Statistics of the longitudinal splitting of proton aurora during substorms

The lower‐latitude boundary of the proton aurora, i.e., the isotropic boundary (IB), marks the transition from a full downgoing loss cone on the poleward side to an empty downgoing loss cone on the equatorward side. A number of authors have correlated this boundary with the amount of stretching in the magnetic field using GOES spacecraft. In this paper, we use 264 substorm events from the IMAGE SI‐12 global proton auroral imager to show that …