The core electron temperature profile between 0.5 and 1.0 AU in the steady‐state high speed solar wind

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Abstract
The radial dependence of the core electron temperature in the steady‐state high speed solar wind between 0.5 and 1.0 AU is measured using Mariner 10 and Imp 6, 7, and 8 instrumentation. The observed temperature profile is best characterized by two separate power‐law functions which apply in the radial ranges between 0.47 and 0.62 AU and between 0.62 and 1.0 AU, respectively. The separate values of the logarithmic temperature derivatives are αi = −1.14±0.24 and α0 = +0.28 ±0.13, respectively. Whereas the inner value of α, αi, agrees with a previous estimate (αc = −0.9±0.1) using data measured within high speed compression zones of simple high speed streams, the outer value of α, α0, is significantly more positive than any previous theoretical or experimental estimate. A likely explanation is a modest thermal coupling between the protons and electrons by way of kinetic instabilities driven by the generally observed, double streaming proton velocity configurations. The core electron temperature of about 1 × 105 K measured using Mariner 10 instrumentation at 0.47 AU is also significantly less than any value predicted by current models of the high speed solar wind at that distance.