The storm-substorm relationship: Current understanding and outlook

The intensification of the ring current during a geospace storm has been one of the key issues in space physics. Substorms have been considered responsible for bringing in particles from the magnetotail, which get trapped on closed drift paths to form a symmetrical ring current. It is now recognized that the ring current develops dominantly from a sustained enhancement of the convection electric field. The magnetic perturbations observed during a storm main phase are then due to a partial ring current, which closes in part through the ionosphere and in part through the magnetopause. An enhanced convection electric field moves the plasma Earth-ward, thus energizing it, and when this field is reduced, the particles become trapped and a symmetric ring current is formed. Substorms, however, are always accompanied by the injection of energetic particles and their contribution to the storm-time ring current is a matter of current debate. Considering the electrodynamic nature of the interaction between different regions of the magnetosphere and the dominantly global nature of its dynamics, storms and substorms are not expected to just co-exist, but the ways in which they influence each other are not clear yet. The Chapman Conference at Lonavala (2001) saw the cementing of a new paradigm for the ring current and the storm-substorm relationship. The accumulating evidence against the substorms being the main constituents of storm main phase and the recognition of the dominant role of partial ring current led to a consensus (Lonavala consensus) marking a turning point in the understanding of the storm-substorm relationship.