Temporal variability of quasi-linear pitch-angle diffusion

Watt, Clare, Allison, Hayley J., Bentley, Sarah, Thompson, Rhys L., Rae, Jonathan, Allanson, Oliver, Meredith, Nigel P., Ross, Johnathan P. J., Glauert, Sarah A., Horne, Richard B., Zhang, Shuai, Murphy, Kyle, Rasinskaitė, Dovilė and Killey, Shannon (2022) Temporal variability of quasi-linear pitch-angle diffusion. Frontiers in Astronomy and Space Sciences, 9. p. 1004634. ISSN 2296-987X

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Official URL: https://doi.org/10.3389/fspas.2022.1004634


Kinetic wave-particle interactions in Earth’s outer radiation belt energize and scatter high-energy electrons, playing an important role in the dynamic variation of the extent and intensity of the outer belt. It is possible to model the effects of wave-particle interactions across long length and time scales using quasi-linear theory, leading to a Fokker-Planck equation to describe the effects of the waves on the high energy electrons. This powerful theory renders the efficacy of the wave-particle interaction in a diffusion coefficient that varies with energy or momentum and pitch angle. In this article we determine how the Fokker-Planck equation responds to the temporal variation of the quasi-linear diffusion coefficient in the case of pitch-angle diffusion due to plasmaspheric hiss. Guided by in-situ observations of how hiss wave activity and local number density change in time, we use stochastic parameterisation to describe the temporal evolution of hiss diffusion coefficients in ensemble numerical experiments. These experiments are informed by observations from three different example locations in near-Earth space, and a comparison of the results indicates that local differences in the distribution of diffusion coefficients can result in material differences to the ensemble solutions. We demonstrate that ensemble solutions of the Fokker-Planck equation depend both upon the timescale of variability (varied between minutes and hours), and the shape of the distribution of diffusion coefficients. Based upon theoretical construction of the diffusion coefficients and the results presented here, we argue that there is a useful maximum averaging timescale that should be used to construct a diffusion coefficient from observations, and that this timescale is likely less than the orbital period of most inner magnetospheric missions. We discuss time and length scales of wave-particle interactions relative to the drift velocity of high-energy electrons and confirm that arithmetic drift-averaging is can be appropriate in some cases. We show that in some locations, rare but large values of the diffusion coefficient occur during periods of relatively low number density. Ensemble solutions are sensitive to the presence of these rare values, supporting the need for accurate cold plasma density models in radiation belt descriptions.

Item Type: Article
Additional Information: Funding Information: This work was enabled by collaborations supported by the Natural Environment Research Council (NERC) Highlight Topic consortium “Rad-Sat,” and the Space Weather Instrumentation, Measurement, Modelling and Risk (SWIMMR) Strategic Priorities Fund consortium “Sat-Risk.” Individual grant numbers are noted below. CW is supported by Science and Technology Facilities Council (STFC) grant ST/W000369/1 and Natural Environment Research Council (NERC) grants NE/P017274/1 (Rad-Sat) and NE/V002759/2 (Sat-Risk). HA is supported by an Alexander von Humboldt Postdoctoral Research Fellowship. IR acknowledges support from STFC grant ST/V006320/1 and NERC grants NE/P017185/1 (Rad-Sat) and NE/V002554/2 (Sat-Risk). NM, RH, SG, and JR would like to acknowledge financial support from the NERC Highlight Topic grant NE/P01738X/1 (Rad-Sat) and the NERC grants NE/V00249X/1 (Sat-Risk) and NE/R016038/1. OA acknowledges financial support from the University of Exeter and from the United Kingdom Natural Environment Research Council (NERC) Independent Research Fellowship NE/V013963/1, as well as NE/P017274/1 (Rad-Sat).
Uncontrolled Keywords: wave-particle interactions, radiation belt, quasilinear, temporal variability, stochastic
Subjects: F300 Physics
F500 Astronomy
Department: Faculties > Engineering and Environment > Mathematics, Physics and Electrical Engineering
Depositing User: John Coen
Date Deposited: 15 Nov 2022 17:16
Last Modified: 15 Nov 2022 17:30
URI: https://nrl.northumbria.ac.uk/id/eprint/50662

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