Statistical study on the occurrence of the ionospheric mid-latitude trough and the variation of trough minimum location over northern hemisphere

Using global TEC data with high spatial and temporal resolution from Sep. , 2014 to Aug. , 2017, the occurrence rate of ionospheric mid-latitude trough and the variation of trough minimum location over Northern Hemisphere was studied statistically. Considering the geomagnetic "historical effects" on the evolution of mid-latitude trough, we introduced one index (here defined as Kp9) calculated from 3-hour Kp index. Through comparing with several other geomagnetic indices, it was found that the linear correlation between the latitude of trough minimum and Kp9 index was the best. And then, the longitudinal difference, seasonal variation, local time variation of the trough occurrence rate and the latitudinal location of mid-latitude trough, as well as the relationship between the location of mid-latitude trough and Kp9 index were analyzed. The results showed that the occurrence of mid-latitude trough is little related to longitude and depend primarily on the season, local time and geomagnetic activity. The occurrence rate of mid-latitude trough in summer is lower than other seasons at midnight and shows a negatively skewed distribution, which there was higher occurrence rate after midnight, and the geomagnetic activity promotes the occurrence of mid-latitude trough. As for the latitudinal distribution of trough minimum, the difference in four longitude bands is not obvious, but monthly variations are different in four longitude bands, of which there was unimodal distribution in 120 degrees W with higher latitude in summer months but lower latitude in winter, and on the contrary in 0 degrees longitudinal band the latitude of trough minimum is lower in summer. In addition, the location of mid-latitude trough is significantly dependent on geomagnetic activity, local time and season. In general, the higher the geomagnetic activity, the lower the latitude of the mid-latitude trough. And there was seasonal difference in the variation of trough minimum with local time. In winter, the location of the trough moves rapidly at dusk with the local time and reaches the lowest latitude before 22:00 LT under the condition of weak geomagnetic activity, and after 22 :00 LT the trough minimum location changes little. Comparatively, the minimum location in equinox months decreases from dusk to midnight, and the minimum location in summer months decreases continuously from dusk even to about 03 :00 LT.