Continual Expansion of the Active-Region Corona Observed by the Yohkoh Soft X-Ray Telescope

Abstract We have found from the observations of the Yohkoh Soft X-ray Telescope (SXT) that the corona above active regions expands occasionally, and almost continually in the cases of “active” active regions. This is contrary to the commonly accepted idea of magnetohydrostatic equilibrium of these regions. The key to this discovery has been a movie representation of the Yohkoh-SXT data, which, for the first time, provides adequate sampling and continuity for this purpose. The movies show ubiquitous expansions above the active regions, with velocities in the range of a few to a few tens km s−1 as measured when they are on the limb. The expansion appears to preserve the overall structure of the active-region corona. We suggest that the expansion may have a physical relationship with the transient loop brightenings found within the active regions. This finding of almost continual expansion of the active-region corona may affect some of the basic ideas concerning active regions, as well as those of the mass-loss from the Sun and Sun-like stars.