Are halo coronal mass ejections special events?

Details Are halo coronal mass ejections special events? Are halo coronal mass ejections special events?

Jun 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006jgra..11106107l&link_type=abstract

Journal of Geophysical Research, Volume 111, Issue A6, CiteID A06107

Physics

Plasma Physics

6

Solar Physics, Astrophysics, And Astronomy: Coronal Mass Ejections (2101), Solar Physics, Astrophysics, And Astronomy: Corona, Solar Physics, Astrophysics, And Astronomy: Radio Emissions, Space Plasma Physics: Shock Waves (4455), Space Weather: Solar Effects

Scientific paper

We revisited the properties of wide coronal mass ejections (CMEs) called halo CMEs. Using the large LASCO/SOHO CMEs data set, from 1996 to 2004, we examined the statistical properties of (partial and full) halo CMEs and compare with the same properties of ``normal'' width (lower than 120°) CMEs. We found that halo CMEs have different properties than ``normal'' CMEs, which cannot be explained merely by the current geometric interpretation that they are seen as halos because they are traveling in the Sun Earth direction. We found that the CME width distribution is formed by, at least, three different populations: Two gaussians: a narrow and a medium distribution centered at ~17° and ~38°, respectively; the narrow population most likely corresponds to the ``true'' observed widths, whereas the medium width population is the product of projection effects. The third distribution corresponds to wider CMEs (80° < W < 210°) which behaves as a power law. Partial and full halo CMEs wider than these do not follow any particular distribution. This lack of regularity may be due to the small number of such events. In particular, we found (and test by a statistical approach) that the number of observed full halo CMEs is lower than expected. The CME speed follows a log-normal distribution, except for the very low speed CME population, which follows a gaussian distribution centered at ~100 km/s and is probably due to projection effects. When the CMEs are divided by width into nonhalo, partial halo, and full halo, we found that the peaks of the distributions are shifted toward higher speeds, ~300, ~400 and ~600 km/s for nonhalo, partial halo, and full halo CMEs, respectively. This confirms that halo CMEs tend to be high speed CMEs. The acceleration of full halo CMEs tends to be more negative compared with nonhalo and partial halo CMEs. We introduce a new observational CME parameter: The final observed distance (FOD), i.e., the highest point within the coronograph field of view where a CME can be distinguished from the background. In other words, the highest CME altitude measured. The FOD for nonhalo CMEs decreases exponentially from ~5 to ~30 R$\odot$ in the LASCO field of view. On the other hand, the FOD of halo CMEs increase with distance. This means that it is more likely to see halo CMEs at large distances (from the Sun) than nonhalo CMEs. These halo CME properties may be explained if the white light wide enhancements (or halo) seen by coronographs correspond to a combination of an expanding (shock) wave which disturbs and/or compresses the ambient material and the CME material itself.

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