Stormtime ring current and radiation belt ion transport: Simulations and interpretations

The authors use a dynamical guiding-center model to investigate the stormtime transport of ring current and radiation-belt ions. They trace the motion of representative ions` guiding centers in response to model substorm-associated impulses in the convection electric field for a range of ion energies. Their simple magnetospheric model allows them to compare their numerical results quantitatively with analytical descriptions of particle transport, (e.g., with the quasilinear theory of radial diffusion). They find that 10-145-keV ions gain access to L approximately 3, where they can form the stormtime ring current, mainly from outside the (trapping) region in which particles execute closed drift paths. Conversely, the transport of higher-energy ions (approximately greater than 145 keV at L approximately 3) turns out to resemble radial diffusion. The quasilinear diffusion coefficient calculated for their model storm does not vary smoothly with particle energy, since their impulses occur at specific (although randomly determined) times. Despite the spectral irregularity, quasilinear theory provides a surprisingly accurate description of the transport process for approximately greater than 145-keV ions, even for the case of an individual storm. For 4 different realizations of their model storm, the geometric mean discrepancies between diffusion coefficients D(sup sim)(sub LL) obtained from the simulationsmore » and the quasilinear diffusion coefficient D(sup ql)(sub LL) amount to factors of 2.3, 2.3, 1.5, and 3.0, respectively. They have found that these discrepancies between D(sup sim)(sub LL) and D(sup ql)(sub LL) can be reduced slightly by invoking drift-resonance broadening to smooth out the sharp minima and maxima in D(sup ql)(sub LL). The mean of the remaining discrepancies between D(sup sim)(sub LL) and D(sup ql)(sub LL) for the 4 different storms then amount to factors of 1.9, 2.1, 1.5, and 2.7, respectively.« less