Adiabatic betatron acceleration by a geomagnetic storm

It is known [Dessler and Karplus, 1961; Fan et al., 1961] that a geomagnetic storm can produce reversible betatron acceleration of trapped radiation. Recent refined measurements [McIlwain, 1966a; Mcllwain, 1966b; Fittius, 1967] have shown that betatron effects are common and fit predictions using models of the storm magnetic field. As accounting for this adiabatic motion is essential to observationalists who wish to isolate crucial nonadiabatic effects, detailed investigation of this mechanism and its relation to magnetic storm fields is desirable. It will be shown in this letter that reversible betatron acceleration in the earth's field can be pictured as the result of two betatrons' acting simultaneously: a gyrobetatron in which the curl of E acts around the circle of gyration, and a drift betatron in which the curl of E acts around the drift circle. This description is of course equivalent to the usual calculation using conservation of the particles' adiabatic invariants. However, as conventional betatrons involve one type of circular motion only, this distinction describes the geomagnetic betatron more naturally and clarifies the connection between the particles' acceleration and the changing magnetic fields.