TIME-DEPENDENT ACCELERATION OF INTERSTELLAR PICKUP IONS AT THE HELIOSPHERIC TERMINATION SHOCK USING A FOCUSED TRANSPORT APPROACH

A time-dependent focused transport approach to modeling diffusive shock acceleration of interstellar pickup ions at the termination shock is discussed. By taking into account time variations in the magnetic field angle, and thus by implication in the shock obliquity and injection speed at the termination shock using Voyager 1 observations as guide, we show that unaccelerated core interstellar pickup protons can be accelerated by the nearly perpendicular termination shock at the heliospheric nose. Many features of anomalous cosmic rays, observed by the Voyager spacecraft at energies below the big spectral dip starting at ∼3 MeV, can be successfully reproduced with this approach. This includes multiple power-law spectral slopes with stable breaking points, power-law spectral slopes that are harder than predicted by standard diffusive shock acceleration, large pitch-angle anisotropies with strong time variations upstream that converge to steady-state isotropy in the heliosheath, upstream pitch-angle anisotropies that peak at ∼1 MeV, episodic strongly anisotropic intensity spikes at the termination shock, and strong spectral volatility upstream that is reduced downstream and almost completely disappears farther downstream.