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(Solar Phys.). It is known that the sources of metric type IV radio bursts generally move outwards with speed of a few to several hundred km s−1 in the solar atmosphere and envelope (e.g., Kundu, 1965; Smerd and Dulk, 1971; Wild and Smerd, 1972). These sources consist of magnetic lines of force being stretched out from the flare regions and relativistic electrons being trapped by these field lines. They are often observed as expanding magnetic bottles (e.g., Smerd and Dulk, 1971). As a result of this expanding motion, the frequency range of type IV burst extends with time to lower frequencies, because of the decrease of both magnetic field intensity and energy of radiating electrons (Dulk, 1970; Sakurai, 1973a). By considering the observed expansion rate of emission frequency range, it seems possible to estimate the pattern of expansion of these radio sources: in doing so, we have analyzed the onset time differences between microwave and metric emissions of type IV bursts by referring to the longitudinal positions of parent solar flares on the solar disk, since the dependence of these differences on parent flare positions in the solar longitude seems to give a clue to estimate a general pattern of the expansion of magnetic bottles. The result thus analyzed is shown in Figure 1. Furthermore, peak flux intensities at metric frequencies for these type IV bursts have been also analyzed as a function of the longitude positions of parent flares (Figure 2). The results shown in Figures 1 and 2 are explained by assuming that, while moving outwards, the magnetic bottle tends to expand a few 10 deg east of the meridian plane which crosses the flare region (Sakurai, 1973a).
