Why Are Halo Coronal Mass Ejections Faster?

TL;DR

This study explains why halo CMEs appear faster than normal CMEs by showing that slower, fainter events are often missed in observations, leading to a bias towards detecting faster, brighter halo CMEs.

Contribution

The paper introduces a simulation-based explanation for the observed higher speeds of halo CMEs, highlighting observational biases due to brightness and detectability.

Findings

Fainter, slower CMEs are often missed in observations.

The average velocity of detectable halo CMEs is around 922 km/s.

Detection bias explains the apparent speed difference between halo and normal CMEs.

Abstract

Halo coronal mass ejections (CMEs) were found to be significantly faster than normal CMEs, which was a long-standing puzzle. In order to solve the puzzle, we first investigate the observed properties of 31 limb CMEs that display clearly loop-shaped frontal loops. The observational results show a strong tendency that slower CMEs are weaker in the white-light intensity. Then, we perform a Monte Carlo simulation of 20 000 artificial limb CMEs that have average velocity of $\sim$523 km s$^{-1}$. The Thomson scattering of these events is calculated when they are assumed to be observed as limb and halo events, respectively. It is found that the white-light intensity of many slow CMEs becomes remarkably reduced as they turn from being viewed as a limb event to as a halo event. When the intensity is below the background solar wind fluctuation, it is assumed that they would be missed by coronagraphs. The average velocity of "detectable" halo CMEs is $\sim$922 km s$^{-1}$, very close to the observed value. It also indicates that wider events are more likely to be recorded. The results soundly suggest that the higher average velocity of halo CMEs is due to that a majority of slow events and a part of narrow fast events carrying less material are so faint that they are blended with the solar wind fluctuations, and therefore are not observed.