Average characteristics of solar wind electron velocity distributions as well as the range and nature of their variations are presented. The measured distributions are generally symmetric about the heat flux direction and are adequately parameterized by the superposition of a nearly bi‐Maxwellian function which characterizes the low‐energy electrons and a bi‐Maxwellian function which characterizes a distinct, ubiquitous component of higher‐energy electrons. An alternate self‐consistent description of the higher‐energy component is presented in terms of an unbound population of hot electrons with energy greater than some breakpoint energy of ≃60 V. The largest‐scale parameter variations appear to come most often in association with high‐speed streams. The salient electron parameter variations associated with these structures are presented and discussed. The mechanism by which interplanetary electrons conduct heat is convection of the hot component relative to the bulk speed. Arguments are presented which favor the local regulation of the solar wind heat flux at 1 AU.

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 -• to 300 km s -•. 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 structure of the magnetic field. 1.For the ...

Comprehensive plasma observations carried out on board the Heos 2 satellite have provided the first systematic description of plasmas in the distant polar magnetosphere. These observations have revealed the presence of a persistent layer of tailward-flowing magnetosheathlike plasma inside of and adjacent to the magnetopause. This region has been termed the 'plasma mantle.' The mantle has been found to extend over the entire surface of the magnetosphere tailward of the polar cusp and northward of the plasma sheet. Vela observations of a 'magnetotail boundary layer' obtained in the vicinity of the plasma sheet by Hones and coworkers refer to the same phenomenon. The salient features of the plasma mantle as provided by Heos measurements from February to December 1972 can be summarized as follows: (l) The mantle was found to be present in over 70% of the passes through the polar magnetosphere in the region described above. (2) Its thickness varies greatly, ranging up to >•4 R•, and does not appear to depend significantly on position or the state of the magnetosphere as measured by Kp. (3) A tailward-directed bulk flow parallel to the local terrestrial magnetic field was nearly always distinctly measurable. It was found to lie usually between 100 and 200 km s -• and was always less than the concurrent flow speed in the nearby magnetosheath. (4) The flow speed in the mantle is positively correlated with the flow speeds in the magnetosheath and solar wind but depends only very weakly, if at all, on distance from the polar cusp, i.e., on XOSM. (5) A narrow region of low density and/or low flow speed plasma, i.e., a 'gap,' 0.1-0.2 R• thick, is frequently observed between the plasma mantle and the magnetopause. (6) The mantle protons are normally significantly cooler along B than perpendicular, i.e., T• < Tl. (7) The proton density, temperature, and bulk speed all tend to decrease gradually with depth inside the magnetopause, but this trend can at times be obscured by fluctuations and magnetopause motions. At the inner edge of the mantle the proton distribution is often very narrow in both energy and angle, i.e., relatively cold, before it finally disappears below the 100-eV threshold of the instrument. It is concluded, after an appraisal of several possible mechanisms, that the most probable cause for the formation of the mantle is the day side merging of terrestrial and interplanetary field lines. now appears that this layer forms almost a complete cover of the magnetosphere with the exception of the day side region between the cusps and at low latitudes in the magnetotail adjacent to the plasma sheet, it will be referred to herein as the plasma mantle. INSTRUMENTATION The observations reported here were obtained with the MPI plasma analyzer flown on the Heos 2 spacecraft. Heos 2 was launched in January 1972 into a highly eccentric polar orbit with initial apogee and perigee heights of 240,194 km (38.7-R• geocentric distance) and 409 km, respectively. The inclination of the orbital plane was approximately 90 ø , and the ...

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Concurrent measurements of electron and proton differential energy spectrums (each spectrum measurement requiring from 0.1 to 0.3 sec) have been obtained near the earth’s bow shock with the Vela 4B electrostatic analyzer. The following results have been derived from an analysis of 26 shock crossings during May and June 1967: (1) The jump in proton temperature is 2 to 4 times greater than the jump in electron temperature. (2) In the magnetosheath the proton temperature is nearly always greater than the electron temperature. (3) Te/Tp, upstream from the shock ranges between 0.6 and 4, and the shock remains well defined over this range. (4) Electron thermalization usually occurs in much less than 3 sec and has been observed to occur in ≈ 0.03 sec. (5) The magnetosheath electron velocity distribution is flat‐topped or sometimes somewhat concave in shape within at least a few RE of the shock. (6) Evidence has been found that electrons are thermalized in a thin region upstream from the region in which most of the proton thermalization occurs. The observed increase in Te/Tp resulting from electron preheating may account for destabilization of electrostatic ion waves that may then produce strong ion heating by nonlinear Landau damping. Representative proton and electron velocity distributions from which the above results were derived are presented.