Energy balance in sources of long-term soft solar X-ray emission: Formation of streamers

We consider solar events accompanied by a soft X-ray emission lasting 0.5-1 day. We analyze in detail the weak event of January 24, 1992, which was detected by the Yohkoh satellite, but not in the integrated solar flux at 1-8 Å. The energy balance in a coronal loop is studied via numerical simulation of the gas dynamical processes taking place during plasma heating near the top of an arcade. In the case of slow heating, the density is initially redistributed over the loop. Then, both the temperature and density vary slowly (with a characteristic time scale of about one hour), reflecting variations of the heating in the tenuous plasma in the presence of an extremely high efficiency of heat transfer. The calculations describe the observational data best for a total energy of 2-3 × 10³⁰ erg, providing evidence that the energy spread for different long-term events is much smaller than the energy spread for impulsive flares. Many of such events are the first stages in the formation of streamers. We also show that energy release in a vertical current sheet takes place in distinct steps, rather than continuously.