A New Approach to Constructing Models of Electron Diffusion by EMIC Waves in the Radiation Belts

Abstract

Electromagnetic ion cyclotron (EMIC) waves play an important role in relativistic electron losses in the radiation belts through diffusion via resonant wave-particle interactions. We present a new approach for calculating bounce and drift-averaged EMIC electron diffusion coefficients. We calculate bounce-averaged diffusion coefficients, using quasi-linear theory, for each individual Combined Release and Radiation Effects Satellite (CRRES) EMIC wave observation using fitted wave properties, the plasma density and the background magnetic field. These calculations are then combined into bounce-averaged diffusion coefficients. The resulting coefficients therefore capture the combined effects of individual spectra and plasma properties as opposed to previous approaches that use average spectral and plasma properties, resulting in diffusion over a wider range of energies and pitch angles. These calculations, and their role in radiation belt simulations, are then compared against existing diffusion models. The new diffusion coefficients are found to significantly improve the agreement between the calculated decay of relativistic electrons and Van Allen Probes data. Plain Language Summary In recent years there have been an increasing number of satellites operating in or traversing the Earth's radiation belts. These belts are composed of charged particles that are largely confined by the Earth's magnetic field, although waves can accelerate and scatter these particles. In the outer belt, electrons can be accelerated up to relativistic energies and pose a threat to satellites. Diffusion-based models are used to simulate the electron population, incorporating the statistical effects of waves on the electrons. Electromagnetic ion cyclotron waves are of particular importance for the relativistic population, effectively scattering them into the atmosphere and removing them from the belts. Previous models of this interaction are based on average plasma and wave observations; however, these do not well represent the range of interactions possible. Here we take a new approach, considering each observation individually to determine their statistical effect. When included into diffusion models, this new approach significantly improves the modeling of the relativistic electron population.

The Implications of Temporal Variability in Wave‐Particle Interactions in Earth's Radiation Belts

“…Diffusion coefficient models could then be constructed using appropriate averages of these individual diffusion coefficients, or stochastic methods such as suggested here. Preliminary studies of these new methods of averaging suggest that they are more effective than previous methods (Ross et al., 2020).…”

Determining the Global Scale Size of Chorus Waves in the Magnetosphere

“…Importantly, when constructing averaged diffusion coefficients, the average should be obtained by combining multiple observation‐specific diffusion coefficients (see Ross et al., 2020; Watt et al., 2019) rather than constructing diffusion coefficients from separately averaged wave parameters and plasma parameters. The results presented in this paper help us to consider how to construct drift‐averages of diffusion coefficients from observations, once individual observation‐specific bounce‐averaged diffusion coefficients have been constructed.…”

On the Variability of EMIC Waves and the Consequences for the Relativistic Electron Radiation Belt Population

“…Ni et al (2015) investigated the sensitivity of electron diffusion to EMIC wave normal angle and found that at L ≤ 5.5 the decay timescales were nearly independent of the wave normal angle model at E ≥ 2 MeV unless a very oblique wave is adopted over all latitudes. We performed calculations using the latitudinal varying wave normal angle model from Ni et al (2015) and found minimal difference in the diffusion coefficients in the domain of interest to the fixed wave normal angle model used by Kersten et al (2014) and Ross et al (2020). Therefore we assume that the waves have a Gaussian distribution in X, X = tan ψ, where ψ is the wave normal angle, centered on ψ = 0 with a width X w = tan 15°.…”

“…Recent studies have highlighted the importance of including the variability in plasma properties and wave spectra in radiation belt modeling, for EMIC waves (Ross et al, 2020) as well as whistler-mode waves (Watt et al, 2019(Watt et al, , 2021 and Ultra Low-Frequency wave (Thompson et al, 2020). Ross et al (2020) developed a new approach for calculating electron diffusion coefficients by averaging observation specific diffusion coefficients (each calculated using the wave spectra and plasma density of each individual EMIC wave observation), rather than averaging the wave spectra and density measurements before calculating diffusion coefficients (e.g., Kersten et al [2014]). This approach, applied to CRRES EMIC observations, significantly improved the agreement between global radiation belt models and Van Allen Probe observations of relativistic and ultra-relativistic electron flux compared to EMIC diffusion models using average wave spectra and plasma density.…”