Sub-Auroral, Mid-Latitude, and Low-Latitude Troughs during Severe Geomagnetic Storms

The dynamics of ionospheric troughs during intense geomagnetic storms is considered in this paper. The study is based on electron density measurements at CHAMP satellite altitudes of 405–465 km in the period from 2000 to 2002. A detailed analysis of four storms with Kp from 5+ to 9− is presented. Three troughs were identified: sub-auroral, mid-latitude, and low-latitude. The sub-auroral trough is usually defined as the main ionospheric trough (MIT). The mid-latitude trough is observed equatorward of the MIT and is associated with the magnetospheric ring current; therefore, it is named the ring ionospheric trough (RIT). The RIT appears at the beginning of the storm recovery phase at geomagnetic latitudes of 40–45° GMLat (L = 1.75–2.0) and exists, for a long time, at the late stage of the recovery phase at latitudes of the residual ring current 50–55° GMLat (L ~ 2.5–3.0). The low-latitude trough (LLT) is discovered for the first time. It forms only during great storms at the latitudes of the internal radiation belt (IRB), 34–45° GMLat (L = 1.45–2.0). The LLT’s lowest latitude of 34° GMLat was recorded in the night sector (2–3 LT). The occurrence probability and position of the RIT and LLT depend on the hemisphere and longitude.