AN EMPIRICAL RECONSTRUCTION OF THE 2008 APRIL 26 CORONAL MASS EJECTION

Abstract

We present a three-dimensional model of the density distribution of a coronal mass ejection (CME) from 2008 April 26. This CME was observed by the two spacecraft composing the Solar Terrestrial Relations Observatory (STEREO), which tracked the CME from near the Sun, into the interplanetary medium (IPM), and all the way to 1 AU. The CME was directed toward STEREO-B and hit that spacecraft on 2008 April 29. The STEREO images of the CME show an internal structure that can be interpreted as having a flux rope shape. The two different perspectives on the event provided by the two STEREO spacecraft allow us to make a particularly strong argument for the flux rope interpretation, and the STEREO data also allow us to study the evolution of the flux rope in the IPM. The flux rope is oriented close to the ecliptic plane, but with the western leg tilted northwards by about 20°. This implies an orientation roughly perpendicular to the neutral line of the active region at the event's point of origin, apparently an unusual geometry given that previous analyses have found that CME flux ropes are usually, but not always, oriented parallel to the neutral lines of their source regions. The CME model also consists of a front out ahead of the flux rope, possibly a shock launched by the flux rope driver. The model density distribution is reasonably successful at reproducing the CME appearance both close to the Sun in coronagraphic images, and far from the Sun in images of the IPM from STEREO's heliospheric imagers. This suggests that self-similar expansion is a reasonable first-order approximation for this particular CME, and also indicates that the flux rope's orientation does not change much during its journey through the IPM.