The Peak Flux Distribution of Solar Radio Bursts

We have investigated the peak flux distribution of 40 years of solar radio burst data as a function of frequency and time over a wide range of frequencies. The bursts were reported by observing stations around the world during 1960-1999, as compiled by the National Geophysical Data Center (NGDC) of the National Oceanic and Atmospheric Administration (NOAA). This period covers three full and two partial solar cycles. We have analyzed the data set to find correction factors for missed events, and find evidence that nearly half of the events were missed by the worldwide network. We obtain power-law fits to the differential (density) (dN/dS in events sfu-1) and cumulative [N(> S) in events] distributions as a function of frequency, time, and phase of the solar cycle. The typical power-law index, ~-1.8, is similar to that found in many hard X-ray studies. The average waiting time between bursts with flux density exceeding 1000 sfu was found to be 6 days at solar maximum, and 33 days at solar minimum. Taking account of missed events, the expected waiting time decreases to 3.5 and 18.5 days, respectively. Bursts of this flux level can cause problems with wireless communication systems. We present tables of fit parameters that can be used to find burst occurrence rates in a number of frequency ranges. We find no significant variation of power-law index from one solar cycle to the next, or with phase of the solar cycle, but we do find significant changes of power-law index with frequency.

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