Plasma and Magnetic Field Characteristics of Solar Coronal Mass Ejections in Relation to Geomagnetic Storm Intensity and Variability

TL;DR

This study investigates how plasma and magnetic field properties of solar CMEs influence the strength and variability of geomagnetic storms, using in situ reconstructions and solar observations to understand complex multi-step storm developments.

Contribution

It introduces new scenarios for multi-step geomagnetic storm development and highlights the role of CME interactions and flux-rope orientations in storm intensity.

Findings

Multi-step storm development mechanisms identified.

Contrasting CME flux-rope configurations can produce intense storms.

Interaction of successive CMEs and high-speed streams can cause 'perfect storm' events.

Abstract

The largest geomagnetic storms of solar cycle 24 so far occurred on 2015 March 17 and June 22 with $D_{\rm st}$ minima of $-223$ and $-195$ nT, respectively. Both of the geomagnetic storms show a multi-step development. We examine the plasma and magnetic field characteristics of the driving coronal mass ejections (CMEs) in connection with the development of the geomagnetic storms. A particular effort is to reconstruct the in situ structure using a Grad-Shafranov technique and compare the reconstruction results with solar observations, which gives a larger spatial perspective of the source conditions than one-dimensional in situ measurements. Key results are obtained concerning how the plasma and magnetic field characteristics of CMEs control the geomagnetic storm intensity and variability: (1) a sheath-ejecta-ejecta mechanism and a sheath-sheath-ejecta scenario are proposed for the multi-step development of the 2015 March 17 and June 22 geomagnetic storms, respectively; (2) two contrasting cases of how the CME flux-rope characteristics generate intense geomagnetic storms are found, which indicates that a southward flux-rope orientation is not a necessity for a strong geomagnetic storm; and (3) the unexpected 2015 March 17 intense geomagnetic storm resulted from the interaction between two successive CMEs plus the compression by a high-speed stream from behind, which is essentially the "perfect storm" scenario proposed by \citet[][i.e., a combination of circumstances results in an event of unusual magnitude]{liu14a}, so the "perfect storm" scenario may not be as rare as the phrase implies.