The two-element interferometer at Hat Creek Observatory was used at 1.3 cm wavelength to study the fine structure of the radio emissive regions on the Sun. Observations of the quiet Sun at 1.3 cm show sudden changes in the fringe amplitude and phase, lasting for typically about 5-8 min. Assuming that these events are identical in nature, a plot of peak amplitude vs the projected baseline at the time of the event suggests emission from a region of angular size of about 10". The corresponding brightness temperature is 50000K. It is possible that these events may be related to the appearance and disappearance of groups of spicules or mottles.
During the period December 13-17, 1972, we used the millimeter wavelength interferometer at the Hat Creek Observatory of the University of California, Berkeley. This interferometer consists of a 10-ft antenna and a 20-ft antenna separated by a distance of 265 m along a baseline approx 50 ~ west of north. The interferometer was operated at 1.3 cm wavelength with a fixed base line. As we know, for complete synthesis, interferometry with variable baseline is required. However, the variation of the projected baseline onto the celestial sphere with hour angle gives maximum coverage in the u, v plane for sources of low declination in the sky (Figure ). Such is the case for the Sun in December when its declination is minimum, around -23 ~ Consequently from observations over a single day, it is possible to carry out limited synthesis of brightness distributions of various regions of the Sun. The objective of the present observations was to look for fine structure in the emissive regions at 1.3 cm wavelength at the limb, in quiet regions and in active regions of the Sun. However, since the primary beams of the antennas were large, our observations covered typically regions of about 10' arc in angular extent. The maximum resolution at 1.3 cm corresponding to the baseline of 20000 wavelengths was 10" arc.
The observations on each day were carried out by tracking specific regions on the Sun -E, W and N -limbs, and the center. On December 17, a region located at 8' to the north of the center was also observed. This region was chosen to represent an active region. Except for December 15, the center of the Sun was relatively quiet. The north limb was quiet throughout the observing period. Some weak plage activity was observed within the large beam area at the east limb on December 16 and 17 and at the west limb on December 16. On each day, the different regions were tracked alternately for a period of 15-30 min each throughout the day.
The interferometer data were in the form of fringe amplitude and phase for each 1 rain of integration along with the corresponding u and v values. A typical interferometer output fringe amplitude and phase are shown in Figure as a function of time,