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

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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.