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

It is paramount from both scientific and societal perspectives to understand the generation of extreme space weather. We discuss the formation of solar superstorms based on a comparative study of the 2012 July 23 and 2017 July 23 eruptions. The first one is Carrington-class, and the second could rival the 1989 March event that caused the most intense geomagnetic storm of the space age. Observations of these events in the historically weak solar cycle 24 indicate that a solar superstorm can occur in any solar cycle and at any phase of the cycle. Recurrent patterns are identified in both cases, including the long-lived eruptive nature of the active region, a complex event composed of successive eruptions from the same active region, and in-transit interaction between the successive eruptions resulting in exceptionally strong ejecta magnetic fields at 1 au. Each case also shows unique characteristics. Preconditioning of the upstream solar wind leading to unusually high solar wind speeds at 1 au is observed in the first case, but not in the latter. This may suggest that the concept of "preconditioning" appears to be necessary for making a Carrington-class storm. We find a considerable deflection by nearby coronal holes in the second case, but not in the first. On the basis of these results, we propose a hypothesis for further investigation that superstorms are "perfect storms" in nature, i.e., a combination of circumstances that results in an event of an unusual magnitude. Historical records of some extreme events seem to support our hypothesis.