Ionospheric ion acceleration by multiple electrostatic waves

Abstract

Observations by Topaz 3 show ionospheric O+ and H+ ions, with ambient energies of around 0.3 eV, to be transversely (to the geomagnetic field) energized, to around 10 eV, within lower hybrid structures composed of broadband large‐amplitude (100–200 mV/m) electrostatic waves. In this paper we show that the energization of O+ ions can be explained by a new nonlinear coherent interaction mechanism involving multiple electrostatic waves propagating across the magnetic field. Low‐energy ions, whose velocities are well below the phase velocities of the waves, are shown to gain energy monotonically increasing in time when averaged over a cyclotron orbit. We examine the properties of this coherent energization mechanism numerically and by an analytical, multiple timescale, analysis. We find, in accordance with observations, that the tail of the O+ distribution is most likely to be energized. The analysis provides the spatial extent, along the geomagnetic field, of the lower hybrid structures needed for the observed energization.