The Impact of Atmospheric Fluctuations on Degree-scale Imaging of the Cosmic Microwave Background

There is currently an active effort to map the spatial variations in the intensity of the cosmic microwave background (CMB), with different experiments utilizing chopped beam, swept beam, and interferometric approaches to minimize systematic errors. Fluctuations in the brightness of the Earth’s atmosphere originating from water vapor are an important source of noise for ground-based instruments. This paper presents a model for the fluctuations and derives simple expressions to predict the contribution of the atmosphere to experimental measurements. Data from the South Pole and from the Atacama Desert in Chile, two of the driest places on Earth, are used to assess the level of fluctuations at each site. The model consists of a layer of turbulence in which the fluctuations follow a modified Kolmogorov power law, with both threeand two-dimensional regimes. The isotropic, three-dimensional case, which applies on small scales, is dominant in most applications. The analysis treats the instruments as window functions that act on the power spectrum of the fluctuations, resulting in a simple pictorial approach analogous to the instrumental window functions that are applied to theoretical models of the CMB angular power spectrum. The South Pole data are from the Python V experiment, and are used to estimate the level of atmospheric fluctuations over 2 months of the summer. The distribution is bimodal, with long periods of very stable conditions (∆Trms < 1 mK in a 6 ◦ strip) broken by occasional periods of much stronger fluctuations (∆Trms > 10 mK), which appear to be associated with clouds. These periods are correlated with a change in wind direction that brings moist air from West Antarctica. Dept. of Applied Physics, California Institute of Technology, Pasadena, CA 91125