STEREO and Wind observations of a fast ICME flank triggering a prolonged geomagnetic storm on 5-7 April 2010

TL;DR

This study combines STEREO and Wind observations to analyze a fast ICME flank that caused a prolonged geomagnetic storm, revealing new insights into ICME structure and interaction with Earth's magnetosphere.

Contribution

It provides the first direct observation of a glancing ICME collision with Earth using STEREO Heliospheric Imagers and challenges existing magnetic cloud modeling assumptions.

Findings

ICME did not decelerate significantly between Sun and Earth.

The ICME flank caused a long storm growth phase.

Magnetic cloud inside the ICME cannot be modeled assuming invariant direction.

Abstract

On 5 April 2010 an interplanetary (IP) shock was detected by the Wind spacecraft ahead of Earth, followed by a fast (average speed 650 km/s) IP coronal mass ejection (ICME). During the subsequent moderate geomagnetic storm (minimum Dst = -72 nT, maximum Kp=8-), communication with the Galaxy 15 satellite was lost. We link images from STEREO/SECCHI to the near-Earth in situ observations and show that the ICME did not decelerate much between Sun and Earth. The ICME flank was responsible for a long storm growth phase. This type of glancing collision was for the first time directly observed with the STEREO Heliospheric Imagers. The magnetic cloud (MC) inside the ICME cannot be modeled with approaches assuming an invariant direction. These observations confirm the hypotheses that parts of ICMEs classified as (1) long-duration MCs or (2) magnetic-cloud-like (MCL) structures can be a consequence of a spacecraft trajectory through the ICME flank.