CALIBRATION OF A FULLY POLARIMETRIC TEMPERATURE RADIOMETER TO OBSERVE THE ZEEMAN-SPLIT EMISSION LINES OF OXYGEN AT 53 GHZ

W. Krochin¹, R. Albers¹, G. Stober¹, A. Murk¹

¹University of Bern

The 60 GHz oxygen band is widely used for space-borne as well as ground-based temperature sounding applications because of the oxygen’s relatively constant volume mixing ratio up to the upper mesosphere and the strong temperature dependence of its microwave emission spectra. For retrievals of tropospheric temperature, channels with bandwidths in the MHz range are used to observe the wings of the emission complex, while for retrieving temperature in the middle atmosphere, fine structure lines are resolved using high-resolution spectrometers with channel widths in the range of a few kHz. However, due to oxygen’s magnetic moment, the Zeeman broadening effect occurs through coupling to the Earth’s magnetic field. In the upper mesosphere, Zeeman broadening dominates over pressure broadening, leading to a natural upper altitude limit for temperature retrievals. To address this issue, we have developed a fully polarimetric radiometer for ground-based temperature sounding (TEMPERA-C) in the Institute of Applied Physics at the University of Bern [1].

The instrument uses an orthomode transducer to split the two orthogonally oriented linear polarization states and guide them into two separate but identical receiver chains. Each chain uses an LNA to amplify the signal and a coherent heterodyne receiver for down-mixing. In the backend, a digital FFT spectrometer calculates a complex spectrum for each polarization. Afterwards, the two spectra are digitally self- and cross-correlated to produce all four components of the Stokes polarization vector. In addition to linear polarization, this technique also allows to measure circular polarization. The spectrometer is tuned to two fine structure emission lines with center frequencies at 53.067 GHz and 53.596 GHz, which are measured simultaneously, each with a bandwidth of 100 MHz and a resolution of 25 kHz. With the fully polarimetric measurements, the upper altitude limit for retrievals of atmospheric temperature profiles can be extended to over 60 km. In addition, an overall increase in retrieval quality measures, such as the values of measurement response and altitude resolution, can be reported [2].

For a test campaign, TEMPERA-C was installed at the High Altitude Research Station Jungfraujoch from March to November 2024. For ground-based temperature sounding in the middle atmosphere, the high altitude of 3571 m a.s.l. is advantageous due to reduced tropospheric influence compared to measurement sites at sea level. In this case, we observed an increase in line strength by a factor of 4 compared to measurements taken in Bern (550 m a.s.l.). For the test campaign, we developed a case-specific calibration method, in which a set of cross-polarization parameters, assumed to remain constant over the campaign, were measured in the laboratory. The instrument gain was measured using a an ambient temperature calibration target and one noise diode for each polarization. Additional phase-imbalance parameters were also measured on-site by making use of the characteristics of the V-Stokes component for the observed transitions.

In 2025 TEMPERA-C was updated with a new mounting that allows for 360° scanning in both azimuth and elevation directions. In this setup, the characteristic dependence of the Q- and V-Stokes components on the angle between the line of sight and the magnetic field lines can be investigated. First results show a maximal V-Stokes amplitude when the instrument is directed parallel to the magnetic field lines, as predicted by simulations.

In my talk, I will present specifications and technical details of TEMPERA-C and discuss our calibration method, as well as the retrieval algorithm and the advantages of fully polarimetric observations for temperature sounding in the middle atmosphere. Furthermore, I will present the results from the measurement campaign at the high-altitude observatory and discuss the benefits of the elevated measurement site for ground-based applications. Finally, I will briefly discuss the polarized Zeeman effect and how this effect can be used for remote sounding of the Earth’s magnetic field with our instrument.

References:

1. Krochin, W., Stober, G., and Murk, A.: Development of a Polarimetric 50-GHz Spectrometer for Temperature Sounding in the Middle Atmosphere, IEEE Journal of selected topics in applied earth observations and remote sensing, 15, https://doi.org/10.48350/186172, 2022

2. Krochin, W., Murk, A., Luder, A., Stober, G.: Operational calibration of a fully polarimetric radiometer for stratospheric temperature retrievals, EGUSPHERE-2025-2561 (Under Review), 2025