Type IV dm bursts: onset and sudden reduction

The effect of collisions of suprathermal electrons with a thermal background plasma is investigated and is shown to cause flattening of a monotonically descending velocity distribution of fast particles. As a result flare-produced energetic electrons that are trapped in a coronal magnetic arch and that are initially distributed in energy according to a power-law, can give rise to an instability of Langmuir waves in the background plasma and the subsequent emission of continuum radiation as observed in type IV dm bursts. To explain the appearance of 'sudden reductions' observed in type IV dm outbursts the effects both of magnetic compressions of the source region and of renewed injections of fast particles on the continuum source are investigated. It is found that the latter process can explain the observed reductions. (Auth.)