Microwave M burst on May 3 1999

We present a remarkable fine structure of "decimetric pulsation" event (DCI M), observed with the 2.60-3.80 GHz spectrometer in Beijing Astronomical Ob servatory on 3 May 1999; a "M" dynamic spectrum has been detected, thanks t o the high time and frequency resolutions. We call this fine structure a mi crowave M burst and propose a plausible model which treats the observed M b urst as an emission produced by the same electron beam travelling along mag netic lines and mirrored; this beam might be originally formed from the pla smoid of a type IV-DCIM burst. The M burst could be another radio evidence of the minor effect on electron beam after the N-burst. It is the first tim e that such a feature has been detected in type IV-DCIM bursts.