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Published November 2012 | public
Journal Article

Large Proton Anisotropies in the 18 August 2010 Solar Particle Event


The solar particle event observed at STEREO Ahead on 18 August 2010 displayed a rich variety of behavior in the particle anisotropies. Sectored rates measured by the Low Energy Telescope (LET) on STEREO showed very large bidirectional anisotropies in 4 – 6 MeV protons for the first ∼ 17 hours of the event while inside a magnetic cloud, with intensities along the field direction several hundred to nearly 1000 times greater than those perpendicular to the field. At the trailing end of the cloud, the protons became isotropic and their spectrum hardened slightly, while the He/H abundance ratio plunged by a factor of approximately four for about four hours. Associated with the arrival of a shock on 20 August was a series of brief (< 10 minute duration) intensity increases (commonly called "shock spikes") with relatively narrow angular distributions (∼ 45∘ FWHM), followed by an abrupt decrease in particle intensities at the shock itself and a reversal of the proton flow to a direction toward the Sun and away from the receding shock. We discuss the STEREO/LET observations of this interesting event in the context of other observations reported in the literature.

Additional Information

© Springer, Part of Springer Science+Business Media. Received: 3 November 2011; Accepted: 28 April 2012; Published online: 6 June 2012. This work was supported by the National Aeronautics and Space Administration (NASA) at Caltech and JPL under subcontract SA2715-26309 from the University of California at Berkeley under NASA contract NAS5-03131, and by NASA award NNX08AK87G. We thank the PLASTIC (NASA contract NAS5-00132), MAG, and SWEA investigators on STEREO for making their data publicly available. This work benefited greatly from discussions at "The Sun-360 Workshop" held at the Christian-Albrechts-Universität in Kiel, Germany in July 2011. We thank B. Dotson for help in analyzing the data and in preparing the figures.

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