Intercycle and intracycle variation of halo CME rate obtained from SOHO/LASCO observations

TL;DR

This study compares the properties and rates of halo coronal mass ejections (HCMEs) between solar cycles 23 and 24, revealing differences in speed, distribution, and activity phases despite similar overall occurrence rates.

Contribution

It provides a detailed analysis of HCME variations across different phases of solar cycles 23 and 24, highlighting cycle-specific differences in properties and activity patterns.

Findings

HCME rate per SSN is 44% higher in cycle 24

Average sky-plane speed is ~16% higher in cycle 23

Maximum phase HCME activity is most contrasting between cycles

Abstract

We report on the properties of halo coronal mass ejections (HCMEs) in solar cycles 23 and 24. We compare the HCMEs properties between the corresponding phases (rise, maximum, and declining) in cycles 23 and 24 in addition to comparing those between the whole cycles. Despite the significant decline in the sunspot number (SSN) in cycle 24, which dropped by 46% with respect to cycle 23, the abundance of HCMEs is similar in the two cycles. The HCME rate per SSN is 44% higher in cycle 24. In the maximum phase, cycle-24 rate normalized to SSN increased by 127% while the SSN dropped by 43%. The source longitudes of cycle-24 HCMEs are more uniformly distributed than those in cycle 23. We found that the average sky-plane speed in cycle 23 is ~16% higher than that in cycle 24. The size distributions of the associated flares between the two cycles and the corresponding phases are similar. The average speed at a central meridian distance (CMD) = 600 for cycle 23 is ~28% higher than that of cycle 24. We discuss the unusual bump in HCME activity in the declining phase of cycle 23 as due to exceptional active regions that produced many CMEs during October 2003 to October 2005. The differing HCME properties in the two cycles can be attributed to the anomalous expansion of cycle-24 CMEs. Considering the HCMEs in the rise, maximum and declining phases, we find that the maximum phase shows the highest contrast between the two cycles.