Analysis of temporal and spatial variation characteristics of midlatitude summer nighttime anomaly in low and middle solar activity period

The spatiotemporal variation characteristics of midlatitude summer nighttime anomaly (MSNA) are essential for ionospheric modeling in the MSNA region. Based on GIM TEC, COSMIC NmF2, COSMIC IEC (Integrate Electron Content), and GPS TEC, this paper analyzes the spatiotemporal variation characteristics of MSNA during the low and middle solar activity period. First, we use the fourth-order Fourier function to fit the TEC, NmF2, and IEC. Second, we calculate an MSNA index and give its spatiotemporal distribution during the low and middle solar activity period. Third, on the basis of GIM TEC and GPS TEC, we describe the spatiotemporal distribution of TEC peak moments from October to January (Oct-Jan) and May to August (May-Aug). Our detailed analysis shows the following: (1) MSNA in the Southern Hemisphere is most significant in Oct-Jan. Moreover, MSNA in the Northern Hemisphere is most significant in May-Aug. (2) MSNA mainly occurs in three regions: South America and Antarctica, Western Europe to eastern North America, and Northeast Asia to the Pacific. (3) When solar activity increases, the MSNA region in the Northern Hemisphere becomes smaller, and the MSNA index is unchanged. Moreover, the MSNA region in the Southern Hemisphere is unchanged, but the MSNA index increases. (4) From Oct-Jan, there are some apparent differences between the east and west sides of the MSNA region of the Antarctic Peninsula. The peak moments of TEC in the west region concentrate before midnight (20:00–22:00 LT), and the peak moments of TEC in the east region concentrate after midnight (00:00–08:00 LT). As the level of solar activity increases, the peak moments in the eastern region are significantly shifted to later local time. (5) As the level of solar activity increases, the incidence of MSNA at three GPS stations (OHI300ATA, YSSK00RUS, and BRST00FRA) shows a decreasing trend.