Full halo coronal mass ejections: Do we need to correct the projection effect in terms of velocity?

TL;DR

This study assesses the impact of projection effects on the observed velocities of full halo CMEs, finding that correction is necessary mainly for slower CMEs near the Sun-Earth line.

Contribution

It provides a systematic analysis of projection effects on FHCMEs using the GCS model, clarifying when velocity correction is essential.

Findings

Projection effect causes some halo CMEs to appear faster or slower.

Most FHCMEs within 45° of Sun-Earth line with speeds under 900 km/s are significantly affected.

Faster CMEs or those far from the disk center are less affected by projection effects.

Abstract

The projection effect is one of the biggest obstacles in learning the real properties of coronal mass ejections (CMEs) and forecasting their geoeffectiveness. To evaluate the projection effect, 86 full halo CMEs (FHCMEs) listed in the CDAW CME catalog from 2007 March 1 to 2012 May 31 are investigated. By applying the Graduated Cylindrical Shell (GCS) model, we obtain the de-projected values of the propagation velocity, direction and angular width of these FHCMEs, and compare them with the projected values measured in the plane-of-sky. Although these CMEs look full halo in the view angle of SOHO, it is found that their propagation directions and angular widths could vary in a large range, implying projection effect is a major reason causing a CME being halo, but not the only one. Furthermore, the comparison of the de-projected and projected velocities reveals that most FHCMEs originating within 45$^\circ$ of the Sun-Earth line with a projected speed slower than 900 km s$^{-1}$ suffer from large projection effect, while the FHCMEs originating far from the vicinity of solar disk center or moving faster than 900 km s$^{-1}$ have small projection effect. The results suggest that not all of FHCMEs need to correct projection effect for their velocities.