Directivity of solar microwave bright regions

Abstract

The slowly varying component of solar radio emission associated with sunspots and plage regions were studied at various frequencies, particularly in the microwave region (CHRISTENSEN et aI., 1957; Cn~ISTENSEN et al., 1960; SWAgUP, 1961; KtJNDtJ, 1965). These studies indicated the main features of the solar active regions. Inferences were drawn regarding the size and shape of the active regions. The directivity of the regions was determined by measuring the flux received from a source during its change in position from limb to limb of the solar disk. Most of these measurements, at or near 10 cm 2, were made during high sunspot-activity period and the results indicated that the source behaves as though it were a disk of small but finite thickness lying parallel to the photosphere.In the present analysis Stanford microwave spectroheliograms (9.1 cm 2) (CRPL solar geophysical data, Part B) were used to investigate the directivity of bright regions of relatively weak intensity observed during 1963-65. The maximum brightness temperature of a source as its position on the solar disk changes from limb to limb was noted for each day and the values were normalized taking the brightness temperature at CMP as unity. The regions were divided into four categories depending on their brightness temperatures. They are:(I) regions with TB <75 • 103K (II) regions with TB>75 x 103 K but < 100 x 103K (III) regions with TB> 100x 103K but <250 • 103K (IV) regions with T B>250 • 103KThe variation of the brightness temperature for the four groups of regions is shown in Figure 1. Each curve represents an average of about 15-20 bright regions. We see that maximum emission occurs about 5 days before CMP for weak regions, and that this peak shifts gradually towards the central meridian with increase in the intensity of the source. For regions whose brightness temperature is >250 • 103K the maximum emission occurs near CMP and the centre-to-limb variation approximates to the cosine law reported by previous workers.The asymmetry observed for relatively weak sources is significant and conspicuous. It will be interesting to see if this behaviour is clearly seen for individual regions. Figure 2 shows some typical examples of radio emission associated with recurrent active regions. We find that in general the individual regions show the same asymmetrical tendency demonstrated by the average curve of 15-20 regions. We also note Solar Physics 4 (1968) 428-431 ; 9 D. Reidel Publishing Company, Dordrecht-Holland