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.

Eleven passes of the ISEE satellites through the frontside terrestrial magnetopause (local time 9 -'17 h; GSM latitude 2 0 -43 0 N) have been identified, where the plasma velocity in the magnetopause and boundary laver was substantially larger than in the magnetosheath. This paper examines the nature of°the plasma flow, magnetic field, and energeticparticle fluxes in these regions, with a view to determining whether the velocity enhancements can be explained by magnetic-field reconnection.

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Observations with the Los Alamos Scientific Laboratory (LASL) plasma probe and the Goddard Space Flight Center (GSFC) magnetometer on the IMP 6 satellite show that the magnetospheric boundary layer, first identified along the flanks of the magnetosphere, is also present at the magnetosphere's sunward surface. The magnetic field lines in this sunward sector of the boundary layer are closed, and the plasma flow has a component transverse to the field. These observations suggest that the boundary layer is a site of continual transfer of plasma, momentum and energy from the magnetosheath to the magnetosphere. These transfer processes supply plasma and magnetic field to the magnetotail. Also, they produce, indirectly, the dawn‐to‐dusk electric field across the polar cap, the field‐aligned currents that border the dayside polar cap, and the soft particle fluxes that characterize the cleft precipitation, including recently reported dawn‐dusk asymmetries of these fluxes. Magnetosheath plasma directly enters the outer few hundred to few thousand kilometers of the magnetosphere's surface to form the boundary layer. There it is enabled to flow across the magnetic field (and approximately parallel to the magnetosphere's surface) by becoming electrically polarized. Leakage of the polarization charge along magnetic field lines to the earth produces the dayside high latitude effects mentioned above. The polarizing current flowing across the boundary layer interacts with the magnetic field to oppose the boundary layer plasma flow, taking up its momentum. In this way the magnetic field lines are pulled downstream. The process described here is independent of the interplanetary magnetic field (IMF) and thus may constitute the principal transfer mechanism during prolonged periods of northward IMF when the magnetosphere is very quiet. It is not clear how the effects of southward IMF are superposed on this process.

Measurements with Los Alamos Scientific Laboratory instrumentation aboard Imp 6, 7, and 8 reveal that approximately one third of all high-speed solar wind streams observed at I A U contain a sharp boundary (of thickness less than --•4 X 104 km) near their leading edge, called a stream interface, which separates plasma of distinctly different properties and origins. Identified as discontinuities across which the density drops abruptly, the proton temperature increases abruptly, and the speed rises, stream interfaces are remarkably similar in character from one stream to the next. A superposed epoch analysis of plasma data has been performed for 23 discontinuous stream interfaces observed during the interval March 1971 through August 1974. Among the results of this analysis are the following: (1) a stream interface separates what was originally thick (i.e., dense) slow gas from what was originally thin (i.e., rare) fast gas; (2) the interface is the site of a discontinuous shear in the solar wind flow in a frame of reference corotating with the sun; (3) stream interfaces occur at speeds less than 450 km s -• and close to or at the maximum of the pressure ridge at the leading edges of high-speed streams; (4) a discontinuous rise by •40% in electron temperature occurs at the interface; and (5) discontinuous changes (usually rises) in alpha particle abundance and flow speed relative to the protons occur at the interface. Stream interfaces do not generally recur on successive solar rotations, even though the streams in which they are embedded often do. At distances beyond several astronomical units, stream interfaces should be bounded by forwardreverse shock pairs; three of four reverse shocks observed at 1 AU during 1971-1974 were preceded within --• 1 day by stream interfaces. Although stream interfaces often occur in close proximity to reversals in direction of the interplanetary magnetic field, the field reversals generally precede the interfaces by 1• hours to 19 days. Approximately 40% of stream interfaces at 1 AU produce si-in the geomagnetic field. It has been suggested previously that stream interfaces are the result of the nonlinear evolution of high-speed streams produced by a smoothly varying temperature elevation in the solar envelope. Our measurements appear to contradict this thesis: many streams do not appear to be smoothly varying close to the sun, and a temperature elevation in the solar envelope is not required to explain the abrupt jumps in plasma properties which occur at stream interfaces. Our observations suggest that many streams close to the sun are bounded on all sides by large radial velocity shears separating rapidly expanding plasma from more slowly expanding plasma. The shear at the leading edge of such streams becomes the stream interface observed at 1 AU; however, momentum transfer across the interface reduces the magnitude of the speed jump across the shear with increasing distance from the sun. The large abrupt increases in electron and proton temperature observed at interfaces are pri...