Loop-top nonthermal microwave source in extended solar flaring loops

Nobeyama Radioheliograph (NoRH) microwave data provide us with unique information about the radio brightness distribution along flaring loops. In particular, it has been found that for several events with extended looplike sources well resolved with NoRH, the brightness maximum at 17 and 34 GHz is located at the top of the corresponding flaring loops. The detailed analysis of these events strongly suggests that the distribution of mildly relativistic electrons along an extended flaring loop must be highly inhomogeneous: accelerated electrons are concentrated in the upper part of the loop. This finding imposes important new constraints on the acceleration/injection mechanisms and on the kinetics of high-energy particles in solar flares.