Return current region aurora: E∥, jz, particle energization, and broadband ELF wave activity

Abstract

In this paper we consider ion heating in downward current, downward electric field, premidnight auroral regions. We show a case study, a test particle model, and a statistical study. The case study and statistical study use electron, ion, wave, and field-aligned current data and include wave spectra and ion pitch angle distributions. The ion pitch angle distribution functions are used to calculate, through the momentum balance equation, a self-consistent local parallel electric field. The test particle model shows ion pitch angle distributions and moments resulting from a "pressure-cooker" arrangement of magnetic and electric fields. This simple model can reproduce the ion pitch angle distributions and the moments of these distributions as seen in the statistical database; the parallel electric field calculated from these moments is consistent with the imposed electric field profile. The statistical database uses data from 23 premidnight auroral Fast Auroral Snapshot crossings. The statistical study shows that the ion to electron energy ratio varies from 1 to 10; the broadband ELF wave power is controlled by the local value of j z - E∥; the relationship between jz and E∥ is controlled by ni, and the wave power near the cyclotron frequency is sufficient to provide the observed ion energies for oxygen in all cases and the observed ion energies for hydrogen for events near the poleward edge. The strong correlation between the wave power and j 2 · E∥ validates the use of the ion distributions as a probe of the local E∥. As expected from theoretical arguments, the current-voltage relationship in these regions is seen to be controlled by the local ionospheric density. Allowing for suitable range in values of the density, the model can be applied to both strong return-current regions within the auroral cavity and weak-field regions at the poleward boundary. Copyright 2002 by the American Geophysical Union.