  The opening-up of the magnetic field during solar eruptive events is often
accompanied by a dimming of the local coronal emission. From observations of
filament eruptions recorded with the Extreme-Ultraviolet Imager on STEREO
during 2008-2009, it is evident that these dimmings are much more pronounced in
19.5 nm than in the lower-temperature line 17.1 nm, as viewed either on the
disk or above the limb. We conclude that most of the cooler coronal plasma is
not ejected but remains gravitationally bound when the loops open up. This
result is consistent with Doppler measurements by Imada and coworkers, who
found that the upflow speeds in a transient coronal hole increased dramatically
above a temperature of 1 MK; it is also consistent with the quasistatic
behavior of polar plumes, as compared with the hotter interplume regions that
are the main source of the fast solar wind. When the open flux reconnects and
closes down again, the trapped plasma is initially heated to such high
temperatures that it is no longer visible at Fe IX 17.1 nm. Correspondingly,
17.1 nm images show a dark ribbon or ``heat wave'' propagating away from the
polarity inversion line and coinciding with the brightened Fe XV 28.4 nm and Fe
XII 19.5 nm post-eruptive loops and their footpoint areas. Such dark ribbons
provide a clear example of dimmings that are not caused by a density depletion.
The propagation of the ``heat wave'' is driven by the closing-down, not the
opening-up, of flux and can be observed both off-limb and on-disk.
