Scintillation noise in widefield radio interferometry

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 similar to 10 degrees) by computing the statistics of Fresnel diffraction from a stochastic plasma, and provide an intuitive understanding. For typical ionospheric conditions (diffractive scale similar to 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 less than or similar to 200 MHz). Consequently, low-frequency wide-field 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.