Supra-arcade Downflows in Long-Duration Solar Flare Events
by David E. McKenzie
Abstract
This report seeks to introduce the reader to a set of observations made
during the decay phase of long duration event (LDE) flares on the Sun. In
a number of events, the soft X-ray images from Yohkoh SXT indicate
a downward directed flow field in the region immediately above the flare
arcade. The prototypical event is the M5 flare of 20 January 1999,
presented by McKenzie & Hudson (1999); since the time that paper was
completed, eleven more events, all related to coronal mass ejections, have
been found in the interval 01 January 1998 to 08 May 1999. As in the 20
January event, the speeds of downflow are 45 -- 500 km/s, lower than both
the freefall speed and the typically assumed Alfven speed. A comparison
is drawn between the SXT observations and the EUV and H-alpha data, where
such are available. Important additions to our knowledge since the first
paper are: (i) the motion is evidenced by both dark and bright (i.e.,
X-ray emitting) features, some of which may have a looplike morphology;
(ii) no cool counterparts have been detected in H-alpha or EUV
observations. The data are compared to the expected appearance according
to two likely (and opposing) interpretations: an "above-the-arcade
coronal rain" interpretation and a "shrinking magnetic flux tube"
interpretation. In the current standard explanation of the formation of
post-CME arcades, motion of the latter type may be related to outflow from
a reconnection site. Movies depicting some of the flare events are on the
accompanying CD-ROM.
Accepted by Solar Physics, February 11, 2000.