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J. Lin (UNH), T. G. Forbes (UNH)
Recently, using data from Yohkoh, Svestka and his co-workers have analyzed giant X-ray arches that appear to be very similar to `post'-flare loops except that they are much larger and have a different pattern of growth. Instead of growing upwards at a rate which decreases with time, the giant arches move upwards at rate which increases with time. Because of this difference, Svestka has suggested that the giant arches may be fundamentally different from `post'-flare loops. Here we show theoretically that this need not be the case. By introducing reconnection with a finite rate into a flux-rope model of coronal mass ejections (CMEs), we show that an increasing rate of upward motion can occur in a loop system if it reaches sufficiently high altitudes.
Our two-dimensional model contains a magnetic flux-rope which loses equilibrium when the photospheric sources of the magnetic field are moved together. Initially there is no current sheet, but once the flux rope loses equilibrium and starts to move upwards, a current sheet appears. As the current sheet grows, reconnection tends to dissipate it. During the early phase of the eruption, the current sheet grows too quickly for the reconnection to have a major effect, but during the very late phase, reconnection eventually starts to dominate and this leads to a rapid shrinkage of the current sheet. During the early phase, the velocity of the lower tip of the current sheet decreases with time, but during the late phase, the opposite is true.
Our model shows that the upward motion of the lower tip of the current sheet is sensitive to the vertical gradient of the Alfvén speed in the corona. Because the Alfvén speed increases with height, reconnection is more effective at high altitudes than at lower altitudes in dissipating the current sheet. We estimate that a loop system associated with the lower tip of the current sheet will start to speed up a few hours after onset if heights in excess of around 105km are reached. Thus, we conclude that the giant arches are essentially the same phenomenon as post-flare loops and show a different pattern of motion only because of the extreme heights at which they occur.
If the author provided an email address or URL for general inquiries, it is a
s follows:
http://www.sr.unh.edu/~jl/