Discussion of paper by E. J. Stegelmann and C. H. von Kenschitzki, ‘On the interpretation of the sudden commencement of geomagnetic storms’

It was pointed out some years ago [Dessler et al. 1960] that a major part of the rise time of a sudden commencement can be accounted for as the period of time in which the enhanced solar wind sweeps over the outer boundary of the geomagnetic field, plus the difference in propagation time from different points on the boundary. The dimensions of the outer boundary of the geomagnetic field are of the order of 105 km, so that an enhanced wind of 103 km/sec characteristically takes about 102 seconds to engulf the field and enhance the compression of the field. Typical propagation times within the field differ by 102 seconds. The basic idea was simply that the compression of the geomagnetic field at the surface of the earth results from the algebraic sum of all the compressive forces exerted by the wind on the outer boundary of the field [Parker, 1958]. Any changes in the compressive forces on the outer boundary are communicated inward at about the Alfvén velocity [Francis et al., 1959]. Enhanced compression of the boundary starts somewhere on the solar side, at which time a slight increase in compression begins to propagate through the geomagnetic field as a hydromagnetic wave. With the passage of time more and more of the boundary is compressed as the enhanced wind sweeps by in space. The rising compression communicates throughout the field with the Alfvén speed, and levels off only after the enhanced wind fully encloses the geomagnetic field.