A Dst contribution to the equatorward shift of the aurora

It is observed that in the course of at least one major magnetic storm, during aurorally quiet pauses, the poleward limit of auoral activity is shifted 10–15° equatorward of its typical non-storm-time limit. The storm-time ring current will contribute to the equatorward shift by expanding the size of the magnetosphere, causing an increase in the magnetic flux in the tail that maps into the aurorally inactive polar cap. We use a new model of the ring current to estimate the size of the ring current effect on the shift in the poleward limit. One calculated example that is probably representative gives a shift of between 5 and 10° corresponding to a Dst in the range from ?300 to ?600 nT.