O’Brien T. P., Y. Y. Shprits, M. B. Moldwin, (2008), Eigenmode analysis of pitch-angle diffusion of energetic electrons in the outer zone, Journal Of Atmospheric And Solar-terrestrial Physics, 70, 1738 – 1744, doi:10.1016/j.jastp.2008.05.011, Dynamic Variability of Earth’s Radiation Belts

## Abstract

We demonstrate a new method of analyzing observed storm-time pitch-angle distributions to obtain information regarding the appropriate choice of the pitch-angle diffusion coefficients. We apply this method to MeV electrons in the outer zone as a diagnostic of the relative contribution of electromagnetic ion-cyclotron (EMIC) waves and whistler-mode hiss and chorus. We assume EMIC and hiss are confined to a plasmaspheric plume (hence, “plume waves”), with chorus prevalent over large portions of the day and night side. First, we determine the eigenmodes and eigenvalues of the pitch-angle diffusion operator predicted by quasilinear diffusion theory and approximate chorus, hiss, or EMIC plasma wave parameters for energetic electrons in the outer zone. Then, by projecting pitch-angle distributions observed by CRRES into the eigenmodes, we determine whether the pitch-angle distributions are consistent with the assumed diffusion process for various relative weighting of chorus and plume waves. Eigenmodes with shorter decay times (i.e., larger negative eigenvalues) ought to represent a comparatively smaller portion of the total flux in the pitch-angle distribution. We show that several observed pitch-angle distributions are consistent with predominantly chorus-driven pitch-angle diffusion, with at most a minor contribution from plume waves.## Authors (sorted by name)

Moldwin O’Brien Shprits## Journal / Conference

Journal Of Atmospheric And Solar-terrestrial Physics## Acknowledgments

The authors thank J. Roeder, M. Schulz, R. Thorne, and J. Albert for helpful discussions, and D. Subbotin for computer assistance. Data for this study were provided by the National Space Science Data Center, the National Geophysical Data Center, and the Kyoto World Data Center. Magnetic field tracing was provided by the ONERA-DESP library. This work was supported in part by NASA LWS TR&T Grant NNXO6AC38G. Work at UCLA was supported by NSF GEM Grant ATM-0603191. Also available as Aerospace Technical Report ATR-2007(5311)-2.## Grants

ATM‐0603191 NNXO6AC38G## Bibtex

@article{OBRIEN20081738,
title = "Eigenmode analysis of pitch-angle diffusion of energetic electrons in the outer zone",
journal = "Journal of Atmospheric and Solar-Terrestrial Physics",
volume = "70",
number = "14",
pages = "1738 - 1744",
year = "2008",
note = "Dynamic Variability of Earth's Radiation Belts",
issn = "1364-6826",
doi = "10.1016/j.jastp.2008.05.011",
url = "http://www.sciencedirect.com/science/article/pii/S1364682608001399",
author = "T.P. O’Brien and Y.Y. Shprits and M.B. Moldwin",
keywords = "Radiation belt, Wave–particle interactions, Pitch-Angle diffusion",
abstract = "We demonstrate a new method of analyzing observed storm-time pitch-angle distributions to obtain information regarding the appropriate choice of the pitch-angle diffusion coefficients. We apply this method to MeV electrons in the outer zone as a diagnostic of the relative contribution of electromagnetic ion-cyclotron (EMIC) waves and whistler-mode hiss and chorus. We assume EMIC and hiss are confined to a plasmaspheric plume (hence, “plume waves”), with chorus prevalent over large portions of the day and night side. First, we determine the eigenmodes and eigenvalues of the pitch-angle diffusion operator predicted by quasilinear diffusion theory and approximate chorus, hiss, or EMIC plasma wave parameters for energetic electrons in the outer zone. Then, by projecting pitch-angle distributions observed by CRRES into the eigenmodes, we determine whether the pitch-angle distributions are consistent with the assumed diffusion process for various relative weighting of chorus and plume waves. Eigenmodes with shorter decay times (i.e., larger negative eigenvalues) ought to represent a comparatively smaller portion of the total flux in the pitch-angle distribution. We show that several observed pitch-angle distributions are consistent with predominantly chorus-driven pitch-angle diffusion, with at most a minor contribution from plume waves."
}