ULF Wave Driven Radial Diffusion During Geomagnetic Storms: A Statistical Analysis of Van Allen Probes Observations

J. K. Sandhu, I. J. Rae, J. R. Wygant, A. W. Breneman, S. Tian, C. E.J. Watt, R. B. Horne, L. G. Ozeke, M. Georgiou, M. T. Walach

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30 Scopus citations


The impact of radial diffusion in storm time radiation belt dynamics is well-debated. In this study we quantify the changes and variability in radial diffusion coefficients during geomagnetic storms. A statistical analysis of Van Allen Probes data (2012–2019) is conducted to obtain measurements of the magnetic and electric power spectral densities for Ultra Low Frequency (ULF) waves, and corresponding radial diffusion coefficients. The results show global wave power enhancements occur during the storm main phase, and continue into the recovery phase. Local time asymmetries show sources of wave power are both external solar wind driving and internal sources from coupling with ring current ions and substorms. Wave power enhancements are also observed at low L values (L < 4). The accessibility of wave power to low L is attributed to a depression of the Alfvén continuum. The increased wave power drives enhancements in both the magnetic and electric field diffusion coefficients by more than an order of magnitude. Significant variability in diffusion coefficients is observed, with values ranging over several orders of magnitude. A comparison to the Kp parameterized empirical model of Ozeke et al. (2014) is conducted and indicates important differences during storm times. Although the electric field diffusion coefficient is relatively well described by the empirical model, the magnetic field diffusion coefficient is approximately ∼10 times larger than predicted. We discuss how differences could be attributed to data set limitations and assumptions. Alternative storm-time radial diffusion coefficients are provided as a function of L* and storm phase.

Original languageEnglish (US)
Article numbere2020JA029024
JournalJournal of Geophysical Research: Space Physics
Issue number4
StatePublished - Apr 1 2021

Bibliographical note

Funding Information:
J. K. Sandhu was supported by NERC Grants NE/P017185/2 and NE/V002554/2. I. J. Rae was supported by NERC Grants NE/P017185/2 and NE/V002554/2. C. E. J. Watt was supported by STFC grant ST/R000921/1 and NERC grants NE/P017274/1 and NE/V002759/2. R. B. Horne was supported by Highlight Topic Grant NE/P01738X/1 (Rad‐Sat). M.‐T. Walach was supported by Natural Environments Research Council grants NE/P001556/1 and NE/T000937/1. The authors thank the EMFISIS instrument team for data provision. The authors thank the EFW team for data provision and the work by the EFW team was conducted under JHU/APL contract 922613 (RBSP‐EFW).

Publisher Copyright:
© 2021. The Authors.


  • Geomagnetic Storms
  • ULF waves
  • Van Allen probes
  • outer radiation belt
  • radial diffusion


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