Results from the DC electric field experiment on the Freja satellite

Abstract

Observations by Freja were the first to show that positive potential structures associated with downward field-aligned electric fields exist in the downward field-aligned current region sometimes connected with black aurora. They form a counterpart to the negative potential structures with upward parallel electric fields observed on auroral field lines. Different mechanisms are likely to contribute to the downward parallel electric fields at different altitude regimes, resulting in an altitude distribution which at present is unknown. Downward parallel electric fields at low altitudes may be generated as a consequence of exceptionally low plasma densities caused by the erosion of ionospheric electrons and perpendicular transport of ions in a diverging electric field. High-resolution data from a satellite crossing through the head of a Westward Travelling Surge reveal a very inhomogenous and complicated picture of the surge, such as narrowly structured, intense (up to 700 mV/m) converging electric fields, intense electron precipitation and balanced field-aligned currents (up to 30 μA/m 2) embedded within an extended region of intense, high-energy electron precipitation, weak electric fields and field-aligned currents. The data suggest that a significant part of the upward surge current is closed by downward currents in the near surroundings. No evidence is found of a pronounced westward electric field with a southward polarization field, which according to some surge models is expected to be found within the high-conductivity surge region. Finally some data regarding subauroral electric fields are presented.