中文摘要：

大量的实验研究表明，在顶部极光区电离层，利用EISCAT非相干散射雷达和Millstone Hill雷达可以观测到不对称、增强的离子声波谱线．考虑到低能H^＋离子束沉降到背景电离层，以及电离层顶部O^＋离子的外流事件，采用含有场向热流项的双麦克斯韦分布描述H^＋离子束的分布函数，进而基于离子-离子双流不稳定性理论来解释增强的离子声波谱线．场向热流的引入可以减小离子声波谱线的不对称现象，这样得到的结果更符合实际．

英文摘要：

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.