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Large solar flares occasionally trigger significant space-weatherdisturbances that affect the technological infrastructures of moderncivilization, and therefore require further investigation. Although these solarflares have been monitored by satellite observations since the 1970s, largesolar flares occur only infrequently and restrict systematic statisticalresearch owing to data limitations. However, Toyokawa Observatory has operatedsolar radio observations at low frequencies (at 3.75 and 9.4 GHz) since 1951and captured the early great flares as solar radio bursts. To estimate themagnitudes of flares that occurred before the start of solar X-ray (SXR)observations with the Geostationary Operational Environmental Satellite (GOES)satellites, we show the relationship between microwave fluxes at 3.75 and 9.4GHz and X-ray fluxes of flares that occurred after 1988. In total, we explored341 solar flares observed with the Nobeyama Radio Polarimeters and ToyokawaObservatory from 1988-2014 and compared them with the SXR observations recordedby the GOES satellites. The correlation coefficient was approximately 0.7.Therefore, the GOES X-ray class can be estimated from the peak flux at 3.75 and9.4 GHz with a large variance and an error of factor of 3 (1 sigma). Thus, forthe first time, we quantitatively estimated the light curves of two early solarflares observed in 1956 February by the Toyokawa solar radio observations usingthe relationship between SXR thermal radiation and microwave nonthermalradiation (Neupert, 1968, ApJ, 153, 59).

Relationship of peak fluxes of solar radio bursts and X-ray class of solar flares: Application to early great solar flares