The Complex Space Weather Events of 2017 September

Interplanetary coronal mass ejections (ICMEs), magnetic clouds (MCs), sheaths, corotating interaction regions (CIRs), solar wind high-speed streams (HSSs), fast forward shocks (FSs), reverse waves (RWs), stream interfaces, and heliospheric current sheet crossings detected upstream of the Earth and their geoeffectiveness are studied during 2017 September. The most intense geomagnetic storm (SYM-H peak = -146 nT) starting on September 7 had a 3-step main phase. A compound interplanetary structure resulting from a FS encountering and compressing the upstream MC southward interplanetary magnetic fields (IMFs) caused the first two steps of the storm. A magnetospheric supersubstorm (SSS: SML < -2500 nT) led to the third and most intense step. The MC and FS were associated with two fast halo CMEs (hCMEs) that erupted on September 4 and 6, respectively. The MC of the second hCME/ICME created another intense storm (SYM-H peak = -115 nT) on September 8. A second SSS was recorded during main phase of this storm. Intense geomagnetically induced currents (GICs) occurred during the SSSs. Two moderate magnetic storms (-50 nT > SYM-H > -100 nT) were caused by sheath and CIR southward IMFs, respectively. Among 6 FSs detected during this period, 3 were associated with ICMEs and 3 with CIRs. The FSs and their associated sheaths caused sudden impulses (SI+s) of magnitude from +11 to +56 nT, and led to magnetospheric relativistic electron flux decreases. The RWs caused SI-s and substorm recoveries by reducing southward IMFs. The HSSs were related to the increase/acceleration of relativistic electron fluxes.