STORMTIME TRANSPORT OF RING CURRENT AND RADIATION BELT IONS

This is an investigation of stormtime particle transport that leads to formation of the ring current. Our method is to trace the guiding-center motion of representative ions (having selected first adiabatic invariants mu) in response to model substorm-associated impulses in the convection electric field. We compare our simulation results qualitatively with existing analytically tractable idealizations of particle transport (direct convective access and radial diffusion) in order to assess the limits of validity of these approximations. For mu less than or similar to 10 MeV/G (E less than or similar to 110 keV at L almost-equal-to 3) the ion drift period on the final (ring-current) drift shell of interest (L almost-equal-to 3) exceeds the duration of the main phase of our model storm, and we find that the transport of ions to this drift shell is appropriately idealized as direct convective access, typically from open drift paths. Ion transport to a final closed drift path from an open (plasma-sheet) drift trajectory is possible for those portions of that drift path that lie outside the mean stormtime separatrix between closed and open drift trajectories. For mu approximately 10-25 MeV/G (110 keV less than or similar to E less than or similar to 280 keV at L almost-equal-to 3) the drift period at L almost-equal-to 3 is comparable to the postulated 3-hr duration of the storm, and the mode of transport is transitional between direct convective access and transport that resembles radial diffusion. (This particle population is transitional between the ring current and radiation belt). For mu greater than or similar to 25 MeV/G (radiation-belt ions having E greater than or similar to 280 keV at L almost-equal-to 3) the ion drift period is considerably shorter than the main phase of a typical storm, and ions gain access to the ring-current region essentially via radial diffusion. By computing the mean and mean-square cumulative changes in 1/L among (in this case) 12 representative ions equally spaced in drift time around the steady-state drift shell of interest (L almost-equal-to 3), we have estimated (from both our forward and our time-reversed simulations) the time-integrated radial-diffusion coefficients D(LL)sim for particles having selected values of mu greater than or similar to 15 MeV/G. The results agree surprisingly well with the predictions (D(LL)ql)) of quasilinear radial-diffusion theory, despite the rather brief duration (almost-equal-to 3 hr) of our model storm and despite the extreme variability (with frequency) of the spectral-density function that characterizes the applied electric field during our model storm. As expected, the values of D(LL)sim deduced (respectively) from our forward and time-reversed simulations agree even better with each other and with D(LL)sim when the impulse amplitudes which characterize the individual substorms of our model storm are systematically reduced.