HR: 1340h
AN: SH13A-0304    [Abstracts]
TI: Are Halo CMEs special events?
AU: * Lara, A
EM: alara@geofisica.unam.mx
AF: Instituto de Geofisica, UNAM, Instituto de Geofisica, D.F., 04510 Mexico
AU: Xie, H
EM: hong.xie@ssedmail.gsfc.nasa.gov
AF: Catholic University of America, Physics Department, Washington D.C., 20010 United States
AU: Mendoza, E
EM: mend@inaoep.mx
AF: Instituto Nacional de Astrofisica Optica y Electronica, Tonantzintla, Puebla, 25200 Mexico
AB: We re-visit the properties of wide coronal mass ejections (CMEs) called halo CMEs. Using the large LASCO/SOHO CMEs data set, from 1996 to 2004, we examine 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 can not be explained by the current geometric interpretation of halos, as CMEs traveling in the Sun Earth direction. We found that the CME width distribution is formed by, at least, three different populations. Two gaussians one narrow and one medium centered at ~17° and ~38°, respectively. It is highly probable, that the narrow population corresponds to ``true'' observed widths, whereas the medium width population is the product of projection effects. The number of wider CMEs (80° < W < 210°) decreases as a power law. After this width, i. e. partial and full halo CMEs, do not follow any particular distribution. This lack of regularity, may be due to the small number of such events. In particular, we found that the number of observed full halo CMEs is lower than the expected. The CME speed follows a log-normal distribution, except for the very low speed CME population, wich follows a gaussian distribution centered at ~100 km/s and probably is due to projection effects. When the CMEs are dividing by width into no, partial and full halo groups we found that the peak of the distributions are shifted towards higher speeds, ~300, ~400 and ~600 km/s for no, partial and full halo CMEs. This confirms that halo CMEs tend to be high speed CMEs. We introduce a new observational CME parameter: the final observed distance (FOD) which is the highest point, inside the coronograph field of view, where the CME can be distinguished from the background. In other words, the highest CME altitude measured. The FOD for no halo CMEs decreases exponentialy from ~5 to ~30 R_ȯ in the LASCO field of view. On the other hand, the FOD of halo CMEs increase with distance, i. e., it is more probable to see a halo CMEs at large distance (from the Sun) than no halo CMEs. These halo CME properties may be explained if the white light wide enhancements (or halo) seen by coronographs correspond to an expanding (shock) wave which is disturbing and/or compressing the ambient material and not only to the CME material by it self.
DE: 7500 SOLAR PHYSICS, ASTROPHYSICS, AND ASTRONOMY
DE: 7513 Coronal mass ejections (2101)
DE: 7851 Shock waves (4455)
SC: SPA-Solar and Heliospheric Physics [SH]
MN: Fall Meeting 2005