Direct observations of plasma upflows and condensation in a catastrophically cooling solar transition region loop
Orange, Norton B.
Chesny, David L.
Oluseyi, Hakeem M.
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Minimal observational evidence exists for fast transition region (TR) upﬂows in the presence of cool loops. Observations of such occurrences challenge notions of standard solar atmospheric heating models as well as their description of bright TR emission. Using the EUV Imaging Spectrometer on boardHinode, we observe fast upﬂows (vλ −10 km s−1) over multiple TR temperatures (5.8 logT 6.0) at the footpoint sites of a cool loop (logT 6.0). Prior to cool loop energizing, asymmetric ﬂows of + 5 km s−1 and −60 km s−1 are observed at footpoint sites. These ﬂows, speeds, and patterns occur simultaneously with both magnetic ﬂux cancellation (at the site of upﬂows only) derived from the Solar Dynamics Observatory’s Helioseismic Magnetic Imager’s line-of-sight magnetogram images, and a 30% mass inﬂux at coronal heights. The incurred non-equilibrium structure of the cool loop leads to a catastrophic cooling event, with subsequent plasma evaporation indicating that the TR is the heating site. From the magnetic ﬂux evolution, we conclude that magnetic reconnection between the footpoint and background ﬁeld is responsible for the observed fast TR plasma upﬂows.