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

Kavli Affiliate: John D. Richardson

| First 5 Authors: Ying D. Liu, Huidong Hu, Rui Wang, Zhongwei Yang, Bei Zhu

| Summary:

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.

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