Variability of Quasilinear Diffusion Coefficients for Plasmaspheric Hiss

In the outer radiation belt, the acceleration and loss of high-energy electrons is largely controlled by wave-particle interactions. Quasilinear diffusion coefficients are an efficient way to capture the small-scale physics of wave-particle interactions due to magnetospheric wave modes such as plasmaspheric hiss. The strength of quasilinear diffusion coefficients as a function of energy and pitch angle depends on both wave parameters and plasma parameters such as ambient magnetic field strength, plasma number density, and composition. For plasmaspheric hiss in the magnetosphere, observations indicate large variations in the wave intensity and wave normal angle, but less is known about the simultaneous variability of the magnetic field and number density. We use in situ measurements from the Van Allen Probe mission to demonstrate the variability of selected factors that control the size and shape of pitch angle diffusion coefficients: wave intensity, magnetic field strength, and electron number density. We then compare with the variability of diffusion coefficients calculated individually from colocated and simultaneous groups of measurements. We show that the distribution of the plasmaspheric hiss diffusion coefficients is highly non-Gaussian with large variance and that the distributions themselves vary strongly across the three phase space bins studied. In most bins studied, the plasmaspheric hiss diffusion coefficients tend to increase with geomagnetic activity, but our results indicate that new approaches that include natural variability may yield improved parameterizations. We suggest methods like stochastic parameterization of wave-particle interactions could use variability information to improve modeling of the outer radiation belt. Plain Language Summary The electrons in Earth's radiation belts exist in a highly rarefied part of space where collisions between particles is very rare. The only way in which the energy or direction of the trapped high-energy electrons can be changed is through interactions with electromagnetic waves. The efficacy of the interaction is a function of the energy and direction of travel of the electrons. In physics-based models of the radiation belts, the efficacy of the wave-particle interactions is captured in diffusion coefficients. These functions are constructed from information about the amplitude and frequency properties of the waves in the interaction, the magnetic field strength, ion composition, and density of the local plasma. We build up collections of observations of these properties from multiple passes of one of the NASA Van Allen probes through the same three small regions of space. The observations display significant temporal variability. We report on the statistical distributions of wave intensity, magnetic field strength and plasma number density and investigate the statistical distribution of the resulting diffusion coefficient. We find that the diffusion coefficients are highly variable and suggest that, by borrowing methods from other branches of geophysics such as numerical weather prediction, we may be able to include this variability in our models and improve the performance of radiation belt simulations.