Brightness Characteristics of Halo CMEs

It is generally accepted that halo Coronal Mass Ejections (CMEs) are regular, low latitude, CMEs traveling towards (or away FROM) the observer. The association between halo CMEs and solar energetic particles events (SEPs) and Type II bursts is higher compared to regular CMEs. Halo CMEs are observed up to large heliocentric distances as compared to the regular CMEs. This is somewhat contradictory to the theory of Thomson scattering because the visibility conditions for halo CMEs are rather poor, assuming that the longitudinal widths of halo CMEs are the similar to those of regular CMEs. In this paper, we present some observational characteristics of halo CMEs, such as high brightness at large distances and symmetry in the velocity space. The CMEs were observed by the Solar and Heliospheric Observatory (SOHO) mission's Large Angle and Spectrometric Coronagraph (LASCO). We also present a possible model that explains the apparent contradiction