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

[1] 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 similar to250,000 identifications of the "optical b2i,'' the magnetic latitude of which we refer to as b2i(Lambda). We intercompare similar to1600 near-simultaneous optical and in situ b2i(Lambda), concluding that the optical b2i(Lambda) is a reasonable basis for an optical equivalent to the MT-index put forward by Sergeev and Gvozdevsky [1995]. Using similar to17,000 simultaneous measurements, we demonstrate a strong correlation between the optical b2i(Lambda) and the inclination of the magnetic field as measured at GOES 8. We develop an empirical model for predicting the GOES 8 inclination, given theuniversal time, dipole tilt, and the optical b2i(Lambda), 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.