Abstract
The excellent article by M. W. Liemohn and A. A. Chan on the radiation belts (see Eos, 88(42), 16 October 2007) is misleading in its implication that the disturbance storm‐time (Dst) index is an indicator of a magnetospheric ring current. That index is formed from an average of magnetic data from three or four low‐latitude stations that have been fallaciously “adjusted” to a magnetic equatorial location under the 1960's assumption [Sugiura, 1964] that the fields arrive from the growth and decay of a giant ring of current in the magnetosphere. In truth, the index has a negative lognormal form [Campbell, 1996; Yago and Kamide, 2003] as a result of its composition from numerous negative ionospheric and magnetospheric disturbance field sources, each having normal field amplitude distributions [Campbell, 2004]. Some partial ring currents [Lui et al., 1987] and their associated field‐aligned currents, as well as major ionospheric currents flowing from the auroral zone to equatorial latitudes, are the main contributors to the Dst index.
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