Diagnostics of radio fine structures around 3 GHz with Hinode data in the impulsive phase of an X3.4/4B flare event on 2006 December 13

On 2006 December 13 during the solar minimum, the superactive region NOAA 10930 at the S05W33 disk location produced an X3.4/4B flare at 02:40 UT. Fine structures were observed in the radio spectra, which included spikes, reverse slope-type III bursts, type-U burst, V-shaped burst, pulsations, zebra patterns, and firstly discovered sub-second spiky zebra-like structures, superimposed on the 2.6-3.8 GHz type IV bursts. The radio fine structures during the impulsive phase of the flare may be closely associated with coronal structures during the magnetic-reconnection process, as revealed by Hinode soft X-ray images. Thus, these microwave fine structure observations may provide very useful diagnostics at the primary energy release sites when they occur in the impulsive flare phase. For this flare event, the estimated coronal magnetic field is about 50-170 G in the rising phase of the flare with a source density of about 1 × 10¹¹ cm^-3. The field strength and plasma density are about 90-200 G and 1.27 × 10¹¹ cm^-3 around the flare maximum.