Study of ionospheric E- and F-region irregularities generated by atmospheric gravity waves

最近的研究表明，电离层中的场向不规则体（FAI）在中纬度E区和F区是活跃的，并具有与大气重力波相关的波浪状结构。本研究的目的是利用中纬度雷达和其他设施观测E区和F区的中纬度FAI，并揭示其产生机制。本研究期间（1995-1997年）正好与被认为是FAI活跃期的太阳活动极小期相吻合，在此期间，还进行了SEEK（九州上空偶发性E实验）项目，这是FAI E区的火箭/雷达协调观测活动。我们参加了SEEK活动，并与国家宇宙开发机构种子岛宇宙中心的通信研究实验室一起操作SRI International的频率捷变雷达（FAR）。FAR观测于1995年8月至10月和1996年7月至9月进行。我们发现，在E区FAI中存在着10-20 mV / m的大极化电场。火箭化学释放实验中观测到的中性风速显示出非常大的垂直切变，可以产生Kelvin-背景大气中的亥姆霍兹不稳定性。SEEK运动的这些结果将作为14篇论文发表在《地球物理研究快报》的特刊上。对于F区FAI，我们发现了100公里尺度的波状结构，类似于帕金斯不稳定性。我们还发现了波长为几十公里的较小结构。从10年的F区FAI观测中，我们可以证实F区FAI的活动与太阳活动有明显的反相关。

Recent research has shown that field-aligned irregularities (FAI) in the ionosphere are active in the midlatitude E-and F-regions, and have wavelike structures associated with atmospheric gravity waves. Purpose of this study was to observe the midlatitude FAI in both E-and F-regions with the MU radar and other facilities, and reveal their generation mechanism. The period of this study (1995-1997) well coincided with the solar minimum condition, which was considered to be active period of the F-region FAI.In this period, the project of SEEK (Sporadic-E Experiment over Kyushu), a rocket / radar coordinated observation campaign for the E-region FAI,was also conducted. We participated in the SEEK campaign, and operated the frequency agile radar (FAR) of SRI International with Communications Research Laboratory in Tanegashima Space Center of National Space Development Agency. The FAR observations were conducted during August-October 1995 and during July-September 1996.From this study, especially from the SEEK campaign, we found that there existed unexpectedly large polarization electric fields of 10-20 mV / m in the E-region FAI.Neutral wind velocity observed with the chemical release experiment from a rocket showed very large vertical shear that could generate the Kelvin-Helmholtz instability in the background atmosphere. These results from the SEEK campaign will be published as 14 papers in a special issue of the Geophysical Research Letter. For the F-region FAI we found 100km-scale wavelike structures that resembles those of the Perkins instability. We found smaller structures with wavelengths of several tens of kilometers as well. From ten-years of F-region FAI observations with the MU radar, we could confirm that activity of the F-region FAI shows clear anti-correlation to the solar activity.