A Measurement of Atmospheric Circular Polarization with POLARBEAR
The Astrophysical Journal(2025)
Abstract
At millimeter wavelengths, the atmospheric emission is circularly polarized owing to the Zeeman splitting of molecular oxygen by the Earth's magnetic field. We report a measurement of the signal in the 150 GHz band using 3 years of observations of the Polarbear project. Although the detectors are sensitive to linear polarization, we can measure the circular polarization because a continuously rotating half-wave plate in the optics converts part of circular polarization into linear polarization. The atmospheric circular polarization signal appears as a modulated signal at twice the frequency of rotation of the half-wave plate. We reconstruct the azimuthal gradient of the circular polarization signal and measure the dependencies on the scanning azimuth and the detector bandpass. We compare the signal with a simulation based on atmospheric emission theory, the detector bandpass, and the half-wave plate leakage spectrum model. We find the ratio of the observed azimuthal slope to the simulated slope is 0.92 ± 0.01(stat)± 0.07(sys), which demonstrates that our measurement is consistent with theoretical prediction. This result validates our understanding of the instrument and reinforces the feasibility of measuring the circular polarization using the imperfection of the half-wave plate. Quantifying atmospheric circular polarization is the first step toward conducting a search for cosmological circular polarization at these wavelengths.
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Cosmic microwave background radiation
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