Tackling Challenges in 21cm Global Spectrum Experiment: the Impact of Ionosphere and Beam Distortion
arxiv(2024)
摘要
The HI 21cm global signal from the Cosmic Dawn and the Epoch of Reionization
(EoR) offers critical insights into the evolution of our Universe. Yet, its
detection presents significant challenges due to its extremely low
signal-to-contamination ratio and complex instrumental systematics. In this
paper, we examine the effects of the ionosphere and antenna beam on data
analysis. The ionosphere, an ionized plasma layer in the Earth's atmosphere,
refracts, absorbs, and emits radio waves in the relevant frequency range. This
interaction results in additional spectral distortion of the observed signal,
complicating the process of foreground subtraction. Additionally, chromatic
variations in the beam can also introduce further contamination into the global
spectrum measurement. Notably, the ionospheric effect, being dependent on the
direction of incoming light, interacts with the instrumental beam, adding
another layer of complexity. To address this, we evaluate three different
fitting templates of foreground: the logarithmic polynomial, the physically
motivated EDGES template, and a SVD-based template. Our findings indicate that
the EDGES and SVD templates generally surpass logarithmic polynomials in
performance. Recognizing the significance of beam chromaticity, we further
investigate specific beam distortion models and their impacts on the signal
extraction process.
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