The Solar and Interplanetary Causes of Superstorms (Minimum <i>Dst</i> ≤ −250 nT) During the Space Age

Abstract

Abstract A comprehensive study of all superstorms (minimum Dst ≤ −250 nT) occurring during the space age (after 1957) and their interplanetary and solar causes has been performed. Most superstorms were driven solely by the sheath preceding an interplanetary coronal mass ejection (ICME) or by a combination of the sheath and an ICME magnetic cloud. Only one superstorm was driven solely by a magnetic cloud. There were two superstorms caused by “compound events” with two magnetic cloud events followed by sheaths (see erratum text at the end of this article). No superstorms in this study were induced by corotating interaction regions. For the interplanetary shocks antisunward of the ICMEs, the shock normal angles were almost all quasi‐perpendicular. Quasi‐perpendicular shocks theoretically cause greater sheath magnetic field intensities than do quasi‐parallel shocks. Larger shock normal angles statistically corresponded to greater superstorm intensities. Ninety percent of the superstorms occurred either near solar maximum or during the declining phase, 8% of the superstorms occurred during the ascending phase, and 2% of the superstorms occurred during solar minimum. Fifty‐four percent of the superstorms were associated with X‐class solar flares, 36% were associated with M‐class flares, and 5% with C‐class flares. All solar flares related to superstorms occurred in active regions, indicating the importance of active regions to superstorms. Most flares were located in the central meridian or slightly west of it as expected. Two superstorms were caused by limb flares, one on the west limb (confirmed) and the other on the east limb (unconfirmed). The confirmed event was a sheath event led by a Mach 4.1 shock.