First measurements of the magnetic field in a coronal hole from RATAN-600 radio observations of the Sun

Based on RATAN-600 spectral-polarization observations of the Sun in the microwave range 2 to 30 cm with a high angular and frequency resolution, we determined the magnetic-field strength in a coronal hole at the lower-corona level for the first time. At wavelengths longer than 8 cm, we detected circularly polarized emission associated with the passage of an isolated equatorial coronal hole across the disk during two solar rotations (CMP on October 12, 1996, and November 9, 1996). The measured degrees of polarization range from 0.2% (lambda = 9 cm) to 3-4% (lambda = 30 cm). The longitudinal magnetic field is calculated in terms of the thermal bremsstrahlung mechanism for the generation of solar-plasma radio emission in a weak magnetic field. Using Stanford photospheric measurements, we conclude that the longitudinal magnetic field in a coronal hole increases with height from ~ 0.2 G at the photospheric level to 7-10 G at the level of generation of the 18-cm emission with the subsequent conservation or with a decrease in this strength to the generation region of the 30-cm emission.