Substorms manifestation at high and mid-latitudes during two large magnetic storm

Abstract

The dynamics of magnetic substorms at high and middle latitudes during two severe geomagnetic storms: on 17March 2015 and on 22–23 June2015has been analyzed. The storms were rather similar: both storms were a result of the solar wind Sheath impact and both storms were characterized by a strong intensity (SYM/Hmin<–200nT). We studied the magnetic substorms during these storms on the base of the INTERMAGNET and IMAGE networks data. The attendant solar wind and Interplanetary Magnetic Field (IMF) parameters were taken from the OMNI data base. The spatial-temporal dynamics of three substorms was studied in detail: at 17:29 UT and at 22:55 UT during the first storm and at 18:33 UT during the second storm. The substorms on 17.03.2015originated during the main storm phase, and the onset of the substorm on 22.06.2015 followed the storm sudden commencement (SSC) of the second storm. All three substorms were characterized by a sharp poleward expansion of the westward electrojet simultaneously with a slower motion to lower latitudes. They were observed also at middle and low latitudes as positive magnetic bays. The westward electrojet reached ~71°CGMLat during the first two substorms and surpassed 75°CGMLat during the third substorm. Therefore, the first two events were “classical” substorms, and the third one –an “expanded” substorm. We suggested that this behavior is related to the different solar wind conditions: the “classical” substorms developed under magnetic cloud (MC) conditions, and the “expanded” –under the Sheath region effect.