Wave-like Formation of Hot Loop Arcades

Abstract

We present observations of hot arcades made with the Mg XII spectroheliograph onboard the CORONAS-F mission, which provides monochromatic images of hot plasma in the Mg XII 8.42 Å resonance line. The arcades were observed to form above the polarity inversion line between active regions NOAA 09847 and 09848 at four successive episodes: at 09:18, 14:13, and 22:28 UT on 28 February 2002, and at 00:40 UT on 1 March 2002. The evolution of the arcades can be described as: a) a small flare (precursor) appeared near the edge of the still invisible arcade, b) the arcade brightened in a wave-like manner - closer loops brightened earlier, and c) the arcade intensity gradually decreased in ≈1 h. The estimated wave speed was ≈700 kms−¹, and the distance between the hot loops was ≈50 Mm. The arcades formed without visible changes in their magnetic structure. The arcades were probably heated up by the instabilities of the current sheet above the arcade, which were caused by a magnetohydrodynamic wave excited by the precursor.