Winter Anomaly and Traveling Ionospheric Disturbances over Japan during DYANA Campaign.

Abstract

The winter anomaly and medium-scale traveling Ionospheric disturbances (MSTIDs) detected during the DYANA campaign in 1990 are analyzed to investigate their characteristics and possible relationship to meteorological phenomena in the lower atmosphere. Anomalous f-min enhancements (FMEs), an indication of the winter anomaly, occurred around January 15 and between January 20 and 29, 1990. For the latter period, the FME region extended down to 31°N. D-region electron density enhancements associated with the FMEs were also recognized from phase measurements of the 10.2kHz Omega signals. The FMEs are not due to solar flares or geomagnetic disturbances, but are related to meteorological processes in the lower atmosphere. Comparison of the FMEs with neutral wind flows observed by the DYANA rockets and MU radar indicates that the FMEs are largest for a few days before the reversal of the stratospheric wind direction from westward to eastward caused by planetary wave activity. There is a 5-day delay between the onsets of the FME and the lower stratospheric (20-hPa) temperature increase. A minor stratospheric sudden warming that appeared around 60°N from February 1 to 14 did not affect the D-region (no FMEs) at least over Japan.A large number of MSTIDs were identified from ionospheric total electron content data. Although a few of these MSTIDs were caused by geomagnetic activities, the majority have sources in the lower atmosphere. We find that daily and diurnal variations of the MSTID occurrence are controlled by F-region electron density (foF2).