Electric field variability and model uncertainty: A classification of source terms in estimating the squared electric field from an electric field model

Joule heating by high‐latitude electric fields is thought to be underestimated by electric field models, and it has been conjectured that the source of the underestimation is “electric field variability,” although the interpretation of this term is not necessarily straightforward. We perform a classification of source terms in estimating the squared magnitude of electric field from an electric field model, and find that the phenomenon summarily referred to as electric field variability canonically decomposes into two distinct components: small‐scale electric field variability, and resolved‐scale model uncertainty. The latter contribution is a statistical estimation uncertainty, and not related to physical small‐scale fluctuations. We argue that the two sources should be characterized separately. An illustration is given in a comparison of the Joule heating measured by the Sondrestrom incoherent scatter radar during a 40‐h period containing a storm, with the Joule heating modeled by the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure, after removing the Sondrestrom data from the AMIE assimilation. A quantitative assessment of the relative importance of the two sources is drawn from a recent satellite study.