Temperature structure of solar active regions

We obtain high-quality temperature maps of solar active regions observed by the Yohkoh Soft X-ray Telescope. The temperatures of active region structures range from 3 MK to 10 MK. The bright X-ray loops do not necessarily have high temperatures, and faint structures sometimes have temperatures as high as 6 MK. The loop structures with shorter lifetimes (less than a few hours) generally have higher temperatures (5-8 MK) than the loops with longer lifetimes (3-4 MK). The plasma with temperature reaching 6-7 MK has cusp structures like solar flares (mini-cusp) or has multiple loop structures in the temperature maps in most cases. The 6-7 MK plasma is transiently heated either by magnetic reconnection at the neutral sheet above the cusp structures, or by magnetic reconnection of multiple loops. The 3-5 MK plasma is more steadily and uniformly heated. Two different mechanisms are apparently involved in the coronal heating.