The frequency ratio of bands of microwave spikes during solar flares

The frequency ratio of narrowband millisecond spikes during 17 solar flares has been measured and analyzed. The observations have been made in the frequency range 0.3-3 GHz with the ETH Zuerich spectrometers. The events have been selected from spectrograms for their harmonic structure. Auto-correlation in frequency yields the following major results: (i) The ratio of the harmonic spike groups is not integer, and therefore the term 'harmonic' is strictly speaking inappropriate. (ii) The ratios are independent of frequency. (iii) The ratios range from 1.06 to 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 results 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 several cases 3, and in one case 4 harmonic bands have been observed. Previously proposed interpretations of harmonic spike emission are discussed in view of the new observations.