The Energy of Magnetic Storms

The extra magnetic energy present in the magnetosphere during a magnetic storm is shown to be the same as if the disturbing field existed without the main field of the Earth: that is, the “joint” energy of the two fields is zero. The disturbance field energy is considered for both the first and the main phase of a storm, on the basis of idealized models of the electric currents that cause the disturbance. The currents, actually distributed throughout volumes, are taken to flow only on surfaces. Numerical estimates of the field energy are given, for a weak storm and for a great storm. The magnetic moment of the field of the ring current is also considered; it is inferred that it is only a small fraction of the Earth's dipole moment. The field energy of magnetic storms is found to be less than the kinetic energy associated with the ring current at the peak of the main phase.