Observations of electrons of energy 125 ev to ∼2 kev with the OGO 1 satellite and of electrons of energy 40 ev to ∼2 kev with OGO 3 (by means of modulated Faraday cup detectors) are used to investigate the low-energy electron population in the magnetosphere within the local-time range ∼17 to ∼22 hours. Intense fluxes of these electrons are confined to a spatial region, termed the plasma sheet, which is an extension of the magnetotail plasma sheet discovered by the Vela satellites and is identified with the soft electron band first detected by Gringauz. The plasma sheet extends over the entire local-time range studied in this investigation, from the magnetospheric tail past the dusk meridian toward the dayside magnetosphere. In latitude it is confined to within 4–6 RE of the geomagnetic and/or solar magnetospheric equatorial plane, in agreement with observations already reported from the Vela satellites; no electron fluxes are detected high above the equator, not even very near the magnetopause. In radial distance the plasma sheet is terminated by the magnetopause on the outside and by a well-defined sharp inner boundary on the inside. The inner boundary has been traced from the equatorial region to the highest latitudes investigated, ∼40°; during geomagnetically quiet periods, it is observed at an equatorial distance of 11 ± 1 RE and appears to extend to higher latitudes along magnetic field lines. Weak or no electron fluxes are found between the inner boundary of the plasma sheet and the outer boundary of the plasmasphere. Detection (by an indirect process) of the very high ion densities within the plasmasphere gives positions for its boundary in good agreement with other determinations. During periods of magnetic bay activity, the plasma sheet extends closer to the earth; the inner boundary of the plasma sheet is then found at equatorial distances of 6–8 RE. This is most simply interpreted as the result of an actual inward motion of the plasma during a bay. In one case, it was possible to associate the beginning of this motion with the onset of the bay and to estimate an average radial speed of ∼12 km/sec, from which an electric field corresponding to ∼48 kilovolts across the magnetospheric tail was inferred. Within the plasma sheet, the electron population is characterized by number densities from 0.3 to 30 cm−3 and mean energies from 50 to 1600 ev and higher, with a strong anticorrelation between density and mean energy, so that the electron energy density (∼1 kev cm−3) and energy flux (∼3 ergs cm−2 sec−1) show relatively little variation. The lower energies and higher densities tend to occur during periods of geomagnetic disturbance. The nonobservation of electrons in regions above the plasma sheet implies an upper limit on the electron number density of 5 × 10−2 cm−3 if their mean energy is assumed to be ∼50 ev (typical of the magnetosheath) and 10−2 cm−3 if the energy is ∼1 kev (typical of the plasma sheet). At the inner boundary of the plasma sheet there is a sharp softening of the electron spectrum with decreasing radial distance but apparently little change in the electron number density. The electron energy density decreases across the inner boundary roughly as ∼exp (distance/0.4 RE) during quiet periods; during times of magnetic bay activity the energy density decreases as ∼exp (distance/0.6 RE), and there is a more complicated spatial structure of density and mean energy.

A survey of low-energy electrons in the evening sector of the magnetosphere with OGO 1 and OGO 3.

It is proposed that the semiannual variation in geomagnetic activity is caused by a semiannual variation in the effective southward component of the interplanetary field. The southward field arises because the interplanetary field is ordered in the solar equatorial coordinate system, whereas the interaction with the magnetosphere is controlled by a magnetospheric system. Several simple models utilizing this effective modulation of the southward component of the interplanetary field are examined. One of these closely predicts the observed phase and amplitude of the semiannual variation. This model assumes that northward interplanetary fields are noninteracting and that the interaction with southward fields is ordered in solar magnetospheric coordinates. The prediction of the diurnal variation of the strength of the interaction at the magnetopause by this model, does not, however, match the diurnal variation of geomagnetic activity as derived from ground-based data.

https://www.leif.org/EOS/JA078i001p00092.pdf

Semiannual variation of geomagnetic activity

Such nonthermal features as temperature anisotropies, heat fluxes, and proton double streams have been observed by a Helios solar probe survey of solar wind three-dimensional proton velocity distributions between 0.3 and 1 AU. It is found that a strong anisotropy in the core of proton distributions, with a temperature that is larger perpendicular rather than parallel to the magnetic field, is a persistent feature of high-speed streams, becoming most pronounced in the perihelion, or about 0.3 AU. Isotropic distributions have been detected only close to, and at, magnetic sector boundaries, and the flattest radial temperature profiles are found in high-speed streams. These observations indicate that local heating or proton heat conduction occurs in the solar wind.

Solar wind protons: Three-dimensional velocity distributions and derived plasma parameters measured between 0.3 and 1 AU

Kinetic plasma physics of the solar corona and solar wind are reviewed with emphasis on the theoretical understanding of the in situ measurements of solar wind particles and waves, as well as on the remote-sensing observations of the solar corona made by means of ultraviolet spectroscopy and imaging. In order to explain coronal and interplanetary heating, the microphysics of the dissipation of various forms of mechanical, electric and magnetic energy at small scales (e.g., contained in plasma waves, turbulences or non-uniform flows) must be addressed. We therefore scrutinise the basic assumptions underlying the classical transport theory and the related collisional heating rates, and also describe alternatives associated with wave-particle interactions. We elucidate the kinetic aspects of heating the solar corona and interplanetary plasma through Landau- and cyclotron-resonant damping of plasma waves, and analyse in detail wave absorption and micro instabilities. Important aspects (virtues and limitations) of fluid models, either single- and multi-species or magnetohydrodynamic and multi-moment models, for coronal heating and solar wind acceleration are critically discussed. Also, kinetic model results which were recently obtained by numerically solving the Vlasov‐Boltzmann equation in a coronal funnel and hole are presented. Promising areas and perspectives for future research are outlined finally.

/pdf/kinetic-physics-of-the-solar-corona-and-solar-wind-bduwadfu6p.pdf

Kinetic Physics of the Solar Corona and Solar Wind

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.

Solar wind electrons

Observations of solar wind interpenetrating ion streams are reviewed. They occur preferentially during periods of low but increasing plasma number density and most likely result from spatial and/or temporal inhomogeneities in the solar wind bulk velocity. The exact nature of these inhomogeneities is not known, but three conceptual models that account for the observations are summarized. Double ion streams are shown to occur during extended time periods during which the proton anisotropy and dimensionless heat flux undergo large coherent variations. This fact provides strong evidence that a major source for solar wind internal ion energy is its bulk convection energy. The observed continuum of proton velocity configurations further suggests that the ion internal energy state at 1 AU is often perturbed by bulk flow spatial and/or temporal inhomogeneities. Self-consistent interactions with the interplanetary wave mix that results from these inhomogeneities as well as other aspects of the solar wind expansion establish preferred proton velocity configurations. These configurations can be summarized by simple statistical relations that couple the two independent third velocity moments to the second moments.

Interpenetrating solar wind streams.

The thermal anisotropies near the peaks of proton distributions are observed to vary characteristically across two simple high speed streams. Before the time of maximum velocity gradient T∥ of the core distribution is greater than T⊥ but afterwards, and persisting throughout the remainder of the high speed region, T∥ < T⊥. This implies that it may not be necessary to invoke extended ion heating close to the sun to explain enhanced ion temperatures associated with high speed streams and that the T-V relation may be primarily the result of local heating near 1 AU.

Evidence for local ion heating in solar wind high speed streams

Twenty-seven-day averages of the solar-wind density and flow speed, observed by Vela 3 and 4 spacecraft between July 1965 and July 1968, are found to vary with the heliographic latitude of observation. High average densities and low average flow speeds were measured near the solar equator; low average densities and high average flow speeds were measured near the northern and southern extremes of the earth's heliographic latitude excursion. Possible instrumental and statistical explanations of this pattern of variations are discussed and found to be unlikely. The variations can be reasonably interpreted in terms of a latitude-dependence in the structure of high-speed solar-wind streams related to solar activity.

Variations of solar‐wind plasma properties: Vela observations of a possible heliographic latitude‐dependence

The stability of a plasma consisting of two, unequal, isotropic, ion beams streaming along a uniform magnetic field has been investigated by numerically solving the full electromagnetic, Vlasov, linear dispersion relation for high-$beta$ plasmas. Three instabilities are found: One is closely associated with the usual Alfven mode and the two others with the ''fast'' or magnetosonic mode. The importance of these instabilities for certain neutral- beam-injection experiments and in the solar wind is emphasized.

Michael D. Montgomery

Papers

Interpenetrating solar wind streams.

Evidence for local ion heating in solar wind high speed streams

Variations of solar‐wind plasma properties: Vela observations of a possible heliographic latitude‐dependence

Electromagnetic ion-beam instabilities in the solar wind

SPATIAL DISTRIBUTION, ENERGY SPECTRA, AND TIME VARIATIONS OF ENERGETIC ELECTRONS (E > 50 kev) AT 17.7 EARTH RADII