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Energetic Electrons at Uranus: Bimodal Diffusion in a Satellite Limited Radiation Belt

Selesnick, R. S. and Stone, E. C. (1991) Energetic Electrons at Uranus: Bimodal Diffusion in a Satellite Limited Radiation Belt. Journal of Geophysical Research A, 96 (A4). pp. 5651-5665. ISSN 0148-0227. http://resolver.caltech.edu/CaltechAUTHORS:20140508-155924671

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Abstract

The Voyager 2 cosmic ray experiment observed intense electron fluxes in the middle magnetosphere of Uranus. High counting rates in several of the solid-state detectors precluded the normal multiple coincidence analysis used for cosmic ray observations, and we have therefore performed laboratory measurements of the single-detector response to electrons. These calibrations allow a deconvolution from the counting rate data of the electron energy spectrum between energies of about 0.7 and 2.5 MeV. We present model fits to the differential intensity spectra from observations between L values of 6 and 15. The spectra are well represented by power laws in kinetic energy with spectral indices between 5 and 7. The phase space density at fixed values of the first two adiabatic invariants generally increases with L, indicative of an external source. However, there are also local minima associated with the satellites Ariel and Umbriel, indicating either a local source or an effective source due to nonconservation of the first two adiabatic invariants. For electrons which mirror at the highest magnetic latitudes, the local minimum associated with Ariel is radially displaced from the minimum L of that satellite by ∼0.5. The latitude variation of the satellite absorption efficiency predicts that if satellite losses are replenished primarily by radial diffusion there should be an increasing pitch angle anisotropy with decreasing L. The uniformity in the observed anisotropy outside the absorption regions then suggests that it is maintained by pitch angle diffusion. The effective source due to pitch angle diffusion is insufficient to cause the phase space density minimum associated with Ariel. Model solutions of the simultaneous radial and pitch angle diffusion equation show that the displacement of the high-latitude Ariel signature is also consistent with a larger effective source. This source may be created by inelastic scattering, leading to diffusion in energy as well as pitch angle.


Item Type:Article
Related URLs:
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http://dx.doi.org/10.1029/90JA02696DOIArticle
http://onlinelibrary.wiley.com/doi/10.1029/90JA02696/abstractPublisherArticle
Additional Information:Copyright 1991 by the American Geophysical Union. (Received September 6, 1990; revised November 5, 1990; accepted December 6, 1990.) Several people at Caltech have made significant contributions to the CRS calibrations and we thank especially M. Looper, A. Cummings, J. Cooper, D. Burke, M. Tran and K. Hargreaves. We also thank B. Mauk for providing electron data from the LECP experiment (S. M. Krimigis, principal investigator). This work was supported y NASA under contract NAS7-918 and grant NGR 05-002-160. The editor thanks J. F. Cooper and L. L. Hood for their assistance in evaluating this paper.
Group:Space Radiation Laboratory
Funders:
Funding AgencyGrant Number
NASANAS7-918
NASANGR 05-002-160
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory1990-09
Record Number:CaltechAUTHORS:20140508-155924671
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20140508-155924671
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:45619
Collection:CaltechAUTHORS
Deposited By: Deborah Miles
Deposited On:09 May 2014 17:15
Last Modified:09 May 2014 17:15

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