turbulent, of a quasi‐parallel, supercritical, turbulent, and of a quasi‐parallel, subcritical, laminar fast‐forward shock wave. The main results of our investigation are as follows: (1) The spectral slopes and powers vary significantly with the length of the data interval analyzed. The difference upstream of the parallel shock was such that the ratio of the axisymmetric spatial diffusion coefficients of a 1‐MeV proton, calculated from the 86.4‐min and 10.8‐min spectra using Morfill and Scholer's (1977) formula, is 11.82. (2) Counterstreaming Alfvén waves were identified immediately upstream and downstream of the quasi‐parallel, supercritical shock. (3) Fast magnetoacoustic waves were identified upstream of the quasi‐parallel, subcritical shock and far upstream of the quasi‐parallel, supercritical shock coinciding with the inclusion of a small transverse discontinuity in the spectral analysis. (4) Compressional turbulence which is not characteristic of either of the magnetoacoustic modes was observed downstream of the parallel, supercritical and quasi‐parallel subcritical shocks. As regards the determination of the parallel diffusion coefficient from power spectra of the observed magnetic field fluctuations, we may conclude from our results that this procedure is susceptible to major errors due to the following reasons: First, there is no complete theory for the diffusion of energetic particles in regions where the MHD fluctuations cannot be labeled Alfvénic or magnetoacoustic. Second, equations for particle motion in a field of counterstreaming Alfvén waves (second Fermi process) are also lacking. Third, even the “Alfvénic” fluctuations are never 100% Alfvénic; i.e., a non‐Alfvénic component is usually present. Such “mixing” is also not considered in the present theories of energetic particle propagation. Fourth and last, it is not at all clear how much data before/after the shock waves should be analyzed to compute values of the parallel diffusion coefficient (κ∥) or mean free path for scattering (λ∥,s) of an energetic particle. We show here that spectral analysis of the fluctuations 10.8 min., 43.2 min. and 86.4 min. upstream of the parallel shock yields

ISSN:0148-0227    

DOI:10.1029/JA092iA03p02243

年代:1987

数据来源: WILEY

摘要:

A data set of 18,000 low‐altitude nightside vector measurements from the Pioneer Venus Orbiter (PVO) magnetometer is used to establish a new upper limit on the intrinsic dipolar magnetic moment of Venus. Various techniques are used to search for coherence in the magnetic observations which might be attributable to such an intrinsic field, with negative results. A similar search is conducted for surface magnetism, that is, geographically correlated magnetic features, also with negative results. The present study represents the best limits that PVO observations can provide and is limited by the amount of low‐altitude nightside data available. We do not expect to obtain a significant amount of new low‐altitude data later in the mission. Our new upper limit to the magnetic moment is 8.4 × 1010T m³ or about 10−5of the terrestrial magnetic moment. This is a factor of 4 lower than the previous up

ISSN:0148-0227    

DOI:10.1029/JA092iA03p02253

年代:1987

数据来源: WILEY

摘要:

We derive perturbation equations for the motion of weakly charged dust grains (q/M1→0, whereqis the grain's electric charge andM1its mass) in planetary rings, with emphasis on the Jovian ring system. Our analysis includes the effects of planetary oblateness and an arbitrarily complex magnetic field expressed in terms of spherical harmonics. The perturbation solutions are shown to agree with numerical integrations for most initial conditions, and to provide a simple means of understanding the nature of charged grain trajectories in terms of forced oscillator models. We identify “Lorentz resonances” where a frequency of the perturbing Lorentz force matches a natural orbital frequency of the grain. The resonances come in pairs, with one on either side of the synchronous orbitrs; asrsis approached, resonances are more closely spaced, and their zones become narrower. Large vertical and horizontal excursions from planar circular orbits are predicted at these locales, and this is confirmed in numerical simulations. The amplitudes of the vertical oscillations at resonance can be 1–2 orders of magnitude larger than those outside such zones. The enhanced vertical excursions mean that for shallow viewing angles the optical depth will decrease as lines of sight approach the Lorentz resonance radii. This then provides a mechanism for explaining some apparent boundaries in dusty rings, such as those of the Jovian halo and, less likely, the outer edges of the Jovian main ring and the gossamer ring. For future use, we list the low‐order resonances that lie at or near several features in the Voyager images. Theoretical ring cross sections and overall ring topology are obtained from averages of the analytical solutions over initial conditions. We find that because of the forced response to perturbations, short time solutions predict a warping of the halo out of the equatorial plane that is not seen in the available images. We then point out the significance of including a slow evolutionary process, such as plasma drag, in the dynamical model. Since the large vertical excursions produced as grains drift through Lorentz resonances apparently survive over the radial evolution time scale, the natural response will dominate the motion in the halo region, and the halo will not be appreciab

ISSN:0148-0227    

DOI:10.1029/JA092iA03p02264

年代:1987

数据来源: WILEY

摘要:

We consider a finite‐sized dust cloud embedded in an infinite plasma. The dust cloud will generally be at a different potential from that of the ambient plasma, and particles which enter the cloud are accelerated or decelerated. We treat the two limiting cases: in case A, particles are not thermalized within the cloud, and in case B, the plasma is thermalized in the cloud interior to form a Maxwellian plasma with densities given by Boltzmann relations as functions of the local cloud potential. Qualitatively, both cases lead to similar results: In a tenuous cloud the dust cloud potential is low while the dust potentials are close to those of single isolated dust particles. In dense dust clouds the cloud potential approaches a maximum while the dust potentials decreases to zero with increasing dust density. However, the exact values of the potentials can differ considerably for the two cases for the same set of other cloud and ambient plasma parameters. Also, for case A we may have several sets of solutions for the two potentials, all giving charge balance in the cloud and current balance to the dust. We find, however, that case A may not easily be formed for all ranges of dust densities because the charging of dust can take a longer time than the time for scattering processes to lead to internal Boltzmann distributions for both plasma particle types. On the other hand, we expect semistable cases intermediate between case A and case B with multiple solutions to be found. Instabilities through the creation of double layers and transitions between two solutions may occur and lead to events in dust clouds. We suggest that such events may possibly occur in cometary comas and in planetary rings and lead to changes in the dust cloud structure. Conditions relevant for multiple solutions will probably be rather short‐lived, at most a few times 10 days, before scattering collisions with diffusion in velocity space turn the plasma velocity distribution into that of case B. In this case there is only one solution for the potentials for each set of cloud and plasma paramet

ISSN:0148-0227    

DOI:10.1029/JA092iA03p02281

年代:1987

数据来源: WILEY

摘要:

The early time dynamics of an AMPTE release in the solar wind is studied using a three‐dimensional particle simulation. The results of the simulation explain the dynamics of the solar wind ions, release ions, and the current system that the release sets up. It is found that, within the limits of charge quasi‐neutrality and the recognition of differential reference frames caused by the inertia of the release ions, the cross‐field motion of the ions can be understood. It is also found in these simulations that the convection of the magnetic field around the release is asymmetric and that the solar wind ion interaction is also asymmetric. From this feature and the correlation between the three‐dimensional and two‐dimensional simulations it is seen that the AMPTE releases produce physics that is somewhat different from that of a planet o

ISSN:0148-0227    

DOI:10.1029/JA092iA03p02289

年代:1987

数据来源: WILEY

摘要:

Within the magnetic ramp of fast collisionless plasma shocks observed with spacecraft instruments and simulated numerically, the magnetic field undergoes an excursion out of the plane of coplanarity (the plane determined by the upstream magnetic field and the shock normal). This rotation is consistently in the direction such that the electrostatic potential jump across the shock, as measured in the de Hoffman‐Teller frame of reference (HTF), is ∼2–6 times smaller than the electrostatic potential jump measured in the normal incidence frame. The preferred direction is consistent with a basic whistler mode transition between the upstream and downstream orientations. The potential jump in the HTF is considerably smaller than the change in bulk flow energy across the shock, confirming the recent suggestion that magnetic forces contribute importantly to the slowing of the plasma in that frame. A further consequence is that suprathermal particles leaking back into the upstream region across the shock do not gain much energy from the cross‐shock electri

ISSN:0148-0227    

DOI:10.1029/JA092iA03p02305

年代:1987

数据来源: WILEY

摘要:

A general analytic theory for the distribution function of particles backstreaming from an arbitrary shock in a magnetized plasma (neglecting wave‐particle scattering) is presented. The theory explicitly shows that abrupt cutoffs in the parallel velocity, where ∂f/∂|v∥|>0, arise naturally in the upstream region, irrespective of the acceleration processes active at the shock. These “cutoff” distributions may provide free energy for wave growth. Two classes of cutoff arise, escape cutoffs due to the particles requiring a characteristic speed to outrun the shock, and edge cutoffs due to the shock having an edge. Analytic expressions for the particle distribution function upstream of finite planar shocks and shocks whose two‐dimensional sections are parabolic are derived. A theory for the electron distribution function upstream from the earth's bow shock and the source of free energy for the observed Langmuir waves is presented. The electron distribution has an escape cutoff in the parallel velocity. This cutoff distribution is the source of free energy for Langmuir wave growth. Distribution functions and the spatial variation of the cutoff velocity are illustrated graphically. The theory is consistent with the spatial variation in the electron beam velocity inferred from indirect observations, assuming the observed electron beams to be the wave‐particle scattered remnants of cutoff distributions. An application of this theory to type II solar radio bur

ISSN:0148-0227    

DOI:10.1029/JA092iA03p02315

年代:1987

数据来源: WILEY

摘要:

A theory for the Langmuir (L) waves observed in the electron foreshock is suggested. Free energy for the Langmuir wave growth is contained in cutoff distributions of energetic electrons streaming from the bow shock. These cutoff distributions drive Langmuir wave growth primarily by the kinetic version of the beam instability, and wave growth is limited by quasi‐linear relaxation. The observed bump‐on‐tail electron distributions are interpreted as the remnants of cutoff distributions after quasi‐linear relaxation has limited the wave growth. Only plausibility arguments for this theory are given since suitable treatments of quasi‐linear relaxation are not presently available. However, it is shown that the wave processes L ± S → L′ and L ± S → T (where S and T denote ion sound and transverse waves, respectively), refraction in steady‐state density structures, diffusion due to interactions with ion sound turbulence, and effects due to wave convection and spatial gradients in the beam velocity, are unable to suppress the beam instability. The theory leads to natural interpretations of the Langmuir electric field waveforms observed and of the decrease in the Langmuir wave electric fields with increasing distance from the foreshock boundary. The theory for the beam instability is reviewed, and previous analytic and numerical treatments of the beam instability are related. A difficulty in recent theoretical interpretations of broadband wave growth below the plasma frequency, relating to the version of the beam instability considered giving rise only to narrow‐band growth, is pointed out and suggestions for resolving thi

ISSN:0148-0227    

DOI:10.1029/JA092iA03p02329

年代:1987

数据来源: WILEY