While the most familiar SADs are dark because of the lack of hot plasma within them,
this is believed to be due to loop shrinkage proceeding faster than the rate at which chromospheric
evaporation can fill the loop with plasma. How often is the opposite situation observed? Consider
the possibility that in some hypothetical flare, the chromospheric evaporation might be especially
vigorous. (We don’t speculate why---maybe the energetic particles impacting the chromosphere are more
energetic, or more populous, than in other flares.) Suppose that at the same time, the field line
shrinkage is slower than “normal” --- this could be due to weak field strength, or greater than normal
drag. In this hypothesized situation, it might be possible to see the evaporated plasma reaching up
into the coronal loop as the loop is still shrinking. Alternatively, if the loop gains mass through a
snowplow effect, the plasma within the loop may become heated as the loop shrinks. The loop gets brighter
and brighter in the X-ray images, as it gets shorter and eventually merges into the top of the arcade.
With that idea in mind, consider the images below, from a flare on 16 August 1998. The solar limb is
the gentle curve starting in the upper right, and the vertical smear is due to pixel saturation. Next
to the arrow, there are two features shrinking into the top of the arcade. They appear to brighten as
they get lower. The enhancement to the emission can be seen more clearly in a light curve: In the
boxed area shown at right, the X-ray signal was summed for each frame in the movie. The evolution of
this sum is shown in the light curve. The two humps in the light curve are due to the two shrinking
features. As the shrinking loops get brighter, the light curve intensity increases; as the loops shrink
out of the little box, the light curve decreases again. We reiterate that this behavior is seen often:
Of 40 flares known to be associated with supra-arcade downflows, half exhibit this kind of bright
loop shrinkage, including the famous 21-April-2002 flare observed by TRACE.
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