Rapid Outer Radiation Belt Flux Dropouts and Fast Acceleration during
the March 2015 and 2013 Storms: Role of ULF Wave Transport from a
Dynamic Outer Boundary
Abstract
We present simulations of the outer radiation belt electron flux during
the March 2015 and March 2013 storms using a radial diffusion model.
Despite differences in Dst intensity between the two storms the response
of the ultra-relativistic electrons in the outer radiation belt was
remarkably similar, both showing a sudden drop in the electron flux
followed by a rapid enhancement in the outer belt flux to levels over an
order of magnitude higher than those observed during the pre-storm
interval. Simulations of the ultra-relativistic electron flux during the
March 2015 storm show that outward radial diffusion can explain the flux
dropout down to L*=4. However, in order to reproduce the observed flux
dropout at L*<4 requires the addition of a loss process
characterised by an electron lifetime of around one hour operating below
L*~3.5 during the flux dropout interval. Nonetheless,
during the pre-storm and recovery phase of both storms the radial
diffusion simulation reproduces the observed flux dynamics. For the
March 2013 storm the flux dropout across all L-shells is reproduced by
outward radial diffusion activity alone. However, during the flux
enhancement interval at relativistic energies there is evidence of a
growing local peak in the electron phase space density at
L*~3.8, consistent with local acceleration such as by
VLF chorus waves. Overall the simulation results for both storms can
accurately reproduce the observed electron flux only when event specific
radial diffusion coefficients are used, instead of the empirical
diffusion coefficients derived from ULF wave statistics.