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 b2iΛ. We intercompare ∼1600 near‐simultaneous optical and in situ b2iΛ, concluding that the optical b2iΛ 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 b2iΛ 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 b2iΛ, 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.