Generation of energetic proton shells during substorms

The circulation of the terrestrial polar wind during dynamical reconfigurations of the geomagnetic field is systematically examined by means of three-dimensional single-particle codes. In these calculations the storm time collapse of the geomagnetic tail is simulated by a gradual evolution of the Mead and Fairfield (1975) model from disturbed to ground state geometry. The polar wind is considered to consist primarily of protons, distributed throughout the magnetosphere at substorm onset consistently with the large-scale plasma convection. The simulations clearly illustrate the “convection surge” accompanying the development of substorms, displaying an overall earthward compression of the H+ populations. Among these, a dense and low-energy core can be identified at close L shells (within L ∼ 7), which appears as a persistent feature during the dipolarization process. Tailward of these inner regions, the simulations demonstrate a pileup of protons initiated in the mid-tail (∼10–15 RE), which experience intense (several keV) accelerations and pitch angle diffusion owing to the transient breaking of their adiabatic invariants. Most notably, a detailed analysis of the trajectory results reveals a sharp earthward boundary for these newly created energetic H+, which corresponds to the postdipolarization image of the ion adiabatic “threshold” in the geomagnetic tail. Further nonadiabatic considerations suggest that this mechanism which supports the “substorm injection boundary” concept depends upon particle mass per charge and may consequently organize distinct ion layers at the inner edge of the plasma sheet.