Explaining the dynamics of the ultra-relativistic third Van Allen radiation belt

Since the discovery of the Van Allen radiation belts over 50 years ago, an explanation for their complete dynamics has remained elusive. Especially challenging is understanding the recently discovered ultra-relativistic third electron radiation belt. Current theory asserts that loss in the heart of the outer belt, essential to the formation of the third belt, must be controlled by high-frequency plasma wave–particle scattering into the atmosphere, via whistler mode chorus, plasmaspheric hiss, or electromagnetic ion cyclotron waves. However, this has failed to accurately reproduce the third belt. Using a data driven, time-dependent specification of ultra-low-frequency (ULF) waves we show for the first time how the third radiation belt is established as a simple, elegant consequence of storm-time extremely fast outward ULF wave transport. High-frequency wave–particle scattering loss into the atmosphere is not needed in this case. When rapid ULF wave transport coupled to a dynamic boundary is accurately specified, the sensitive dynamics controlling the enigmatic ultra-relativistic third radiation belt are naturally explaine

Three-dimensional test simulations of the outer radiation belt electron dynamics including electron-chorus resonant interactions

We present results from our three-dimensional (3-D) simulations using the Salammbô electron radiation belt physical model. We have run steady state and dynamic storm test case simulations to study the effect of electron-chorus resonant interactions on the radiation belt electron dynamics. When electron-chorus interactions are introduced in the code outside the plasmasphere, results show that a seed population with a kappa distribution and a characteristic energy of 2 keV is accelerated up to a few MeV in the outer radiation belt. MeV electron fluxes increase by an order of magnitude during high magnetic activity conditions especially near L* ∼ 5 and for equatorial mirroring particles. We have also performed a parametric study of various important parameters to investigate how our results could be influenced by the uncertainty that characterizes their values. Results of this study show that if we consider higher values of the radial diffusion coefficients, different initial states, and different boundary conditions, we always observe a peak in the L* profile of the MeV electrons when electron-chorus interactions are included

Mechanisms for the acceleration of radiation belt electrons

During the declining phase of the solar cycle fast solar wind streams produce corotating interaction regions (CIRs) that drive moderate geomagnetic storms. These storms often have an unusually long recovery phase and produce high fluxes of relativistic electrons. Here we investigate the physical mechanisms responsible for accelerating electrons to relativistic energies inside the outer radiation belt. We review the most important electron acceleration and loss mechanisms, and present global simulations that combine radial diffusion with acceleration and loss by whistler mode chorus waves. We show that acceleration by chorus waves alone can increase the -MeV electron phase space density between 4.5 < L < 6.5 by up to three orders of magnitude. When radial diffusion and wave acceleration are included accelerated electrons are transported both inwards and outwards and increase the phase space density by a factor of 10 between 3.5 < L < 7. At lower energies of similar to 0.1 to a few hundred keV, chorus waves cause electron precipitation that enhances inward radial diffusion. We conclude that chorus wave acceleration and loss play a major role in the dynamics of the outer radiation belt. We suggest that during the declining phase of the solar cycle Alfvenic wave activity in the fast solar wind provides continuous inward transport of similar to 1-100 keV electrons inside the magnetosphere which maintains whistler mode wave power long enough to accelerate electrons up to similar to MeV energies, and drives radial diffusion to fill up the entire outer radiation belt

Simulation of the acceleration of relativistic electrons in the inner magnetosphere using RCM-VERB coupled codes

Radiation belt dynamics have been modeled by the modified Fokker-Planck diffusion equation with sources from the low-energy plasma sheet population and losses to the atmosphere and magnetopause. We perform a coupled simulation of the Rice Convection Model (RCM) and Versatile Electron Radiation Belt (VERB) code. The RCM models magnetospheric convection and provides a low-energy electron seed population for the VERB diffusion code simulations of the Earth's radiation belts. VERB simulations are driven by the realistic time-dependent electron seed population and by the Kp index, which is used to specify rates of diffusion by ultralow frequency (ULF) and very low frequency wave activity and, therefore, diffusion processes. Radial diffusion is produced by ULF waves, while pitch angle and energy diffusion are produced by chorus waves outside the plasmasphere and by hiss waves inside the plasmasphere. The results of the simulation indicate that storm time enhanced magnetospheric convection combined with radial diffusion can bring electrons with tens of keV energy close to the Earth and can affect electron fluxes at 3-4 R(E). These electrons can be further accelerated locally by chorus waves to MeV energies. Furthermore, outward radial diffusion smooths out the peak of the high-energy fluxes and produces MeV electron enhancement around geosynchronous orbit (6-7 R(E)) despite the absence of local electron acceleration in that region. Our coupled simulations indicate that local acceleration in the inner magnetosphere may be a dominant source of relativistic electrons that reach geosynchronous orbit

Origin of energetic electron precipitation >30 keV into the atmosphere

Energetic electrons are deposited into the atmosphere from Earth's inner magnetosphere, resulting in the production of odd nitrogen (NOx). During polar night, NOx can be transported to low altitudes, where it can destroy ozone, affecting the atmospheric radiation balance. Since the flux of energetic electrons trapped in the magnetosphere is related to solar activity, the precipitation of these electrons into Earth's atmosphere provides a link between solar variability and changes in atmospheric chemistry which may affect Earth's climate. To determine the global distribution of the precipitating flux, we have built a statistical model binned by auroral electrojet (AE) index, magnetic local time (MLT), and L shell of E > 30 keV precipitating electrons from the Medium Energy Proton and Electron Detector (MEPED) on board the NOAA Polar Orbiting Environmental Satellites (POES) low-altitude satellites NOAA-15, NOAA-16, NOAA-17, and NOAA-18. We show that the precipitating flux increases with geomagnetic activity, suggesting that the flux is related to substorm activity. The precipitating fluxes maximize during active conditions where they are primarily seen outside of the plasmapause on the dawnside. The global distribution of the precipitating flux of E > 30 keV electrons is well-correlated with the global distribution of lower-band chorus waves as observed by the plasma wave experiment onboard the Combined Release and Radiation Effects Satellite (CRRES) satellite. In addition, the electron precipitation occurs where the pitch angle diffusion coefficient due to resonant interaction between electrons and whistler mode chorus waves is high, as calculated using the pitch angle and energy diffusion of ions and electrons (PADIE) code. Our results suggest that lower-band chorus is very important for scattering > 30 keV electrons from Earth's inner magnetosphere into the atmosphere

Simulation of the outer radiation belt electrons near geosynchronous orbit including both radial diffusion and resonant interaction with Whistler-mode chorus waves

We present the first simulation results for electrons in the outer radiation belt near geosynchronous orbit, where radial diffusion and resonant interactions with whistler-mode chorus outside the plasmasphere are taken into account. Bounce averaged pitch-angle and energy diffusion rates are introduced in the Salammbô code for L ≤ 6.5, for electron energies between 10 keV and 3 MeV and fpe/fce values between 1.5 and 10. Results show that an initial seed population with a power law (Kappa) distribution and a characteristic plasmasheet energy of ∼5 keV can be accelerated up to a few MeV, for 4.5 < L < 6.6 and give a steady state profile similar to the one obtained from average satellite measurements. For a Kp = 4 magnetic storm simulation MeV electron fluxes increase by more than a factor of 10 on a timescale of 1 day. We conclude that whistler-mode chorus waves can be a major electron acceleration process at geostationary orbit