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

Abstract

Physics-based radiation belt models of electron behavior often focus on the wave-particle interactions that accelerate and scatter particles or contribute to radial diffusion. These models make considerable use of quasilinear theory to describe the wave-particle interactions (e.g., Lyons et al., 1972; Ripoll et al., 2020) and can be used to study the flux of high-energy electrons on a range of time scales, from single storms (e.g.,

Determining the Global Scale Size of Chorus Waves in the Magnetosphere

“…This suggests that when constructing a drift-average, an average of observation-specific bounce-averaged coefficients is likely to work well, since the effective timescales for electrons drifting through chorus wave regions are very short. Further numerical experiments, similar to those in Watt et al (2021), but using an appropriate distribution of chorus diffusion coefficients constructed as in Watt et al (2019) should confirm whether the timescales indicated in Figure 7c are short enough to allow for straightforward averaging.…”

“…The temporal scale size of chorus waves has not been studied statistically and quantitatively before. Recent numerical experiments demonstrate that, the temporal variability is very important for diffusion models due to the wave‐particle interactions (Thompson et al., 2020; Watt et al., 2019, 2021). Figures 5a–5d show the temporal correlations are proportional to time lag ( e −0.0067 *Δ t ) and drops to 0.5 when Δ t = 10 s, which means the statistically temporal scale size of chorus wave only lasts ∼10 s, and could be negligible compared to the electron diffusion timescales.…”

“…Numerical experiments demonstrate that the solution of Fokker-Planck equation for the diffusion model is sensitively dependent on the temporal variability of the diffusion coefficient (Thompson et al, 2020;Watt et al, 2021), which will be affected by the spatial and temporal variations of the wave and plasma parameter. Watt et al (2021) performed multiple numerical experiments of bounce-averaged diffusion, and indicated that rapid variations (with timescales of ∼2 min) of whistler-mode wave diffusion coefficients resulted in solutions of the Fokker-Planck equations that were similar to results obtained from an averaged diffusion coefficient. For much longer timescales (∼6 hr), results from averaged experiments did not match well with the results from experiments with temporally varying coefficients.…”

“…Traditional pitch angle diffusion models use parameterized diffusion coefficients that are mostly constructed from averaged plasma and wave properties (e.g., Fok et al., 2011; Horne et al., 2013; Tu et al., 2013). While, recent numerical experiments (Watt et al., 2019, 2021) indicate that the spatial and temporal variability of diffusion coefficients have universal importance for the diffusion process. Therefore, it is necessary to identify the temporal and spatial variation scale of wave characteristics to determine whether averaged diffusion coefficients can adequately capture the diffusion process, and to provide crucial information for use when constructing such averages.…”

“…In spite of these advances, the global spatial scale size of chorus waves in a physical coordinate system and its dependence on locations are still unclear. Specifically, the temporal scale size of chorus waves has not been studied statistically before, which has been revealed recently to be a significant factor for the radiation belt diffusion (Watt et al., 2021).…”