Anisotropic thermal electron distributions in the solar wind

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Abstract
Solar wind thermal electrons are normally nearly isotropic, with a temperature ratio, T∥/T⊥, of 1.2 or less. This study presents ISEE 3 observations of unusually anisotropic (T∥/T⊥ = 1.5 to 4) electron distributions in the solar wind near 1 AU. The highly anisotropic events generally share the following characteristics: (1) they have unusually low density, and (2) they are located in the rarefaction regions on the trailing edges of high‐speed streams. Correlation of the electron temperature ratio with other solar wind parameters over a full range of T∥/T⊥ shows a distinct negative correlation with electron density and reveals that the anisotropies are caused by high T∥ rather than by low T⊥. A simple numerical model illustrates that these effects can be described by the competing processes of solar wind expansion and isotropization via Coulomb self‐collisions. However, disagreement between model predictions and observations suggests the need for consideration of other mechanisms, including wave‐particle interactions, for repartitioning of electron energy between the parallel and perpendicular components. The periods of extreme electron anisotropy tend to be coincident with intervals of double ion beams, suggesting similar causal mechanisms for the two phenomena. We conclude that certain aspects of the thermal anisotropies can be explained in a simple manner but that complete understanding will require analysis of additional factors such as collisions in nonthermal distributions and the radial evolution of solar wind structures. In the appendix, analysis of the alignment of the major axes of the electron distributions with the observed magnetic field directions demonstrates that calculation of T∥ and T⊥ self‐consistently from the electron measurements is justified.

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