PROPERTIES OF SINGLE-PARTICLE DYNAMICS IN A PARABOLIC MAGNETIC REVERSAL WITH GENERAL TIME-DEPENDENCE

We investigate single charged particle dynamics in the earth's magneto tail by examining a reversal with simple spatial dependence (the field lines are parabolic) but with general time dependence, which includes the associated induction electric field. The parabolic spatial depend ence has, in static models, been shown by previous authors to imply th at various regular and stochastic regimes of behavior exist, with part icles remaining in a given regime for all time t. Here we show that, i n general, time dependence yields transitions in behavior between the various regimes of regular and stochastic behavior. We identify three independent parametric coordinates which are functions of the field sp atial and temporal scales, and the particle gyroscales, and time, whic h will indicate the regime of particle behavior at any given t. For sp ecific time dependent field models these parametric coordinates can be inverted to directly give the timescales for transitions between one regime of behavior and another in terms of the field and particle spat ial and temporal scales. These parametric coordinates have no direct a nalogue in static models. For a general thinning and folding sheet (re presenting the magnetotail magnetic field close to the center plane ju st before substorm onset) this characterization implies that stochasti c dynamics may only occur over a finite time period and may not occur under certain circumstances. This may have implications for our unders tanding of the role played by single particle dynamics in the destabil ization or reconfiguration of the magnetotail current sheet associated with substorms.