Solar observations at millimeter wavelengths

Solar radiations at millimeter wavelengths consist of a basic component of thermal radiation from an undisturbed sun, a slowly varying component originating from active regions on the sun, and a solar burst component. Eclipse measurements of the brightness distribution have led to a quiet sun model of the chromosphere containing small jet-like spicules. High-resolution millimeter-wave measurements of the solar disk show that the slowly varying component is enhanced radiation from bright spots correlating with plage areas. An increase in brightness of a plage area is observed as early as one day before a plage region develops a flare. Both gradual-rise-and-fall bursts and outbursts have been observed at 4.3 mm. The large outbursts occur in the flash phase of a solar flare. Some of these outbursts had higher flux at 4.3 mm than at longer wavelengths of 3.15 cm and 9.4 cm.