Magnetic Properties at Footpoints of Hot and Cool Loops

Abstract

Observations of the solar corona with Yohkoh, the Solar and Heliospheric Observatory, and the Transition Region and Coronal Explorer (TRACE) have revealed that individual coronal loops of active regions have their own temperatures from 1 to 5 MK. The hot (2-5 MK) Soft X-Ray Telescope (SXT) loops appear to require more heating energy than the cool (1-2 MK) EUV loops. We investigate the photospheric magnetic signature for the hot and cool loops with the Advanced Stokes Polarimeter. In contrast to the cool loops, the hot loops observed with the SXT are usually diffuse, resulting in ambiguous identification of their footpoint locations. We use TRACE moss structure, which we confirm is low-lying EUV emission at the footpoints of the hot loops. Footpoints of both loops have magnetic fields whose strength is 1.2-1.3 kG, and the orientation is almost vertical to the surface. A significant difference is discovered in the magnetic filling factor, which is defined by the fraction of a pixel filled with a magnetized atmosphere. The footpoints of the hot loops have a lower filling factor than the footpoints of the cool loops. We suggest that braiding of coronal magnetic fields is more efficient at the footpoints of the hot loops than at the footpoints of the cool loops as a result of the combination of the lower filling factor and higher horizontal velocity.