Temporal variability of quasi-linear pitch-angle diffusion

Watt，Clare E.J.1; Allison，Hayley J.2; Bentley，Sarah N.1; Thompson，Rhys L.3; Rae，I. Jonathan1; Allanson，Oliver4; Meredith，Nigel P.5; Ross，Johnathan P.J.5; Glauert，Sarah A.5; Horne，Richard B.5; Zhang，Shuai6; Murphy，Kyle R.1,7; Rasinskaitė，Dovilė1; Killey，Shannon1

2022-10-13

10.3389/fspas.2022.1004634

Frontiers in Astronomy and Space Sciences   Impact Factor & Quartile

2296-987X

2296-987X

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.

quasilinear radiation belt stochastic temporal variability wave-particle interactions

SCI

English

Others

Natural Environment Research Council[NE/P017274/1 NE/V002759/2 NE/P017185/1 NE/V002554/2 NE/P01738X/1 NE/V00249X/1 NE/R016038/1];Central Laser Facility, Science and Technology Facilities Council[ST/W000369/1 ST/V006329/1];Science and Technology Facilities Council[ST/W000369/1 ST/V006329/1];

Astronomy & Astrophysics

Astronomy & Astrophysics

WOS:000876505600001

FRONTIERS MEDIA SA

2-s2.0-85140625057

Scopus

1.Department of Mathematics,Physics and Electrical Engineering,Northumbria University,Newcastle upon Tyne,United Kingdom
2.GFZ German Research Centre for Geosciences,Potsdam,Germany
3.Department of Mathematics and Statistics,University of Reading,Reading,United Kingdom
4.Department of Mathematics,University of Exeter,Penryn,Penryn/Cornwall Campus,United Kingdom
5.British Antarctic Survey,Cambridge,United Kingdom
6.Department of Earth and Space Sciences,Southern University of Science and Technology,Shenzhen,China
7.Department of Astronomy,University of Maryland,College Park,United States

Watt，Clare E.J.,Allison，Hayley J.,Bentley，Sarah N.,et al. Temporal variability of quasi-linear pitch-angle diffusion[J]. Frontiers in Astronomy and Space Sciences,2022,9.

Watt，Clare E.J..,Allison，Hayley J..,Bentley，Sarah N..,Thompson，Rhys L..,Rae，I. Jonathan.,...&Killey，Shannon.(2022).Temporal variability of quasi-linear pitch-angle diffusion.Frontiers in Astronomy and Space Sciences,9.

Watt，Clare E.J.,et al."Temporal variability of quasi-linear pitch-angle diffusion".Frontiers in Astronomy and Space Sciences 9(2022).