Global distribution of nighttime medium-scale traveling ionospheric disturbances seen by Swarm satellites

The activity of nighttime medium-scale traveling ionospheric disturbance (MSTID) in middle latitudes (±25°-40° magnetic latitudes) is investigated using the measurements of the electron density by Swarm satellites. Fractional electron density fluctuation is used as a proxy of MSTID and the MSTID distribution is derived from the observations in December 2013-January 2017. The MSTID activity shows a semi-annual 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 (near 100 %) of MSTIDs around 150°E longitude in June-August. The longitudinal and annual distribution of MSTID shows a hemispheric symmetry, which we interpret as the demonstration of conjugacy in MSTIDs.