The Spatiotemporal Evolution of the Midlatitude Trough and Subauroral Ion Drifts during a Geomagnetic Storm Observed by Multiple DMSP Satellites

We investigate the spatiotemporal evolution of Subauroral Ion Drifts (SAIDs) and a midlatitude trough during a geomagnetic storm over storm timescales. Using data from multiple Defense Meteorological Satellite Program (DMSP) satellites, we analyze the evolution of a SAID event and its connection to the midlatitude trough. During the main phase of an intense geomagnetic storm (Dst $<$-100 nT), the SAID exhibited an equatorward movement of approximately 2.5$^{\circ}$ in magnetic latitude (MLAT), while the width of the high-speed channel associated with the SAID broadened by 1.2$^{\circ}$ in MLAT over a 4-hour period. As the original channel broadened, another flow channel emerged nearby and formed a new trough. The midlatitude trough associated with the SAID evolved through three stages: (1) a mild gradient structure emerged, (2) a sharp gradient appeared near the ionospheric projection of the plasmapause, and (3) a persistent deep ionization trough was observed near the density minimum. Two peaks were found in the AE index during the appearance of the flow channels and a midlatitude trough in the SAID event. These results suggest that the evolution of the flow channels and a midlatitude trough associated with the SAID are closely linked to geomagnetic storm conditions and substorm activity. Our study highlights the importance of multi-satellite observations in capturing these dynamic processes.