Fundamental aspects of substorm onset

We now know that substorm onset occurs within the near-Earth plasma sheet in the region associated with the Harang electric field reversal. It is also known that substorm onset is generally associated with a reduction in the large-scale convection driven by the solar wind, and that this reduction is imparted to the ionosphere a few minutes prior to substorm onset. This is the same time relative to onset that current wedge formation has been inferred to initiate. These are critical aspects of substorm onset, and they can be explained by considering the balance between sources and losses in the plasma sheet continuity equation. In this equation, the source is earthward convection, and it increases with the strength of convection. The loss is particle divergence driven by magnetic drift, which depends on the gradient in plasma pressure in the direction normal to the gradient in flux tube volume. This loss increases with plasma pressure and leads to development of the Harang reversal during the substorm growth phase. We show that a reduction in convection that reduces the source term to less than the loss term can lead to current wedge formation, and thus to substorm onset, within the Harang reversal region. We suggest how this leads to a time-scale for current wedge formation that decreases as the current wedge forms, as required to account for the rapid, non-linear growth of the current wedge that occurs during the substorm expansion. OBSERVATIONAL ASPECTS A set of well-defined observational features of substorms that give fundamental information for understanding substorms have been obtained during the last decade. These include: 1. Current wedge formation initiates in the pre-midnight to midnight MLT sector of the inner plasm