Climatological study of Nighttime Winter Anomaly by GRACE satellite and ground-based observations

The Ionospheric Nighttime Winter Anomaly (or seasonal anomaly) is a persistent feature in the Northern Hemisphere for the American sector and the Southern Hemisphere for the Asian longitude sector under Low Solar Activity (LSA) conditions. During the Nighttime Winter Anomaly, the mean ionization level is higher in the winter nights compared to the summer nights. The aim of this paper is to investigate the occurrence of Nighttime Winter Anomaly during LSA periods utilizing measurements of vertical total electron content (VTEC) obtained through the International Global Navigation Satellite System (GNSS) Service (IGS) during the years 2009 and 2020 for several stations located in Asian and American sectors. The NWA phenomenon was also studied through the use of electron density measurements taken from the GRACE satellite in 2009. The Midlatitude Summer Nighttime Anomaly (MSNA) is visible on the GRACE satellite in the American Southern Hemisphere and the Okhotsk Sea Anomaly (OSA) with the highest ionization in the late evening hours in the northern summer values of the Asian sector.