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ABSTRACT  We compute the circularly polarized signal from atmospheric molecular oxygen. The polarization of O 2  rotational lines is caused by the Zeeman effect in the Earth's magnetic field. We evaluate the circularly polarized emission for various sites suitable for cosmic microwave background (CMB) measurements: the South Pole and Dome C (Antarctica), Atacama (Chile) and Testa Grigia (Italy). We present and discuss an analysis of the polarized signal within the framework of future CMB polarization experiments. We find a typical circularly polarized signal ( V  Stokes parameter) of  ∼50–300  μK at 90 GHz looking at the zenith. Among the sites, Atacama shows a lower polarized signal at the zenith. We present maps of this signal for the various sites and we show typical elevation and azimuth scans. We find that Dome C presents the lowest gradient in polarized temperature: ∼0.3 μK deg −1  at 90 GHz. We also study the frequency bands of observation: around  ν≃ 100  GHz and  ν≃ 160  GHz, we find the best conditions because the polarized signal vanishes. Finally, we evaluate the accuracy of the templates and the signal variability in relation to our knowledge of and the variability of the Earth's magnetic field and atmospheric parameters.

A template of atmospheric O 2 circularly polarized emission for cosmic microwave background experiments