Identification of Earth-directed partial halo coronal mass ejections

Abstract

The very weak storm-effectiveness of frontside partial halo coronal mass ejections (FPH CMEs) suggests that many, if not most, FPH CMEs are not Earth-directed. Based on the cone model (Zhao et al., 2002) we put forward a method to identify Earth-directed partial halo CMEs from FPH CMEs. 1. The Earthward direction of halo-type CMEs CMEs with apparent angular span greater than 120◦ are termed as halo-type CMEs in what follows. The threshold of 120◦ is selected somewhat arbitrary (see next section). The Earthward direction of a halo-type CME is a necessary condition for the halotype CME to be storm-effective. Identification of Earth-directed halo-type CMEs is thus an important task in the research of the space weather. By “Earthward direction” or “Earth-directed” here we mean that there is a component of the halo-type CME that is able to hit the Earth, though the central axis of the CME may not be aligned with the line-of-sight. Halo-type CMEs are interpreted as the result of the Thompson scattering of Sun’s white light along the line-of-sight caused by a broad shell or bubble of dense plasma which is ejected in the frontside or backside of the Sun (Howard et al., 1982). Observations show that limb CMEs propagate radially beyond the first couple of solar radii, and that many limb CMEs maintain their angular width constant. Therefore halo-type CMEs with angular span of 360◦ and having associated near surface activities, the frontside full halo (FFH) CMEs, are expected to be Earth-directed because the halos of FFH CMEs cover Sun’s disk center, the Earth’s projection in Sun’s disk. The fact that all FFH CMEs observed in 1997 have associated interplanetary counterparts near the Earth and geomagnetic storms (Zhao and Webb, 2003) confirms that FFH CMEs are indeed Earthdirected. The storm-effectiveness for both FFH and FPH CMEs significantly weaker than for FFH CMEs alone (See Table 1) suggests, however, that many, if not most, FPH CMEs are not Earth-directed. It may improve the space weather forcasting if we can identify Earth-directed partial halo CMEs from all FPH CMEs. 2. Identifying Earth-directed partial halo CMEs There are studies for identifying Earth-directed partial halo CMEs. Webb et al used the angular span of 140◦ as a threshold for selecting partial halo CMEs and found that the FPH CMEs between December 1996 and June 1997 are storm-effective when the source region in the low corona is within ∼ 0.5 radii of the Sun’s disk center. Lyons et al. selected the angular span that exceeds 100◦ and encompasses one or the other of the solar poles. Thompson et al used a similar criterion but with a higher threshold of