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.
