Akebono observations of electron temperature anisotropy in the polar wind

Abstract

We report Akebono observations of anisotropic electron velocity distributions in the sunlit polar wind, in which the averaged energy (temperature) of the thermal energy electrons is higher in the upward magnetic field direction than in the downward and perpendicular directions. In the 1500–4000 km altitude region, the observed upward to downward temperature ratio, Teu/Ted, was in the range of 1.5 to 2. The observed downward and perpendicular temperatures were similar; Ted/Te⊥ ≃ 1. The heat flux associated with the observed thermal energy electron velocity distribution was upward and on the order of ∼10−2erg cm−2s−1, and was a factor of 5 larger than that of the atmospheric photoelectrons above 10 eV. In this altitude region, the higher‐energy (> a few eV) photoelectrons are essentially collisionless. In contrast, the ambient and lower‐energy photoelectrons remain collisional because of their larger Coulomb collision cross sections. In the steady state, an ambipolar electric field is required to maintain quasi‐neutrality along the field line. It is suggested that the ambipolar electric field and the Coulomb collisions modify the velocity distributions of the thermal energy electrons and the photoelectrons, resulting in the observed temperature anisotropy. The upward heat flux associated with the observed temperature anisotropy dominates any downward heat flux due to electron temperature gradients that may be present in the polar wind plasma and demonstrates the important role of the photoelectrons in the dynamics of the polar wind.