Variations of the radiation signatures of noise storm-emitting sunspot groups during a solar cycle

Abstract

The flux density of a noise storm continuum is known to depend on importance parameters of the associated sunspot group, e.g., its total area A. A study of the continua at 287, 234, 113, and 64 MHz, however, reveals in case of the two cycles Nos. 20 and 21 that the radiation signatures of sunspot groups, with a value of A kept fixed, vary systematically with time indicating regular changes of relevant parameters of the overlying loop systems with the phase of the solar cycle. A trend of intense continua at high frequencies (for definition, cf. Figures 1, 2(b)) to occur preferably during the first activity maximum of a solar cycle has been obtained in either case suggesting a decrease of the emissivity of sunspot groups with time. Vice versa, intense continua at lower frequencies (for definition, cf. Figures 1, 2(b)) were mainly observed during the later phase of both cycles. The latter effect is shown to be attributed rather to a long-term variation of the spectral characteristics of the type-I continua than to an enhanced number of intense type-III continua. From the result obtained it follows that non-potential loops extending to great heights into the corona or developing at least conditions favourable for the generation of an intense type-I continuum even at the frequencies < 100 MHz tend to occur more frequently above sunspot groups during the later phase of a solar cycle than above the comparable groups of its first activity maximum.

Furthermore, characteristic periods have been found for both cycles during which the emissivity, especially of the very large sunspot groups, was significantly diminished with reference to the comparable groups of the adjacent time intervals.