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1. chinaXiv:201605.01577 [pdf]

PLASMA AND MAGNETIC FIELD CHARACTERISTICS OF SOLAR CORONAL MASS EJECTIONS IN RELATION TO GEOMAGNETIC STORM INTENSITY AND VARIABILITY

Liu, Ying D.; Hu, Huidong; Wang, Rui; Yang, Zhongwei; Zhu, Bei; Liu, Yi A.; Luhmann, Janet G.; Richardson, John D.
Subjects: Geosciences >> Space Physics

The largest geomagnetic storms of solar cycle 24 so far occurred on 2015 March 17 and June 22 with D-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 Liu et al. (i.e., a combination of circumstances results in an event of unusual magnitude), so the "perfect storm" scenario may not be as rare as the phrase implies.

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