Measurement of Electron Temperature Anisotropy to within 35 Rs

Adiabatic theory predicts that as the solar wind moves away from the Sun, electron temperature should increase in the direction parallel to the magnetic field, becoming highly anisotropic by 1 AU. Observations show on the contrary that electron temperatures remain relatively isotropic over this distance, suggesting some scattering mechanism, such as plasma instabilities driven by the temperature anisotropy, counteracts the magnetic focusing. During the first two orbits of Parker Solar Probe, the Solar Probe Analyzers (SPAN) measured electron distribution functions down to ~35 solar radii, allowing the characterization of the scattering process starting from near its onset. We compute, along with other important plasma diagnostics, the temperature anisotropy of the thermal and suprathermal solar wind electrons using a combination of numerical integration and model fitting, and discuss the dependence of these measurements on heliocentric distance and local plasma properties.