Early Evolution of Coronal Active Regions Observed with theYohkohSoft X‐Ray Telescope. I. Expansion Velocity

Abstract

We study the early evolution of active regions in the corona by analyzing 33 emerging flux regions (EFRs) observed with the soft X-ray telescope (SXT) aboard Yohkoh during the period from 1992 February to 1996 May. We examine the time variation of the size (projected area) and the total soft X-ray intensity of the EFRs, and find that the time-averaged apparent velocity of the expansion of most of the EFRs is less than 2 km s-1 during the very early phase (t < 6-14 hr after the birth of the EFRs). The average expansion velocity of 33 EFRs is 1.5 km s-1. This expansion velocity is much lower than the rise velocity of emerging magnetic loop in the upper chromosphere, inferred from Hα observations of arch filament systems as well as MHD simulation and theory. Some possibilities to account for this discrepancy are discussed.