A Comparative Study of 2017 July and 2012 July Complex Solar Eruptions: Are Superstorms Homologous in Nature?

Abstract:

An extreme space weather storm is termed a low-probability, high-consequence event, otherwise called a solar superstorm. We discuss whether solar superstorms are homologous in nature (i.e., similar in formation, evolution and structure), based on a comparative study of the 2017 July 23 and 2012 July 23 eruptions. Although they occurred in a historically weak solar cycle, both produced very strong magnetic fields inside the ejecta at 1 AU, and if they had hit the Earth they would have generated unusually severe geomagnetic storms. By combining multi-point imaging and in situ observations, we find that both of them are complex events composed of consecutive eruptions. For both cases the strong ejecta magnetic fields at 1 AU were caused by the in-transit interaction between the closely launched eruptions. These two cases also show different features. The 2012 July event was associated with upstream solar wind preconditioning by earlier eruptions and resulting high solar wind speeds at 1 AU, while for the 2017 July case this effect was not observed. The 2017 July event was deflected by a nearby coronal hole in the low corona, which was not seen in the 2012 July case. Despite these differences, both can be classified as “perfect storms”, i.e., a combination of circumstances results in an event of unusual magnitude.