2.3 Halo CMEs

Because of their increased sensitivity, field of view and dynamic range, the SOHO/LASCO and STEREO/COR coronagraphs now frequently observe halo CMEs, which appear as expanding, circular brightenings that completely surround the coronagraphs’ occulting disks (Figure 4View Image). Observations of associated activity on the solar disk are necessary to help distinguish whether a halo CME was launched from the front or backside of the Sun relative to the observer. This has had limited success, as frontsided CMEs that do not have a solar surface association can be mistaken for backsided events. Halo CMEs are important for three reasons:
  1. The source regions of frontside halo CMEs are likely to be located within a few tens of degrees of Sun center from the perspective of the observer (Cane et al., 2000; Webb, 2002Jump To The Next Citation Point; Gopalswamy, 2004Jump To The Next Citation Point; Gopalswamy et al., 2010b). Thus, these regions can be studied in greater detail than for most CMEs which are observed near the limb, but at the cost of reduced information about the CME itself because of projection. In recent years several CMEs have been observed by the “three eyes” of STEREO-B, LASCO and STEREO-A by a variety of viewing points, thus reducing this latter problem (e.g., Howard and Tappin, 2008Jump To The Next Citation Point; Wood and Howard, 2009Jump To The Next Citation Point; Robbrecht et al., 2009bJump To The Next Citation Point; Möstl et al., 2009, 2010Jump To The Next Citation Point; Patsourakos and Vourlidas, 2009Jump To The Next Citation Point).
  2. Lacking significant deflections in the interplanetary medium, frontside halo CMEs should travel with part of their structure approximately along the Sun-observer line, so their internal material can be sampled in-situ by the observer.
  3. When they are Earth-directed (i.e., observed as halos by spacecraft on the Sun-Earth line like SOHO), they are the key link between solar eruptions and major space weather phenomena such as geomagnetic storms and solar energetic particle events. This “geoeffectiveness” of halo CMEs depends on the source location on the disk. CMEs that are aligned near the relative disk center tend to be more geoeffective while those nearer the relative solar limb are less so. This center-to-limb variation of the geoeffectiveness has been documented (e.g., Gopalswamy et al., 2007Jump To The Next Citation Point). The vast majority of the most intense geomagnetic storms of Cycle 23, for example, were caused by halo CMEs (Gopalswamy, 2010a). Three spacecraft, SOHO, Wind and ACE, provide solar wind measurements upstream of Earth, and the twin STEREO spacecraft provide similar measurements from their perspectives drifting away from the Sun-Earth line.

Partial and full halo CMEs occur at a rate of about 10% that of all CMEs, but 360° halo CMEs are only detected at a rate of ∼ 4% of all CMEs. It has been documented (e.g., Gopalswamy et al., 2010aJump To The Next Citation Point) that halo CMEs appear to be faster and more energetic than non-halo CMEs. This, of course, does not imply that halo CMEs are somehow physically different, but rather it shows that even with LASCO some CMEs are not detected. LASCO does not observe faint (weak) CMEs near Sun center. Studies investigating this include those involving post-eruptive arcades (Tripathi et al., 2004Jump To The Next Citation Point), interplanetary transients and shocks (Cane and Richardson, 2003; Howard and Tappin, 2005), and heliospheric imagers (Howard and Simnett, 2008Jump To The Next Citation Point). All found that between 3 – 7% of the studied CME-associated events were not associated with LASCO CMEs. Howard and Simnett (2008Jump To The Next Citation Point) further deduced that around 15% of interplanetary transients observed far from the Sun by SMEI were associated with either very weak CMEs or with those that had measurement problems, e.g., related to the large height and time separations between the LASCO and SMEI fields of view. However, this study did not exclusively involve halo CMEs.

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