High resolution interferometry of the Sun at 3.7 cm wavelength

Abstract

Interferometric observations of the Sun at a wavelength λ = 3.7 cm and an effective angular resolution of θ = 7 or 15″ are presented. When active regions are observed, circularly polarized radiation is found with an angular size of θ ≈ 15″, an effective temperature of T ≈ 5 × 105K, and 20 to 30% circular polarization. This S-component of solar radio radiation is interpreted in terms of the theory of gyroradiation and gyroresonant absorption. Prior to the onset of a solar flare, an additional S-component is observed with an angular size of θ ≲ 7″, an effective temperature of T ≥ 106 K, and 90 ± 10% circular polarization. A small scale, quasi-periodic component of solar radio radiation is also observed to be coming from all over the solar disk; and this component is found to be less than 10% circularly polarized. The angular sizes, θ, and periods, P, of this component lie in the ranges 7″ ≲ θ % 37″ and 180 s ⪞ P ≲ 750 s. The observed modulation in flux density, ΔS, lies in the range 20 f.u. ≲ ΔS ≲ 200 f.u. (1 f.u. = 10-26 Wm-2 Hz-1 or 10-23 e Hz-1) and the brightness temperature fluctuations, ΔT, lie in the range 103K ≲ ΔT ≲ 105K. This component of solar radio radiation is thought to be the free-free radiation (bremsstrahlung) of temperature fluctuations associated with velocity oscillations in the chromosphere-corona transition region. High resolution observations of impulsive microwave bursts show that some of the radiation is linearly or circularly polarized, has an angular size θ ≤ 7″, and a peak brightness temperature of T ≳ 3 × 106K. This component of solar radio flares is interpreted in terms of the theory of gyro-synchrotron radiation, and the density of accelerated electrons is found to be greater than 100 cm-3. The size of the emitting region is, however, comparable to that of the feet of magnetic dipoles rather than the region between them. Phase shifts in the interferometer pattern observed during an impulsive burst are interpreted in terms of hydrodynamic waves travelling at a velocity of 7000 km s-1, but further observations are needed to confirm the result.