| 20 May 2026
On the Feasibility of Remotely Sensing the Earth's Magnetic Field Using a Ground-Based Fully Polarimetric Microwave Radiometer
Abstract. The Zeeman effect affects millimeter-wave emission lines from atmospheric oxygen through the interaction between the molecular magnetic dipole moment and the geomagnetic field, leading to characteristic line splitting and polarization signatures. Depending on the total angular momentum quantum number, the split components may appear as distinct peaks or as effective line broadening when unresolved. These signatures are observable in molecular oxygen emissions from the stratosphere and mesosphere, where pressure broadening is weak compared to the troposphere. The resulting line shape5 depends on magnetic field strength, viewing geometry, and polarization. The impact of the magnetic field on the line-spectra provides a potential pathway to remotely sense geomagnetic field perturbations. In this study, we examine the feasibility of such measurements using two oxygen fine-structure transitions at 53.067 GHz and 53.596 GHz. Radiative transfer simulations and synthetic retrievals indicate sensitivity to magnetic field perturbations at altitudes between 30 and 75 km. The method is further evaluated using fully polarimetric observations from the Campaign Temperature Radiometer (TEMPERA-C) during a10 geomagnetic storm associated with a strong solar flare in January 2026. Applying the developed inversion scheme, we retrieve a perturbation of the vertical magnetic field component of −130 ± 62 nT for the event.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Annales Geophysicae.
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General comments
This study evaluates the feasibility of remotely sensing the geomagnetic field at stratospheric and mesospheric altitudes by exploiting the magnetic field's influence on the spectral lines of thermal emission from molecular oxygen, which, via the Zeeman effect, splits these spectral lines and alters the radiation's polarization characteristics. These spectra depend strongly on the geomagnetic field, since oxygen's magnetic dipole couples to it producing measurable spectral shifts. Using the relation between the magnetic field and spectral characteristics, a method is proposed for remote monitoring of geomagnetic variations. The method was validated through observations using the TEMPERA-C radiometer, targeting two oxygen fine-structure transitions at frequencies around 53 GHz. Implementing this technique to monitor magnetic field disturbances requires a full-polarization microwave radiometer capable of detecting oxygen emissions at altitudes of 30–75 km. The authors performed atmospheric radiative transfer modeling to retrieve the characteristics of magnetic field disturbances. Observations conducted during a geomagnetic storm show a disturbance of −130 ± 62 nT in the vertical component of the magnetic field. This result demonstrates the robustness of the proposed method, which is practically the only technique for direct, continuous measurement of geomagnetic field variations at mesospheric altitudes. When scaled up, such data will significantly improve geomagnetic field models for altitudes inaccessible to satellite measurements, particularly under disturbed conditions, making this work both interesting and important.
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