Supersubstorms and Conditions in the Solar Wind

This article examines the effect of various large-scale solar-wind structures and streams on the occurrence of a special type of substormsthe so-called supersubstorms (SSS), which are very intense substorms defined by the indices SML <-2500 nT and AL < -2500 nT. The analysis covers 131 cases of SSS events from observations at the SuperMAG stations in 1998-2016 and 26 cases of SSS events from the IMAGE network. Analysis of the dependence of SSS events on different types of solar wind and different geomagnetic disturbances shows that these events are mainly observed during the approach to the Earth's magnetosphere of solar-wind magnetic clouds (MC) (42%) and SHEATH plasma compression regions ahead of MCs or ahead of EJECTA. Supersubstorms may sometimes occur during EJECTA (8.3%). Thus, SSS events are caused by interplanetary coronal mass ejections and are, in fact, unassociated with high-speed streams from coronal holes. It is shown that SSS events mainly occur during magnetic storms (Dst < -50 nT). In the rare cases (13.4%) of SSS observations during intervals with Dst > -50 nT, the events occur mostly right after the sudden onset (SC) of a storm (11%) and, very rarely, happen late in the storm recovery phase (1.2%). The space weather conditions associated with SSS events differ sharply from those associated with other types of high-latitude substorms, such as polar and expanded substorms.