Global Characteristics of Field-Aligned Acceleration Processes Associated with Auroral Arcs.

Abstract

To study the global characteristics of field-aligned acceleration processes associated with auroral arcs, we have analyzed the particle and aurora image data obtained from the DMSP-F6 and -F7 satellites. The intensities of auroral emissions for 5577-Å, 6300-Å and N2 (1PG) have been calculated using the observed electron fluxes and the two-stream electron transport code to show that the fluxes of precipitating electrons are sufficient to produce visible auroral arcs. Then, the density and thermal energy of these electron fluxes at magnetospheric altitude have been estimated by fitting the accelerated Maxwellian distribution function to the observed electron energy spectra and by evaluating the effect of electron heating during the acceleration. We have obtained global distributions of the field-aligned potential difference, magnetospheric electron density and thermal energy, electron heating rate during the acceleration and auroral emission intensities for magnetic quiet and active periods, respectively. It is found that the field-aligned potential difference increases as magnetospheric electron density and adiabatic field-aligned conductivity decrease, suggesting the constant-current generator for the magnetosphere-ionosphere current circuit. The magnetospheric source electrons of dayside arcs are characterized by high density and low thermal energy, particularly in the quiet period. It is suggested that accelerated electron precipitation events observed on the lower latitude side of the cusp region are connected to the LLBL, while those observed on the higher latitude side of the cusp region are connected to the plasma mantle. In the quiet period, the region of latter events expands to higher latitude region associated with the expansion of the plasma mantle into the magnetospheric lobe region. The nightside events which show low electron density and high thermal energy at the magnetospheric source region appear to have originated from the plasma sheet region. The morning side events are a mixture of high density events and low density events. We consider that the former events have a source in the LLBL or the plasma mantle, while the latter events have a source in the plasma sheet which expands to the tail flank region. These different characteristics of magnetospheric source electrons for different magnetic local time and magnetic latitude seem to produce the variety of auroral arcs on global scale.