CURRENT-VOLTAGE RELATIONSHIP IN THE DOWNWARD AURORAL CURRENT REGION

In the auroral zone of the earth currents flow along magnetic field li nes. In the downward current region currents are mainly carried by upf lowing electrons from the ionosphere. Because of the low plasma densit y along auroral field lines, substantial currents in the range of micr oamps per square meter require substantial potential drops parallel to the magnetic field in the range of a few hundred to a few thousand vo lts. The current-voltage relation along such magnetic field lines can be determined for simple profiles of the background ion density by inv oking the condition of charge neutrality. For typical parameters, the current density is found to be a few times larger in the downward curr ent region compared to currents in the upward current region for simil ar potential drops. Thus potential drops up to a few thousand volts an d the consequent acceleration of ionospheric electrons up to keV energ ies, such as has been observed by the FAST satellite, are a necessary consequence of the observed current densities in the downward auroral current region.