The effect of the initial phase of a magnetic storm upon the outer Van Allen belt

We present in this note the results of calculations of the longitudinal invariant of the motion of a charged particle trapped in a magnetic dipole field and in a compressed dipole field. The longitudinal adiabetic invariant [Chandrasekhar, 1958; Chew, Goldberger, and Low, 1955], has already been used to advantage in describing other geophysical phenomena. Auroral isochasms have been satisfactorily described with the longitudinal invariant for the earth's main field [Vestine and Sibley, 1959]. Also, using the invariant [Dessler and Karplus, 1960], the gap in the outer Van Allen electron belt at 1.9×104 km may be accounted for by the Capetown magnetic anomaly.