The Morphology and Oscillations of Nightside Mid-Latitude Ionospheric Trough at Designated Longitudes in the Northern Hemisphere

The mid-latitude ionospheric trough (MLIT), an anomaly in the ionosphere's F layer caused by various mechanisms, affects radio wave propagation. In this study, we investigated the morphology and oscillations of the MLIT using global Global Positioning System total electron content map data between 1 January 2018, and 31 December 2020. The MLIT position varies longitudinally, reaching its farthest equatorward at 60$°$W and its farthest poleward at 30$°$E. The MLIT occurrence rates peak during the winter and equinoxes and dip in summer, while seasonal variations in MLIT position vary across longitude bands. Heightened geomagnetic activities, quantified by the SME6 index, promote MLIT occurrence, especially during pre-midnight hours in summer and equinoxes, and shift the MLIT equatorward, particularly during midnight and post-midnight hours. The MLIT position shows clear local time variation, with a gradual decrease before midnight, stabilization afterward, and a minor resurgence around dawn. Wavelet analysis reveals three distinct periodic components in the MLIT position: 27, 13.5, and 9, with the 27-day period being the most persistent. Cross-wavelet and wavelet coherence analyses suggest that solar wind (SW) velocity variations precede changes in the MLIT position. The main factors responsible for the equatorward movement of MLIT are the electric fields in high-speed SW that enhance the ionospheric convection pattern, and the intensified geomagnetic activities induced by interplanetary shocks.