Variations in field-aligned plasma velocity with altitude and latitude in the auroral zone: EISCAT observations and the physical interpretation

Two days of EISCAT data (69.6°N, 19.2°E, obtained during the Northern Hemisphere summer period) were used to study the variation of field-aligned plasma velocity with altitude and latitude. During periods of intense frictional heating an electron density hole forms at about 260 km. The corresponding profile of field-aligned velocity shows downward ion flows above this height and upward flows below it. The magnitudes of both velocities are typically in the range 50-80 ms^-1 and result from a balance between; diffusion, neutral atmosphere expansion, gravity and the effects of a meridional neutral wind. As the electron density hole refills, a decrease in both the upward and downward diffusion velocities is observed. After a short period the flow becomes upward at all heights, with velocities typically 70 ms^-1. Large upward ion fluxes are observed near the polar cap/auroral zone boundary in the midnight-dawn sector. These are larger than required to support the light ion outflow at greater heights and occur in a region where low altitude transverse acceleration mechanisms are common. However, the observed velocities are much higher than those expected if the main mechanism causing the upflowing thermal ions was diffusion from lower altitudes to fill the hole left by upflowing energised ions at greater heights. Recent model results show that large upward fluxes of ions will occur in response to ion heating. This is consistent with the observations presented in this paper and the particularly good agreement between the temporal and spatial morphology of the frictional heating and upflowing ions favours this mechanism to explain the observations.