Scintillation noise power spectrum and its impact on high redshift 21-cm observations

TL;DR

This paper analyzes how ionospheric scintillation noise affects low-frequency 21-cm cosmology experiments, showing its spectral properties and implications for different array configurations in detecting the Epoch of Reionization signal.

Contribution

It derives scintillation noise power spectra considering data processing effects and compares its impact on minimally redundant and fully redundant radio arrays.

Findings

Scintillation noise is comparable to thermal noise in minimally redundant arrays.

In fully redundant arrays, scintillation noise dominates but can be removed due to its coherence.

Scintillation noise is confined to the wedge in the power spectrum, affecting EOR measurements.

Abstract

Visibility scintillation resulting from wave propagation through the turbulent ionosphere can be an important sources of noise at low radio frequencies ($\nu\lesssim 200$ MHz). Many low frequency experiments are underway to detect the power spectrum of brightness temperature fluctuations of the neutral-hydrogen $21$-cm signal from the Epoch of Reionization (EOR: $12\gtrsim z\gtrsim 7$, $100\lesssim \nu \lesssim 175$ MHz). In this paper, we derive scintillation noise power-spectra in such experiments while taking into account the effects of typical data processing operations such as self-calibration and Fourier synthesis. We find that for minimally redundant arrays such as LOFAR and MWA, scintillation noise is of the same order of magnitude as thermal noise, has a spectral coherence dictated by stretching of the snapshot $uv$-coverage with frequency, and thus is confined to the well known wedge-like structure in the cylindrical ($2$-dimensional) power spectrum space. Compact, fully redundant ($d_{\rm core}\lesssim r_{\rm F} \approx 300$ m at $150$ MHz) arrays such as HERA and SKA-LOW (core) will be scintillation noise dominated at all baselines, but the spatial and frequency coherence of this noise will allow it to be removed along with spectrally smooth foregrounds.