Characteristics and Importance of "ICME-in-Sheath" Phenomenon and Upper Limit for Geomagnetic Storm Activity

Abstract

As an important source for large geomagnetic storms, an "ICME-in-sheath" is a completely shocked interplanetary coronal mass ejection (ICME) stuck in the sheath between a shock and host ejecta. Typical characteristics are identified from coordinated multi-sets of observations: (1) it is usually short in duration and lasts a few hours at 1 AU; (2) its solar wind parameters, in particular the magnetic field, seem to keep enhanced for a large range of distances; and (3) common ICME signatures are often lost. The host ejecta could be a single ICME or a complex ejecta, being fast enough to drive a shock. These results clarify previous misinterpretations of this phenomenon as a normal part of a sheath region. The "ICME-in-sheath" phenomenon, together with a preconditioning effect, produced an extreme set of the magnetic field, speed and density near 1 AU in the 2012 July 23 case, all around their upper limits at the same time. This is probably the most extreme solar wind driving at 1 AU and enables us to estimate the plausible upper limit for geomagnetic storm activity. With an appropriate modification in the southward field, we suggest that a geomagnetic storm with a minimum $D_{\mathrm{st}}$ of about $-2000$ nT could occur in principle. The magnetopause would be compressed to about 3.3 Earth radii from the Earth's center, well inside the geosynchronous orbit.

arXiv commentAccepted for publication in The Astrophysical Journal Letters

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Cited by in corpus (6)

• Extreme solar events
• Detection of small magnetic flux ropes from the third and fourth Parker Solar Probe encounters
• Polytropic behavior in the structures of Interplanetary Coronal Mass Ejections
• Dependence of great geomagnetic storm intensity ($\Delta$SYM-H$\le$-200 nT) on associated solar wind parameters
• Categorization of Coronal Mass Ejection Driven Sheath Regions: Characteristics of STEREO Events
• Estimation of the solar wind extreme events