A Theory of Polar Geomagnetic Storms

Summary This is the third of a series of papers outlining a hydromagnetic theory of geomagnetic storms. The first two dealt with the storm‐time variations Dst in terms of isotropically propagating hydromagnetic compression (first phase) and rarefaction (main phase) of the geo‐magnetic field and its plasma. The three phases of the longitude‐dependent disturbance Ds , the first phase or Sc , the main phase and the preliminary reverse are all accounted for in terms of hydromagnetic waves whose Poynting fluxes are directed along the magnetic lines of force. These disturbances originate from “electromagnetic friction” between the solar wind and geomagnetic lines of force near 06 and 18h local time. These lines of force are bent out of their meridian planes and cause two pairs of “twist” waves to reach the Earth, being focused into relatively confined regions at high latitudes. These twists are stable or force‐free and result in space‐charge accumulations in the lower ionosphere where the resultant Hall current accounts for the observed disturbances. During the main phase the twists are maintained by a deformation of the geomagnetic field which was described in the first two papers (the geomagnetic “tail”). Thus the tail theory accounts for both Dst and Ds main phase. It is difficult to see how any ring‐current theory or other theory dependent on trapped corpuscular radiation could account for the asymmetric features of a storm. The mechanism also provides the beginning of a theory of aurorae.