Modeling the Depletion and Recovery of the Outer Radiation Belt During a Geomagnetic Storm: Combined MHD and Test Particle Simulations

Abstract

During geomagnetic storms the intensities of the outer radiation belt electron population can exhibit dramatic variability. Deep depletions in intensity during the main phase are followed by increases during the recovery phase, often to levels that significantly exceed their prestorm values. To study these processes, we simulate the evolution of the outer radiation belt during the 17 March 2013 geomagnetic storm using our newly developed radiation belt model (Conservative Hamiltonian Integrator for Magnetospheric Particles) based on test particle and coupled 3‐D ring current and global magnetohydrodynamic (MHD) simulations, and driven solely with solar wind and F10.7 flux data. Our approach differs from previous work in that we use MHD information to identify regions of strong, bursty, and azimuthally localized earthward convection in the magnetotail where test particles are then seeded. We validate our model using in situ Van Allen Probe electron intensities over a multiday period and show that our model is able to reproduce meaningful qualitative and quantitative agreement. Analysis of our model enables us to study the processes that govern the transition from the prestorm to poststorm outer belt. Our analysis demonstrates that during the early main phase of the storm the preexisting outer belt is largely wiped out via magnetopause losses, and subsequently, a new outer belt is created during a handful of discrete, mesoscale injections. Finally, we demonstrate the potential importance of magnetic gradient trapping in the transport and energization of outer belt electrons using a controlled numerical experiment.

Rapid Outer Radiation Belt Flux Dropouts and Fast Acceleration During the March 2015 and 2013 Storms: The Role of Ultra‐Low Frequency Wave Transport From a Dynamic Outer Boundary

“…This indicates that for this storm outward radial diffusion to the magnetopause acting alone can explain the observed flux drop across all L‐shells without the need for any other additional loss processes. These radial diffusion simulations of the flux dropout during the March 2013 storm are also consistent with the test particle simulations presented in Sorathia et al (). In addition, the steady inward motion of the observed outer radiation belt flux during the pre‐storm interval, before 17 March, is also reproduced by our radial diffusion simulation.…”

Solar Wind Ion Entry Into the Magnetosphere During Northward IMF

“…Our methodology builds on the approach described in detail in Appendix 1 of Sorathia et al (). Combining global MHD and test particle simulations allows us to simultaneously capture both global and mesoscale dynamics, for example, Kelvin‐Helmholtz vortices, that can facilitate solar wind entry and drift physics beyond the drift of the MHD flow, which we find leads to dawn‐dusk asymmetry and particle energization during the entry process.…”

“…Combining global MHD and test particle simulations allows us to simultaneously capture both global and mesoscale dynamics, for example, Kelvin‐Helmholtz vortices, that can facilitate solar wind entry and drift physics beyond the drift of the MHD flow, which we find leads to dawn‐dusk asymmetry and particle energization during the entry process. In the remainder of this section we describe the salient details of our simulation pipeline, which consists of global electromagnetic fields that are generated using the Lyon‐Fedder‐Mobarry (LFM) MHD code (Lyon et al, ) to simulate the Earth's magnetosphere during northward IMF, an ensemble of test particles used to model a thick “wall” of solar wind created upstream of the Earth's bow shock using our test particle code CHIMP (Sorathia et al, ), and the magnetic field topology diagnostics we use to identify particle entry.…”

Earth's Van Allen Radiation Belts: From Discovery to the Van Allen Probes Era

“…The inner zone proton belt is produced by a combination of Cosmic Ray Albedo Neutron Decay (CRAND; Singer, 1958), which occurs when cosmic rays scatter off the neutral atmosphere producing neutrons that decay with a 15-min half-life. Solar energetic protons associated with flares and coronal mass ejections (CMEs; Reames, 2001) also become trapped in the Earth's magnetic field (Hudson et al, 1997;Kress et al, 2005;Selesnick et al, 2008).…”