Interpretation of optical substorm onset observations

The ionospheric location of substorm onset is generally found to be at the most equatorward arc in the poleward portion of the diffuse aurora. The observation that most activity occurs in this region provides a reference from which the source region in the magnetotail may be assessed. This reference can be examined in two ways. First, magnetic field mappings of these onset locations to the equatorial plane suggest that the onset is associated with processes quite near the Earth. For example, for 14 cases the average GSM X value was found to be ≈ −7.8 R_E. However, this identification is based on a static magnetic field model and while these results are consistent with some earlier findings there is not sufficient confidence in this technique to discriminate between topological regions in the magnetotail. A second way to examine the ionospheric onset location is in relation to the open/closed field line boundary. It is evident from Viking satellite images that optical substorm expansions can occur well equatorward of the poleward extent of emissions, both during quiet and active periods. There is no reason to suspect that this poleward region of emissions is not on closed field lines and that the onset location is therefore unrelated to the open/closed field line boundary, a result consistent with some (but not all) near-Earth mechanisms but only under some conditions with the distant tail boundary layer theory.