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The evolution of solar wind strahl with heliospheric distance

Graham, GA; Rae, IJ; Owen, CJ; Walsh, AP; Arridge, CS; Gilbert, L; Lewis, GR; ... Waite, JH; + view all (2017) The evolution of solar wind strahl with heliospheric distance. Journal of Geophysical Research: Space Physics , 122 (4) pp. 3858-3874. 10.1002/2016JA023656. Green open access

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Abstract

Field-aligned beams of suprathermal electrons, known as “strahl,” are a frequently observed constituent of solar wind plasma. However, the formation and interplanetary evolution of the strahl electron populations has yet to be fully understood. As strahl electrons travel away from the Sun, they move into regions of decreasing magnetic field strength and thus are subject to adiabatic focusing. However, the widths of strahl pitch angle distributions observed at 1 AU are significantly broader than expected. Previous investigations have found that the average observed strahl pitch angle width actually increases with heliocentric radial distance. This implies that strahl electrons must be subjected to some form of pitch angle scattering process or processes, details of which as of yet remain elusive. In this paper, we use Cassini electron measurements to examine strahl beams across a distance range of approximately 8 AU, from its Earth Flyby in 1999 until its insertion into orbit around Saturn in 2004. We find that, in general, there is a relatively constant rate of broadening of strahl pitch angle distributions with distance between ∼1 and 5.5 AU. Our results from beyond this distance indicate that the strahl population is likely to be completely scattered, presumably to form part of the halo. We find multiple energy dependences at different radial distances implying that there are multiple strahl scattering mechanisms in operation.

Type: Article
Title: The evolution of solar wind strahl with heliospheric distance
Open access status: An open access version is available from UCL Discovery
DOI: 10.1002/2016JA023656
Publisher version: http://dx.doi.org/10.1002/2016JA023656
Language: English
Additional information: ©2017. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Space and Climate Physics
URI: https://discovery.ucl.ac.uk/id/eprint/1546737
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