Persistence of the ring current, 1958–1984

Statistical evidence for a solar cycle modulation in storm‐time ring current particle lifetimes is presented. Linear regression of Dst with respect to ap(τ), and an optimisation of τ to maximise correlation, gives characteristic persistence times for each year through 2.5 solar cycles. These average persistences vary between less than 10 h at solar minimum (1962, 1975) to greater than 16 h at solar maximum (1970, 1981). The analysis is biassed toward the later stages of recovery when the rate of decay has reduced, but the result is not a function of mean current strength or frequency of large storms. Increased abundance of O+ is postulated as the most probable explanation of longer particle lifetimes at solar maximum, this strongly supports the proposition that a large fraction of the ring current is of ionospheric origin, and underlines the importance of dynamic coupling between the ionosphere and magnetosphere during disturbed periods.