Geoeffectiveness of halo CMEs

TL;DR

This study analyzes the properties of halo coronal mass ejections (HCMEs) observed by SOHO to determine which parameters predict their potential to cause severe geomagnetic storms, emphasizing the importance of velocity over width.

Contribution

The paper introduces a technique to derive HCME parameters from observations and demonstrates their relation to geoeffectiveness, highlighting the role of velocity in storm severity.

Findings

Fast HCMEs (>1000 km/s) from near-central solar regions can cause severe geomagnetic storms.

HCME width does not correlate with storm strength.

The developed technique enables better space weather forecasting.

Abstract

Halo coronal mass ejections (HCMEs) originating from regions close to the center of the Sun are likely to be geoeffective. Assuming that the shape of HCMEs is a cone and they propagate with constant angular widths and velocities, at least in their early phase, we have developed a technique (Michalek et al. 2003) which allowed us to obtain the space speed, width and source location. We apply this technique to obtain the parameters of all full HCMEs observed by the Solar and Heliospheric Observatory (SOHO) mission's Large Angle and Spectrometric Coronagraph (LASCO) experiment until the end of 2002. Using this data we examine which parameters determine the geoeffectiveness of HCMEs. We show that in the considered period of time only fast halo CMEs (with the space velocities higher than $\sim 1000{km\over s}$ and originating from the western hemisphere close to the solar center could cause the severe geomagnetic storms. We illustrate how the HCME parameters can be used for space weather forecast. It is also demonstrated that the strength of a geomagnetic storm does not depend on the determined width of HCMEs. This means that HCMEs do not have to be very large to cause major geomagnetic storms.