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

TL;DR

This paper characterizes the 'ICME-in-sheath' phenomenon, identifies its unique features, and estimates the upper limit of geomagnetic storm activity, highlighting its significance in extreme space weather events.

Contribution

It provides the first detailed observational characterization of 'ICME-in-sheath' events and estimates the maximum potential severity of geomagnetic storms they can cause.

Findings

'ICME-in-sheath' is usually short-lived and shows sustained magnetic field enhancement.

2012 July 23 event exemplifies extreme solar wind conditions near upper limits.

A geomagnetic storm with Dst of about -2000 nT is theoretically possible.

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 $D_{\rm st}$ 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.