The effects of variable ionospheric and plasmaspheric Faraday rotation on low frequency radio arrays

Low frequency arrays such as the Murchison Widefield Array (MWA), which use fixed dipoles operating at meter wavelengths and longer, have very large fields of view and exhibit strong cross-polarization response due to geometric effects, as well as the characteristics of the electronics. The linearly polarized sky at these frequencies is bright and complex in structure, demanding accurate polarimetric calibration for high fidelity imaging. This calibration is made more challenging by high-amplitude time- and direction-dependent ionospheric and plasmaspheric Faraday rotation. In this contribution we describe the relevant ionospheric behaviors, and discuss approaches for dealing with the resulting effects.