S. Yashiro et al., EARLY EVOLUTION OF CORONAL ACTIVE REGIONS OBSERVED WITH THE YOHKOH SOFT-X-RAY TELESCOPE - I - EXPANSION VELOCITY, The Astrophysical journal, 493(2), 1998, pp. 970
We study the early evolution of active regions in the corona by analyz
ing 33 emerging flux regions (EFRs) observed with the soft X-ray teles
cope (SXT) aboard Yohkoh during the period from 1992 February to 1996
May. We examine the time variation of the size (projected area) and th
e total soft X-ray intensity of the EFRs, and find that the time-avera
ged apparent velocity of the expansion of most of the EFRs is less tha
n 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 eme
rging magnetic loop in the upper chromosphere, inferred from Her obser
vations of arch filament systems as well as MHD simulation and theory.
Some possibilities to account for this discrepancy are discussed.