Polarized horseshoes around sunspots at 6 centimeter wavelength

Abstract

Horseshoe-shaped structures associated with sunspot penumbrae have been detected in 6 cm synthesis maps of circular polarization taken with the Westerbork synthesis radio telescope. The high degree of circular polarization (rho/sub c/ = 95%) of the horeshoes requires gyroresonant emission. In fact, the horeshoe structures were predicted from the theory of the gyroresonance emission of individual sunspots. The absence of polarized emission above the sunspot umbrae is also explained by gyroresonance theory. In sharp contrast to the polarized emission, the total intensity of the 6 cm radiation is enhanced above the sunspot umbrae and exhibits a remarkable correlation with the longitudinal magnetic field of the underlying photosphere. Brightness temperatures of T/sub B/roughly-equal10/sup 6/ K are found above sunspot umbrae in coronal regions where the longitudinal magnetic field strength Hiota = 600--900 gauss. Temporal instability precludes the detection of cool material above sunspot umbrae in synthesis maps which are only sensitive to the much more stable and long-lived hot material.