Intensity Calibration for the Mingantu Spectral Radioheliograph Images

The MUSER, a new generation of solar-dedicated radio imaging spectroscopic telescope, has achieved high-time, high-angular, and high-frequency resolution imaging of the Sun within the 0.4–15 GHz ultra-broadband frequency range. The radio brightness temperature is a critical parameter in describing solar physical processes. It plays a significant role in studying various radio radiation mechanisms, the solar magnetic field, and the acceleration of non-thermal particles during solar bursts, making brightness temperature calibration essential for radioheliograph images. This paper introduces a method suitable for calibrating the intensity of radioheliograph images. Given that solar radio images contain structural information about the solar disk, the radio radius and intensity of the quiet solar disk can be obtained by fitting Bessel function of the first kind with the visibilities of short baselines from the radioheliograph. Subsequently, the Rayleigh-Jeans law and the daily solar radio flux are used to calculate the calibration factor $G_c$ for the daily image, thereby achieving image intensity calibration of the radioheliograph. Applying this method to actual observation data from MUSER, including scenarios such as the quiet Sun and solar radio bursts, shows that the error of the daily calibration factor $G_c$ is within 10% of its value, and the obtained brightness temperature of the quiet Sun strongly correlates with results from other studies. These results indicate the method's feasibility and effectiveness.