The Origin of Postflare Loops

We apply a tracking technique, previously developed to study motions in the outer corona by Sheeley, Walters, Wang, and Howard, to 195 Å filtergrams obtained with the Transition Region and Coronal Explorer (TRACE) satellite and obtain height-time maps of the motions in the hot (10-20 MK) plasma clouds above postflare loop systems. These maps indicate the following two main characteristics. (1) Within the plasma cloud, the motions are downward at speeds of approximately 4 km s^-1. The cloud itself grows with time, its upper layers being replenished by the arrival and deceleration of fast inflows and its lower layers disappearing when they cool to form the tops of new postflare loops. (2) Early in these events, the inward motions are turbulent, showing a variety of dark elongated features resembling "tadpoles" and some bright features. Later, the inflows are visible as dark collapsing loops, changing from initially cusp-shaped features to rounder loops as they move inward. Their speeds initially lie in the range 100-600 km s^-1 but decrease to 4 km s^-1 in about 3 minutes, corresponding to an average deceleration ~1500 m s^-2. Combining these observations with similar observations obtained at reconnection sites in the outer corona by the Large Angle Spectrometric Coronagraph (LASCO), we conclude that postflare loops are the end result of the formation, filling, deceleration, and cooling of magnetic loops produced by the reconnection of field lines blown open in the flare. The formation of collapsing loops occurs in the dark tadpoles; the filling of these initially dark loops occurs via chromospheric evaporation, which also contributes to the deceleration of the loops; and the radiative cooling ultimately resolves the loops into sharply defined structures.