Geomagnetic storm sudden-commencement rise times

Adopting the view that the sudden commencement of a geomagnetic storm (SC) is the result of the impact on the geomagnetic field of an abrupt solar-plasma front, the form of the SC observed on the surface of the earth is investigated. A model is constructed to represent the shape of the geomagnetic field boundary as perturbed by the solar plasma. Calculations (carried out in the equatorial plane for simplicity) show that, regardless of how abrupt may be the impact of a solar-plasma front on the geomagnetic field, the variation in hydromagnetic transit times from different positions on the boundary, down to a point on the surface of the earth, yields SC rise times of several minutes at ground level. These times are in agreement with the observed SC rise times.