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 auror a. The observation that most activity occurs in this region provides a reference from which the source region in the magnetotail may be asse ssed. This reference can be examined in two ways. First, magnetic fiel d mappings of these onset locations to the equatorial plane suggest th at the onset is associated with processes quite near the Earth. For ex ample, for 14 cases the average GSM X value was found to be almost-equ al-to -7.8 R(E). However, this identification is based on a static mag netic field model and while these results are consistent with some ear lier 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 op en/closed field line boundary. It is evident from Viking satellite ima ges that optical substorm expansions can occur well equatorward of the poleward extent of emissions, both during quiet and active periods. T here is no reason to suspect that this poleward region of emissions is not on closed field lines and that the onset location is therefore un related to the open/closed field line boundary, a result consistent wi th some (but not all) near-Earth mechanisms but only under some condit ions with the distant tail boundary layer theory.