Refractive Interstellar Scintillation of Extra-galactic Radio Sources I: Expectations

Surveys for transient and variable phenomena can be confounded by the presence of extrinsic variability such as refractive interstellar scintillation (RISS). We have developed an all-sky model for RISS which can predict variability on a variety of timescales, survey locations, and observing frequencies. The model makes use of Halpha intensity maps to probe the emission measure along the line of sight, convert this to a scattering measure, and finally a scintillation strength. The model uses previously developed and long understood physics along with (indirect) measurements of the electron content and distribution within the Milky Way. We develop a set of expectations that are useful in the planning of future surveys for transient and radio variability, and demonstrate that the 1-GHz sky is a poor predictor of the variable nature of the $100$-MHz sky. Interestingly, the correlation between the incidence of variability and Galactic latitude which has been seen at 1GHz, is reversed at 100MHz. We compare the predictions of our model to a low-frequency radio survey that was conducted with the Murchison Widefield Array, and find good qualitative agreement. We discuss the implications, current limitations, and future development of the model. The model has been implemented in a Python code and is available on GitHub/Zenodo.