Statistics and Classification of the Microwave Zebra Patterns Associated with Solar Flares

The microwave zebra pattern (ZP) is the most interesting, intriguing, and complex spectral structure frequently observed in solar flares. A comprehensive statistical study will certainly help us to understand the formation mechanism, which is not exactly clear now. This work presents a comprehensive statistical analysis on a big sample with 202 ZP events collected from observations at the Chinese Solar Broadband Radio Spectrometer at Huairou and the Ondrejov Radiospectrograph in Czech Republic at frequencies of 1.00 - 7.60 GHz during 2000 - 2013. After investigating the parameter properties of ZPs, such as the occurrence in flare phase, frequency range, polarization degree, duration, etc., we find that the variation of zebra stripe frequency separation with respect to frequency is the best indicator for a physical classification of ZPs. Microwave ZPs can be classified into 3 types: equidistant ZP, variable-distant ZP, and growing-distant ZP, possibly corresponding to mechanisms of Bernstein wave model, whistler wave model, and double plasma resonance model, respectively. This statistical classification may help us to clarify the controversies between the existing various theoretical models, and understand the physical processes in the source regions.