Global and local equatorward expansion of the ion auroral oval before substorm onsets

[1] The temporal variation of the equatorward boundary of the proton aurora/high-energy ion precipitation is a manifestation of diurnal and seasonal (i.e., dipole tilt) effects as well as magnetic activity. In particular, during the substorm growth phase this boundary moves equatorward, an effect due primarily to thinning and earthward motion of the cross-tail current in the inner magnetosphere as the field evolves toward a more stretched topology. Recent advances in monitoring this boundary using ground-based instruments have opened up the possibility of following its temporal evolution across several hours in local time. This in turn allows one to explore whether this magnetotail stretching is a global or local phenomenon. We have examined this boundary evolution during the growth phases of 68 substorms over the Canadian sector. We use the equatorward boundary of SuperDARN E region echoes as a proxy for the proton auroral boundary as described by Jayachandran et al. (2002b). We find that in 21 of the 68 substorms the equatorward motion of the auroral boundary is restricted to several hours of local time in the evening sector. In the remaining 47 substorms, the equatorward motion was global so that the boundary retained its shape throughout the growth phase. Our results indicate dramatically different growth phase phenomenology in these two classes of substorms. In one, the growth phase involves stretching in the inner magnetosphere that is most pronounced around the onset meridian. In the other, the stretching extends many hours in local time away from the onset meridian.