Flux density measurements of radio bursts of proton-producing flares and nonproton flares

Radio spectra of the maximum flux density of three of the proton events of 1966 have been analyzed in detail. Bursts from these proton-producing flares had distinct characteristics in the microwave portion of the spectrum and a typical shape in their flux density spectrograms. The spectral shape consists of the following: (1) High flux densities at meter wavelengths; (2) high flux densities at centimeter wavelengths; and (3) A minimum of flux density in the decimeter region. At all wavelengths, but especially at the meter wavelengths, high flux density persists over a relatively long time (type IV burst). From the form shown on the flux density radio spectrogram, these characteristics are termed the ‘U’-shaped spectrum. The majority of earlier radio proton (PCA) events very clearly had this shape. Even the smaller PCA's, often discounted on lists of principal events, appear to have radio spectra similar in shape to the larger events. The recent nonproton flare of optical importance 4B (February 13, 1967) did not display this spectrum. Its radio spectrum and low intensity, moreover has not been found among PCA events. It appears, therefore, that for proton events with PCA effects traceable to flares on the visible disk of the sun, the U-shaped radio spectrum with high flux densities at meter and centimeter wavelengths is the ‘preferred spectrum’