Observations of Core Fallback during Coronal Mass Ejections

Abstract

White-light observations made with the Large Angle and Spectrometric Coronagraph (LASCO) during the present solar maximum have revealed a multitude of faint, inward-moving features at heliocentric distances of r ~ 2-6 R☉. Most of these structures appear to originate above r ~ 3 R☉ and may be signatures of the closing-down of magnetic flux at the boundaries of coronal holes or in the aftermath of coronal mass ejections (CMEs). Here, we present observations of a different type of inflow, in which material within the bright core of a CME collapses back toward the Sun after rising to heights of r ~ 2.5-6 R☉. We have identified roughly 20 such fallback events during 1998-2001. The core structures, which have the form of loops or concave-outward flux ropes, ascend into the coronagraph field of view beyond 2 R☉ with speeds of ~100-400 km s-1 but return with speeds of only ~50-200 km s-1. The initial deceleration rates of ~20-100 m s-2 are comparable to the local gravitational deceleration GM☉/r2 but continually decrease with time. The associated CMEs tend to be impulsive but relatively slow, with the leading front moving outward at ~250-450 km s-1 and often showing some deceleration. It is thus not surprising that some fraction of the core material fails to reach escape speeds, remaining bound to the Sun by gravitational and magnetic tension forces. We suggest that the dynamical behavior of the core may be determined in part by momentum exchanges with the background medium, which consists of ongoing outflows of CME material, ambient solar wind, and inflow streams. In particular, we attribute the asymmetry of the up-down trajectories to the action of such drag forces, whose direction changes from inward to outward as the core decelerates.