Some Aspects of Type-IV Continuum Radiation on Decimeter Wavelengths

The flare associated continuum bursts of Type IV in the 500–1000 Mc/s range in general consist of a smooth continuum with different variabilities superimposed upon it (Kundu and Spencer, 1963). These variabilities can take various forms. They are often short-duration fast-drift bursts which appear to be different in nature from the high frequency components of meter-wave bursts of Type III. The intense fast-drift elements which are the most dominant radiation in some dm-λ continuum events are sometimes so blended together as to make the continuum radiation appear to be composed of a succession of fast-drift bursts of fine structure. Even in such events, the smooth background continuum appears clearly whenever the superimposing fast-drift elements are weak or absent (Figure la). Occasionally the fast-drift elements occur in the form of “sprays” (Figure lb). Sometimes drifting patches occur over short periods in course of the continuum event. The individual patches, usually of duration less than 1/2 minute, often drift at the rate of several Mc/s per sec from low to high frequencies over bandwidths of the order of 200–400 Mc/s in the 500–1000 Mc/s range (Figure 1c). In addition, intermediate-drift bursts are occasionally superimposed upon the continuum (Young et al. 1961); these are short duration (< 1 sec at one frequency) bursts drifting from high to low frequencies at rates of about 30 Mc/s per second. Often the superimposing variabilities are so mixed-drifting patches, fine-structure bursts, or a combination of both-as to make the continuum event appear rather complex. However, in the later stages of such events, the continuum becomes very smooth with practically no variabilities superimposed upon it. Because of the superimposing variabilities, the dm-λ Type IV appears more variable than the corresponding cm-λ and meter-λ Type IV.