High‐ and low‐altitude observations of adiabatic parameters associated with auroral electron acceleration

Abstract

Electron density and temperature, adiabatic thermal current, and field‐aligned conductivity have been estimated on the basis of observations by the Defense Meteorological Satellite Program (DMSP) satellites above the auroral oval and from measurement by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module (IRM) satellite in the near‐Earth plasma sheet. We found that the estimated densities are comparable between these two satellites, while the temperatures obtained from an accelerated Maxwellian fitting procedure used on the DMSP spectra are far lower than those measured by IRM in the near‐Earth plasma sheet. From this temperature discrepancy we conclude that the accelerated electrons do not come from the equatorial plane of the magnetosphere but from the region just above the field‐aligned potential difference at an altitude of a few RE. The DMSP data show that a large field‐aligned potential difference, which accelerates auroral electrons downward, is formed in the region with low field‐aligned conductivity. The IRM data show that the field‐aligned conductivity decreases with increasing XGSM distance, increasing AE index, and after earthward high‐speed flow passage. The adiabatic thermal current estimated from the IRM data is found to be not enough to supply typical auroral current. Though the mechanism that produces field‐aligned potential difference has not been identified yet, these results suggest that field‐aligned potential difference is formed to keep the balance between the field‐aligned current generated by magnetospheric processes and the current carried away from the generator region by accelerated electrons.