Analysis of the remote sensing mechanism and influencing factors of microwave radiometers for the Earth's magnetic field

The remote sensing of the geomagnetic field via microwave radiometry represents an innovative approach to rapidly acquire global geomagnetic field distribution data, which is pivotal for both research and practical applications of the geomagnetic field. This paper delves into the underlying principles of atmospheric remote sensing to elucidate the mechanisms of geomagnetic remote sensing. We analyze the Zeeman effect on atmospheric oxygen spectral lines, the resultant spectral line splitting, and polarization induced by the geomagnetic field. Utilizing a vector radiative transfer model expressed in Stokes parameters, we simulate the Zeeman effect of atmospheric oxygen molecules and the consequent impact on the brightness temperature of microwave radiation. Simulations, based on International Geomagnetic Reference Field (IGRF) data, demonstrate that brightness temperature variations can reach tens of Kelvin within the global geomagnetic field intensity range of approximately 40 μT, with these variations being significantly correlated to the global geomagnetic field intensity distribution. We further simulate the effects of observation angle, frequency band, and magnetic field intensity on the brightness temperature of fully polarized radiation. The findings indicate that the sensitivity of brightness temperature to magnetic field information is most pronounced within the MHz range near the oxygen spectral line center of THz bands. Due to the distinct characteristics of Zeeman splitting spectral lines, the brightness temperature of different polarizations and their sensitivity to magnetic field intensity vary across different oxygen absorption bands. At the oxygen absorption peak of 61 ~ 773 GHz, the sensitivity to magnetic field intensity changes is approximately 2 K/μT. Additionally, the angle between the radiation vector and the magnetic field vector significantly influences the brightness temperature characteristics of different polarization states. Notably, the T3 and T4 polarization brightness temperatures exhibit the strongest signal when the radiation vector is parallel to the magnetic field vector, with T4 polarization showing greater sensitivity to magnetic field changes. These insights underscore the necessity for a comprehensive analysis of various parameter configurations in the future design of spaceborne microwave radiometer-based geomagnetic field remote sensing detection schemes to identify the optimal detection combination.