Many theoretical studies using different collision models have been devoted to calculating ion velocity distributions in the high latitude ionosphere. In the presence of strong convection electric fields, the ions can be accelerated to drift speeds which exceed the thermal speed of the neutral gas leading to velocity distribution functions distorted from the Maxwellian shape.

With EISCAT we are able to measure three independent components of the ion velocity vector and temperature in the same scattering volume. The ion temperature observed at about 300 km during strong enhancements of the ion velocity is characterized by different temperatures parallel and perpendicular to the magnetic field with T⊥>T∥. For an electric strength of 50 mV/m, an anisotropy factor of about 1.5 was measured.