Substorms on a contracted auroral oval

Abstract

The high-latitude magnetic substorms observed at geomagnetic latitudes higher 70° MLAT under the substorm absence at the lower latitudes are known as “substorms on the contracted oval” or “polar” substorms. Such substorms appear during quiet or weakly disturbed space weather conditions. The study of 254 “polar” substorms, recorded at the Scandinavian IMAGE magnetometer chain during the winter seasons of 2010–2020, confirmed a tendency to occur in the late evening (∼19–23 MLT), that is a bit earlier than the “normal” substorms (22–24 MLT). It was found that before the onset of “polar” substorms, like “normal” substorms, there is an increase in the PC-index indicating an energy input into the magnetosphere. We established that “polar” substorms, like “normal” substorms, are accompanied by positive mid-latitude magnetic bays, demonstrating a substorm current wedge (SCW) development.Several “polar” substorms are examined in detail. The ionospheric electrojets and field-aligned currents (FAC) distribution was studied basing on the AMPERE satellites measurements. We found that “polar” substorm onsets are associated with an enhancement of FACs in a localized evening area. Thus, “polar” substorms exhibited the properties, typical for “normal” substorms, so, they could be referred as a specific type of substorms developing under rather quiet space weather conditions.