A numerical simulation for the omega band formation

Abstract

[1] The formation of the omega bands and torch structures in the recovery phase of a substorm is numerically simulated. The kinetic energies of auroral particles are substantially provided by nonadiabatic acceleration in the tail current sheet. The magnetic drift flux (in the adiabatic sense) of the plasma sheet ions increases with decreasing invariant latitude; it starts to increase significantly around a certain magnetic shell, which is assumed, as a first approximation, to be the interface demarcating the nonadiabatic and adiabatic regions in the tail current sheet. Region 1 field-aligned currents can be generated in the situation that this interface is inclined with respect to the direction of the average magnetic drift velocity. As long as the energy density of injected particles does not significantly change with time, the region 1 current remains stable and no electrostatic waves with long wavelengths (>100 km at the ionospheric height) grow. Since the nonadiabatic particle acceleration is assumed to weaken during the (early) recovery phase of a substorm, the flux-tube-integrated energy density of injected particles attains a latitudinal profile unstable to the hybrid Kelvin-Helmholtz/Rayleigh-Taylor instability. While injected particles with less kinetic energies are transported to magnetic shells at lower latitudes, the unstable region expands in latitude so that long-wavelength waves develop in the plasma sheet. Then omega bands similar to those observed commonly in the substorm recovery phase form. The main characteristics of the omega bands in the simulation are shown to be consistent with observations.