A thermal mechanism for generation of small‐scale irregularities in the ionospheric E region
A thermal mechanism for generation of small‐scale irregularities in the ionospheric E region
复制标题
电离层 E 区小尺度不规则性产生的热机制
DOI：
10.1029/1999ja900415
复制
发表时间：
2000
影响因子：
--
通讯作者：
M. Kelley
中科院分区：
文献类型：
--
作者：
L. Kagan;M. Kelley
我们提出了一种由于中性风驱动的热不稳定性而在 E 区域产生小规模不规则性的机制。中性风引起的极化电场由于离子中性碰撞而引起摩擦加热。在等离子体耗尽区域，这导致等离子体压力增强并且等离子体被挤出，从而增强等离子体耗尽。当包括热平流时，不稳定的阈值低于中纬度E地区常见的中性风。导致不稳定性增长的另一个因素是离子中性碰撞频率对离子温度的依赖性。由于离子-中性能量交换和热平流过程中离子冷却速率降低，这也导致中性风阈值更小。在标准的双流体处理中，热理论预测了产生小规模不规则性的两个优选高度范围：93 至 102 公里之间以及接近 110 公里高度。我们发现，热过程可能是中层和高层大气雷达以及频率捷变雷达在九州零星E实验（SEEK）活动期间分别针对3.2米和6.1米场对准不规则性观测到的连续和“厚”连续回波的原因。如果不规则伸长率强于双极扩散引起的伸长率，则该理论可以解释 SEEK 活动在高达 150 公里高度处观察到的 6.1 米回波。预测的相速度远小于离子声速，这使我们能够将不稳定性与 2 型雷达回波联系起来。
We present a mechanism for small-scale irregularity generation in the E region due to a thermal instability driven by the neutral wind. The polarization electric field induced by the neutral wind causes factional heating due to ion neutral collisions. In plasma-depleted regions this leads to enhanced plasma pressure and plasma is forced out, enhancing the plasma depletion. When heat advection is included, the threshold for instability is lower than neutral winds commonly observed in the midlatitude E region. Another factor contributing to the instability growth is the dependence of the ion neutral collisional frequency on ion temperature. This also results in a smaller threshold neutral wind due to both a reduced ion cooling rate during ion-neutral energy exchange and heat advection. In a standard two-fluid treatment the thermal theory predicts two preferable altitude ranges for small-scale irregularity generation: between 93 and 102 km and near 110 km altitude. We find that the thermal processes may be responsible for the continuous and “thick” continuous echoes observed by the middle and upper atmosphere radar and by the frequency agile radar during the Sporadic E Experiment over Kyushu (SEEK) campaign for 3.2-m and 6.1-m field-aligned irregularities, respectively. The theory may explain the SEEK campaign observations of 6.1-m echoes at as high as 150 km altitude if the irregularity elongation is stronger than that due to ambipolar diffusion. The predicted phase velocities are much smaller than the ion acoustic velocity, which allows us to associate the instability with the type 2 radar echoes.