Thermal tide observations from ground-based measurements of the Zeeman-split emission lines of oxygen at 53 GHz

Several studies have shown the importance of solar tides for the dynamics in the MLT region. The solar tidal modes generated
in the troposphere and stratosphere increase in amplitude as they propagate vertically, transporting energy and momentum
to higher layers and enhancing layer mixing. The energy and momentum deposition by wave breaking alters the angular
momentum and kinetic energy budget and forces the global circulation in the MLT.

The majority of observations of solar tides have been derived from satellite data. Temperature and wind measurements from
satellites in geostationary orbits have been successfully used to derive tidal amplitudes around the equator. At higher latitudes,
however, the temporal resolution of the derived data product is limited by the orbital geometry of the satellites. With a revis-
iting time of several hours, the data set must be sampled over long periods to derive spectral components with periods of 8,
12, or 24 hours. In contrast, ground-based observations provide a comparably high time resolution of 0.5-1 hours, which is
suitable for investigating the short-time variability of solar tides. Observations of tidal amplitudes derived from ground-based
measurements using meteor radar systems, LIDARs, and microwave radiometers, are reported but are rare.

TEMPERA-C is a newly developed fully polarimetric ground-based microwave radiometer for temperature observations in the middle atmo-
sphere. It is designed to measure the four Stokes components of the Zeeman-split fine structure emission line of oxygen at 53
GHz. Compared to single polarized instruments, TEMPERA-C has an increased altitude coverage for temperature retrievals
with an upper limit of 60 km. By resolving the Zeeman-split emission line with a digital correlator with high frequency
resolution, retrievals of magnetic field features are possible. However, the calibration of a fully polarimetric instrument is more
complex than in the case of single polarization.

For a test campaign, TEMPERA-C measured continuously from March to November 2024 at the Jungfraujoch high-altitude
research station. In my presentation, I will focus on how thermal tides and other wave modes can be derived from this dataset.
I will also introduce the instrument, present a simplified calibration method, and discuss the influence of the Earth’s magnetic
field on the measured spectra.