摘要:

Sime et al. (1984, 1985) have argued that two common characteristics of looplike coronal mass ejections are inconsistent with interpretation of such loops as shock waves moving through the corona. These are the cessation of lateral motions of the loop sides to form nearly radial, stationary bright columns or “legs” in the late stages of many mass ejections and the pushing aside or bending of pre‐existing coronal features well outside the loop sides. Both are contrary to the notion of a propagating wave with a shock at its front and in disagreement with quantitative models of shock propagation in the corona. We report here Solar Maximum Mission (SMM) coronagraph observations of a looplike coronal mass ejection that occurred on July 6, 1980, and that was atypical in the two characteristics mentioned above. The sides of the loop moved laterally while they were visible and did not stop to form stationary “legs.” A group of raylike features initially outside the mass ejection loop were not pushed aside or bent until the laterally moving loop sides passed over them. We thus propose the July 6, 1980, mass ejection as a candidate for identification as a shock wave. Quantitative analysis of the motion of the bright loop is also consistent with such an identification. The loop top moves outward with an apparent radial speed of ∼1000 km s−1, faster than all but one other mass ejection observed by SMM in 1980 and probably faster than the Alfvén speed in the corona. The loop sides move laterally at a speed of ∼800 km s−1and show no evidence for a significant deceleration. The excess mass in the bright loop is estimated at ≤2 × 1014g, comparable to the expected error in its measurement and well below the value found to be typical of c

ISSN:0148-0227    

DOI:10.1029/JA092iA02p01049

年代:1987

数据来源: WILEY

摘要:

A previous model has shown that in order to account for the charge state distribution in the low‐speed solar wind a high coronal temperature is necessary and that this temperature peak goes together with a peak ofnα/npin the corona. In the present paper, one of the assumptions made previously, i.e., that coronal electrons are Maxwellian, is relaxed, and a much cooler model is presented, which could account for the same oxygen charge states in the solar wind due to the inclusion of non‐Maxwellian electrons. Also, due to a different choice of the coronal magnetic field geometry, this model would show no enhancement of the coronalnα/np. Results of the two models are then compared, and observational tests to distinguish between the two scenarios are proposed: comparison of directly measured coronalTeto charge state measurements in the solar wind, determination of the coronalnα/np, measurement of ion speeds in the acceleration region of the solar wind, and measurement of the frozen‐in silicon charge state dist

ISSN:0148-0227    

DOI:10.1029/JA092iA02p01057

年代:1987

数据来源: WILEY

摘要:

A model is constructed for the evolution of an interstellar pickup ion distribution in the solar wind. The model assumes that the ions are immediately isotropized at ionization and follows the subsequent development of the distribution function as the particles are convected with the solar wind. The effects of energy diffusion in an ambient wave field with a power law spectrum, adiabatic deceleration in the expanding solar wind, and continual addition of newly ionized particles are all included in the model. An analytical expression describing the evolution of the distribution function in phase space velocity and heliocentric radius is obtained. The distribution quickly approaches an asymptotic shape in phase space which depends on the relative efficiency of the energy diffusion process compared to that of adiabatic deceleration. At large distances from the sun the density of pickup ions falls asr−1in this model. The distribution function at large distances and for large particle speed υ has the formF∼ υ−3/2exp (−αυ³−γ), where γ is the index of the power law wave spectrum and α is a constant related to the energy diffusion coefficient. The asymptotic shape should describe the distribution of pickup ions in the outer heliosphere and could be used as an input distribution for a model of the anomalous component of cosmic rays. Comparison of this work with the recent observation of He+at 1 AU implies that the energy diffusion process is very

ISSN:0148-0227    

DOI:10.1029/JA092iA02p01067

年代:1987

数据来源: WILEY

摘要:

The shapes of three typical examples of electron distribution functions, which have been observed by Helios 2 in the solar wind, are analyzed and compared with theoretical predictions. We have considered a distribution function with a “narrow strahl” (narrow beam), which is extremely anisotropic and skewed with respect to the magnetic field direction at particle energies above 100 eV, a distribution function with a “broad strahl” (broad beam), which is less anisotropic and skewed, and finally a nearly isotropic distribution function which, however, shows a slight bidirectional anisotropy. The main results are as follows: (1) For each distribution function we may discern a “break,” i.e., a sudden change in the slope of the distribution function, separating the “core” at lower energies from the “halo” at larger energies. For the anisotropic distributions a significant break is observed in velocity directions opposite to the strahl and perpendicular to it but not along the strahl. Here the energy of the break (breakpoint energy) may be determined both by the interplanetary electrostatic potential and by collisions. In contrast, for the nearly isotropic distribution function, a significant break is observed for all velocity directions, and the breakpoint energy may be determined by collisions only. (2) The strahl observed at larger energies in the anisotropic distribution functions can be qualitatively explained by existing theoretical approaches describing the propagation of electrons in the solar wind. However, at least for the distribution function with the broad strahl as well as for the nearly isotropic distribution function, the halo electrons should be scattered by unknown anomalous scattering processes, which do not show a strong energy dependence. (3) For the anisotropic distribution functions we find a velocity shift between the peak of each distribution function and the solar wind bulk velocity, which is typically 100 km s−1to 300 km s−1. This shift is drastically reduced compared to the shift predicted by exospheric theory, indicating strong frictional processes between electrons and ions. However the results do not settle the question whether this friction is provided by the combined action of wave‐particle interactions and Coulomb collisions or by Coulomb collisions only. For the nearly isotropic distribution function this shift is probably not significantly different from zero. In this case it may be determined by some anomalous processes and/or trapping in closed magnetic field structures. (4) For the anisotropic distribution functions the heat flux is carried mainly by the strahl. For the nearly isotropic distribution function most of the heat flux is carried by the core electrons. For this distribution, part of the halo electrons carry heat flux in the opposite direction, and the total heat flux is probably not significantly different from zero. (5) The pitch angle distribution in the energy regime of the halo may provide some indications for the global struct

ISSN:0148-0227    

DOI:10.1029/JA092iA02p01075

年代:1987

数据来源: WILEY

摘要:

Electron distribution functions with unusual features, which have been observed on rare occasions in the solar wind by the Helios probes, are presented. Two examples show a strong symmetric bidirectional anisotropy in the energy regime of the halo up to particle energies of 800 eV (double‐strahl distributions). Another example shows an unusually strong bidirectional anisotropy in the energy regime of the core (below 150 eV). The infrequently observed double‐strahl distributions provide evidence that magnetic field loops can exist in the solar wind where electrons are trapped. In addition, they provide evidence that in the case of electrons trapped in closed magnetic field structures the break in the energy spectrum separating the core from the halo is produced only by collisions. On the other hand, the class of distribution functions with strongly anisotropic cores indicates that in the case of “open” magnetic field lines the break between core and halo is largely determined both by the interplanetary electrostatic potential and by col

ISSN:0148-0227    

DOI:10.1029/JA092iA02p01093

年代:1987

数据来源: WILEY

摘要:

Variations of electron distribution functions in the solar wind have been investigated using the electron data observed aboard Helios 2 during the first four months of its mission in 1976 in the distance range between 0.3 and 1 AU. In particular, variations across the sector structure of the interplanetary magnetic field and across the plasma stream structures have been studied. It has been found that there is a strong correlation between the electron properties and the sector structure of the magnetic field. Within the interior of magnetic sectors the electron distribution functions are extremely anisotropic and skewed with respect to the magnetic field direction at high particle energies (above 100 eV); i.e., the distribution functions have a narrow strahl (beam) directed along the magnetic field away from the sun. Often a slight indication of a second strahl directed along the magnetic field toward the sun has been found. Toward sector boundaries the electron distribution functions become less anisotropic and less skewed. Right at sector boundaries the electrons are relatively cool, and their distributions are nearly isotropic, often showing a slight bidirectional anisotropy. These observations have been interpreted to indicate that scattering of electrons with energies above 100 eV is weak within the interior of magnetic sectors but anomalous scattering increases drastically toward sector boundaries for all energies in the halo regime up to several hundred eV. There is evidence for closed magnetic field structures, probably occurring frequently in the solar wind, where the magnetic field lines should usually be connected to the sun outside sector boundaries but may or may not be disconnected from the sun at sector boundaries.

ISSN:0148-0227    

DOI:10.1029/JA092iA02p01103

年代:1987

数据来源: WILEY

摘要:

Until now, studies of shock drift acceleration (SDA) at fast mode collisionless shocks have been confined to particles with initial energies greater than 10 keV. We present for the first time the results of test particle calculations of the interaction of low‐energy (1–8 keV) protons with both strong and weak oblique shocks. Since it is expected that the details of the shock structure will be important, we use a one‐dimensional hybrid, fully self‐consistent simulation to provide a realistic, time‐varying model of the shock fields. We follow the evolution, in these fields, of an isotropic, monoenergetic population of particles released just upstream of the shock. Our results indicate that SDA is a viable mechanism at oblique shocks for initial energies down to those normally classified as superthermal. We find that there is a minimum initial energy below which particles are not reflected; this threshold energy increases with both θBn(the shock normal angle) and MA(the Alfvén Mach number), and therefore implies that superthermal ions are more readily reflected as θBnand/or MAdecreases. We suggest that the reflection of superthermal to very mildly energetic (around 1 keV) ions at the earth's bow shock could provide an explanation for the most energetic field‐aligned beams observed in the earth's foreshock. We have compared our results with those from a simplified model of a finite width shock transition, and we find that, statistically, it provides remarkably good agreement with the hybrid simulation results. The greatest differences between the two models appear for transmitted ions whose initial energy

ISSN:0148-0227    

DOI:10.1029/JA092iA02p01119

年代:1987

数据来源: WILEY

摘要:

The neutral gas emitted by comets is partly photoionized along its path. The interaction of the ions with the solar wind leads to observable particle and wave effects in the ambient plasma. These are described in the present paper.

ISSN:0148-0227    

DOI:10.1029/JA092iA02p01131

年代:1987

数据来源: WILEY

摘要:

The high‐resolution plasma electron and magnetic field data sets from the ICE tail traversal of comet Giacobini‐Zinner have been combined to make a detailed study of the draped Giacobini‐Zinner magnetotail in general, and its field‐reversing current sheet in particular. The geometry of the magnetotail at the time of the ICE crossing is determined and is shown to be consistent with a circular tail cross section rotated 10.5° in the normal sense of aberration and 9.9° above the ecliptic plane, bisected by a cross‐tail current sheet which is rotated 43° out of the ecliptic about the solar wind velocity vector. MHD continuity, momentum, and energy equations are combined with the plasma and field observations to determine unmeasured plasma properties at ICE and upstream at the average point along each streamline where the cometary ions are picked up. The ion temperature, beta, and flow speed at ICE range from 1–1.5 × 106K, 1–4, and −20 to −30 km s−1, respectively, in the draped lobes to ∼1.2 × 105K, up to ∼40, and ∼−20 km s−1in the current sheet. Upstream at the average pickup locations, the flow velocity, ion temperature, density, and ion source rates range from ∼−75 km s−1, ∼4 × 106K, ∼20 cm−3, and ∼1.5 cm−3s−1in the regions upstream from the lobes to ∼ −12 km s−1, ∼1 × 105K, 200–600 cm−3, and ∼3.6 cm−3s−1in the prime mass‐loading region upstream from the current sheet. Gradients in the plasma properties at the edges of this region are quite strong, and the diameter of the region is ∼1500 km, in good agreement with the expected size scale of the cometary ionopause; implications of our inferred plasma properties in this region are examined. The derived and measured flow velocity and measured plasma density at ICE are combined to calculate the transport rate of ions past the plane of the ICE encounter, and we calculate that ∼2.5–5 × 1028ions per second (neutrals per second in the steady state

ISSN:0148-0227    

DOI:10.1029/JA092iA02p01139

年代:1987

数据来源: WILEY

摘要:

An accurate analytical theory of the absorption of energetic magnetospheric particles by an inert satellite is developed for the case in which the radiusbof the satellite is much less than the equatorial gyroradiusrgof the particles and in whichrgis in turn much less than the radiusrof the satellite's orbit. In previous interpretations of Pioneer 11 observations an estimate of the lifetime against absorption of energetic protons (E>80 MeV) at Saturn's ring G has been made. This lifetime is used in the framework of the absorption theory to establish an upper limit on the sizes of shepherding satellites associated with the ring. The resulting upper limit, ignoring the absorption of the optically observed particulate matter, is ∑i= 1Nbi2= 6 km2(uncertain by a factor of 2) for an assemblage ofNsatellites of various radiibi. It is noted that ring G lies outside the Roche limit. No shepherding satellites at ring G were detected optically by Voyagers 1 and 2, though the lower limit of sensitivity of this technique wasb≈ 10 km for an albedo exceeding 0.1 and the searches were not comprehensive in longitudinal cover

ISSN:0148-0227    

DOI:10.1029/JA092iA02p01153

年代:1987

数据来源: WILEY