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Interpretation on the Origin of Enhanced Ion Acoustic Line Spectra in the High Ionosphere

Xue Kun;Wu Jian; Xu Bin; Guo Lixin   

  • Received:1900-01-01 Revised:1900-01-01 Online:2010-01-15 Published:2010-01-15

Abstract: A number of observations showing enhanced ion acoustic echoes observed in the topside auroral ionosphere by means of both the EISCAT and Millstone Hill radars have been reported in the literature. The received power, often asymmetric, is extremely enhanced by up to 1 or 2 orders of magnitude above usual values, and it is mostly contained in one of the two ion acoustic lines. The instability processes have been proposed in many literatures to explain these phenomena, and one of the theories is the ion-ion two-stream instability theory, which is driven by large relative drifts between two or more ion species, for example O+ and a beam of H+. As long as the drift velocity is of the order of the species thermal velocity, the ion acoustic fluctuation level can be enhanced well above the thermal level, leading to an enhancement in one or both ion lines. The high relative drifts require a sufficient acceleration of H+, which might be possible at high altitudes. In this paper, the low energy H+ ion precipitation and the O+ ion outflow events are considered. The H+ ion distribution function described by the bi-Maxwellian contains the field-aligned heat flow, then the enhanced ion acoustic echoes can be interpreted using the ion-ion two-stream instabilities. The asymmetry is weakened due to the consideration of field-aligned heat flow.