Abstract
X-ray emitting hot plasmas of a temperature 2-12 keV, associated with galaxy clusters, constitute the most dominant form of baryons in the universe. The plasma is thought to be magnetized at least to 0.1-1 nT, although the β value is relatively high at 10-100. The plasma is confined by gravity of each cluster in a stable manner, but the member galaxies are moving through it with an approximately transonic speed. Therefore, the plasma must be in a magnetohydrodynamically turbulent condition, where frequent magnetic reconnection is expected to take place. Mainly based on observations with the ASCA X-ray Observatory, evidence for such magnetic reconnection is accumulating.
