The Peculiar Behavior of Halo Coronal Mass Ejections in Solar Cycle 24

TL;DR

Cycle 24 exhibits more halo CMEs with a broader source longitude distribution despite lower sunspot activity, likely due to decreased heliospheric pressure allowing greater CME expansion.

Contribution

This study reveals the unusual increase and distribution change of halo CMEs in cycle 24, linked to heliospheric pressure differences, contrasting prior expectations based on sunspot numbers.

Findings

Halo CMEs are more abundant in cycle 24 despite lower sunspot numbers.

The source longitude distribution of halo CMEs is flatter in cycle 24.

Average CME speed and flare size are unchanged between cycles.

Abstract

We report on a remarkable finding that the halo coronal mass ejections (CMEs) in cycle 24 are more abundant than in cycle 23, although the sunspot number in cycle 24 has dropped by about 40%. We also find that the distribution of halo-CME source locations is different in cycle 24: the longitude distribution of halos is much flatter with the number of halos originating at central meridian distance >/=60 degrees twice as large as that in cycle 23. On the other hand, the average speed and the associated soft X-ray flare size are the same in the two cycles, suggesting that the ambient medium into which the CMEs are ejected is significantly different. We suggest that both the higher abundance and larger central meridian longitudes of halo CMEs can be explained as a consequence of the diminished total pressure in the heliosphere in cycle 24 (Gopalswamy et al. 2014). The reduced total pressure allows CMEs expand more than usual making them appear as halos.