Auroral electrojet configuration during substorm growth phase

[1] The spatial configuration of the auroral electrojets during the growth phase of classical substorms is investigated. Electrojet intensities are determined from measurements of the convection electric field and calculated height integrated conductivity. Both the westward and eastward electrojets decrease in latitudinal width towards similar to 21 - 23 MLT where they both are terminated. The latitudinal widths of the eastward electrojet as a function of MLT, however, show significant scatter. This scatter is shown to be effectively minimized by organizing the data by the local time distance to the future optical onset location rather than using magnetic local time. We find that the future optical onset will be located in the region of overlapping eastward and westward electrojets. The fact that the future onset location better organizes the data suggests that during the growth phase the magnetosphere is organizing itself for an onset at an already determined MLT as seen at ionospheric altitudes. Neither the IMF By nor IMF clockangle provide a simple explanation of the variation in the onset location.