资源环境科技发展态势分析平台

The activity of nighttime medium-scale traveling ionospheric disturbance (MSTID) in middle latitudes (+/- 25 degrees-40 degrees magnetic latitudes) is investigated using the measurements of the electron density by Swarm satellites. Fractional electron density fluctuation is used as a proxy for MSTIDs, and the MSTID distribution is derived from the observations in December 2013-January 2017. The MSTID activity shows a semiannual variation in most longitudes with the primary and secondary peaks during the June and December solstices, respectively. The distinguishing feature in the MSTID distribution is the high occurrence rate of MSTIDs around 150 degrees E longitude in June-August. The longitudinal and annual distribution of MSTIDs shows a hemispheric symmetry, which we interpret as the demonstration of conjugacy in MSTIDs.