THE FREQUENCY RATIO OF BANDS OF MICROWAVE SPIKES DURING SOLAR-FLARES

The frequency ratio of narrowband millisecond spikes during 17 solar f lares has been measured and analyzed. The observations have been made in the frequency range 0.3-3 GHz with the ETH Zurich spectrometers. Th e events have been selected from spectrograms for their harmonic struc ture. Autocorrelation in frequency yields the following major results: (i) The ratio of the harmonic spike groups is not integer, and theref ore the term 'harmonic' is strictly speaking inappropriate. (ii) The r atios are independent of frequency. (iii) The ratios range from 1.06 t o 1.54 with a prominent peak at 1.39+/-0.01. The cross-correlation of the time profile of harmonic groups peaks at zero lag, demonstrating a tight relation between individual spikes in associated groups. The re sults require an emission mechanism that can produce harmonic emission at a ratio 5:7 or 1:1.4 and vary significantly from this value. In se veral cases 3, and in one case 4 harmonic bands have been observed. Pr eviously proposed interpretations of harmonic spike emission are discu ssed in view of the new observations.