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Tim Howard   Submitted: 2008-02-01 13:16

One of the main sources of uncertainty in quantifying the kinematic properties of coronal mass ejections (CMEs) using coronagraphs is the fact that coronagraph images are projected into the sky plane, resulting in measurements which can differ significantly from their actual values. By identifying solar surface-source regions of CMEs using X-ray and Hα flare, and disappearing filament data, and through considerations of CME trajectories in 3D geometry, we have devised a methodology to correct for the projection effect. We outline this method here. The methodology was automated and applied to over 10000 CMEs in the CDAW (SOHO LASCO) catalog spanning 1996-2005, in which we could associate 1961 CMEs with an appropriate surface event. In the latter subset, de-projected speeds, accelerations and launch angles were determined to study CME kinematics. Our analysis of this subset of events reconfirms some important trends - notably that previously uncovered solar cycle variation of CME properties are preserved, CMEs with greater width have higher speeds and that slower CMEs tend to accelerate while faster CMEs tend to decelerate. This points out that statistical trends in CME properties - recovered from plane-of-sky measurements - may be preserved even in the face of more sophisticated 3D measurements from spacecrafts such as STEREO, if CME trajectories are predominantly radial. However, our results also show that the magnitude of corrected measurements can differ significantly from the projected plane-of-sky measurements on a case-by-case basis and that acceleration is more sensitive to the de-projection process than speed. Average corrected speed and acceleration tend to be a factor of 1.7 and 4.4 higher than their projected values, with mean corrected speed and acceleration magnitudes being on the order of 1000 km s-1 and 50 m s-2, respectively. We conclude that while using the plane-of-sky measurements may be suitable for studies of general trends in a large sample of events, correcting for projection effects is mandatory for those investigations which rely on a numerically precise determination of the properties of individual CMEs.

Authors: Howard, T.A., Nandy, D., and Koepke, A.C.
Projects: SoHO-LASCO

Publication Status: In Print (online)
Last Modified: 2008-02-03 09:23
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