On the Global Ionospheric Diurnal Double Maxima Based on GPS Vertical Total Electron Content

Ionospheric diurnal double maxima (DDM) is a twin-peak pattern in the ionospheric electron density/total electron content (TEC) during the daytime. Understanding the characteristics of DDM is essential to study the physical mechanisms of the ionosphere. In this paper, the vertical TEC data (VTEC) in 2019-2020 derived from 537 globally distributed GPS stations were used to investigate the DDM phenomenon. The results reveal that the occurrence rate of DDMs is roughly quasi-symmetrical about the magnetic equator. In the northern hemisphere, it first increases, then decreases, and finally increases with the increase of magnetic latitude. The DDM phenomenon also exhibits significant seasonal variation. It mainly appears in summer/winter in the northern/southern hemisphere, and the valley and the second peak usually appear earlier in winter and later in summer. According to the difference in the magnitude of the two peaks of DDM, the DDM phenomenon is mainly manifested as the front peak significant type or the posterior peak significant type. The probability of the former shows a M-shaped variation with the increasing longitude in the middle and high latitudes of the northern hemisphere, and an inverted V-shaped variation in the high latitudes of the southern hemisphere within 180°W~60°W. The probability of the posterior peak significant type shows a trend opposite to the front peak significant type in each area. The occurrence time of DDM structures is usually about one hour later in low-latitude regions than other regions, and the duration is usually shorter than in other regions. The relative magnitude of the DDM’s twin peaks in low-latitude regions is usually smaller than that of other regions.