Scintillation noise in widefield radio interferometry

TL;DR

This paper analyzes how ionospheric turbulence causes scintillation noise in widefield radio interferometry, significantly impacting low-frequency observations and requiring new calibration strategies.

Contribution

It derives the statistical expressions for scintillation noise in widefield interferometry and discusses its implications for low-frequency radio observations.

Findings

Ionospheric scintillation noise can dominate at frequencies below 200 MHz.

The paper provides formulas for visibility variance due to plasma turbulence.

Scintillation noise's coherence properties affect mitigation strategies.

Abstract

In this paper, we consider random phase fluctuations imposed during wave propagation through a turbulent plasma (e.g. ionosphere) as a source of additional noise in interferometric visibilities. We derive expressions for visibility variance for the wide field of view case (FOV$\sim10$ deg) by computing the statistics of Fresnel diffraction from a stochastic plasma, and provide an intuitive understanding. For typical ionospheric conditions (diffractive scale $\sim 5-20$ km at $150$ MHz), we show that the resulting ionospheric `scintillation noise' can be a dominant source of uncertainty at low frequencies ($\nu \lesssim 200$ MHz). Consequently, low frequency widefield radio interferometers must take this source of uncertainty into account in their sensitivity analysis. We also discuss the spatial, temporal, and spectral coherence properties of scintillation noise that determine its magnitude in deep integrations, and influence prospects for its mitigation via calibration or filtering.