An index of impulsiveness for 2800 MHz impulsive solar noise bursts

The initial growth of intensity, I, of the impulsive solar noise burst observed at 2800 MHz has been fitted with a parabolic curve of the form I=ct ²and the quantity c taken as the index of impulsiveness. Two groups of bursts comprising 85% of all impulsive bursts observed in 1962–63 and 1966 were selected for study. A good fit has been obtained for bursts having peak flux density up to 20 flux units, while for more intense bursts, the average observed growth is more rapid than the parabolic rate. The distribution of the index in the range 0.1 to more than 100 shows two peaks, one for c values 1–10 and another apparent peak for those with c greater than 100. The index is independent of the peak intensity of the burst and its position on the solar disc, while there is a small trend, indicating that shorter bursts are more impulsive than longer duration events. The more easily derived linear rate of rise, b = Peak Flux/Interval from start to peak is related to the parabolic impulsive index by b = 1.86 c ^0.57. The non-linear rate of expansion of a flaring volume suggested by Pneuman when applied to explain the parabolic rise of microwave bursts indicates that the impulsiveness of bursts is inversely related to the contained magnetic field.