  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 A 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 arcades all evolved in the same way: 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 $\approx$ 1 h. The
estimated wave speed was $\approx$ 700 km s$^{-1}$, and the distance between
the hot loops was $\approx$ 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 an
MHD wave excited by the precursor.
