Ground‐based optical determination of the b2i boundary: A basis for an optical MT‐index

The equatorward boundary of the proton aurora corresponds to a transition from strong pitch angle scattering to bounce trapped particles. This transition has been identified as the b2i boundary in Defense Meteorological Satellite Program (DMSP) ion data [ Newell et al. , 1996 ]. We use ion data from 29 DMSP overflights of the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) Meridian Scanning Photometer (MSP) located at Gillam, Canada, to develop a simple algorithm to identify the b2i boundary in latitude profiles of proton auroral (486 nm) brightness. Applying this algorithm to a ten year set of Gillam MSP data, we obtain ∼250,000 identifications of the “optical b2i,” the magnetic latitude of which we refer to as b2i Λ . We intercompare ∼1600 near‐simultaneous optical and in situ b2i Λ , concluding that the optical b2i Λ is a reasonable basis for an optical equivalent to the MT‐index put forward by Sergeev and Gvozdevsky [1995] . Using ∼17,000 simultaneous measurements, we demonstrate a strong correlation between the optical b2i Λ and the inclination of the magnetic field as measured at GOES 8. We develop an empirical model for predicting the GOES 8 inclination, given the universal time, dipole tilt, and the optical b2i Λ , as determined at Gillam. We also show that in terms of information content, the b2i boundary is an optimal boundary upon which to base such an empirical model.