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Published March 22, 2016 | Published
Book Section - Chapter Open

Solar energetic particle anisotropies and insights into particle transport


As solar energetic particles (SEPs) travel through interplanetary space, their pitch-angle distributions are shaped by the competing effects of magnetic focusing and scattering. Measurements of SEP anisotropies can therefore reveal information about interplanetary conditions such as magnetic field strength, topology, and turbulence levels at remote locations from the observer. Onboard each of the two STEREO spacecraft, the Low Energy Telescope (LET) measures pitch-angle distributions for protons and heavier ions up to iron at energies of about 2-12 MeV/nucleon. Anisotropies observed using LET include bidirectional flows within interplanetary coronal mass ejections, sunward-flowing particles when STEREO was magnetically connected to the back side of a shock, and loss-cone distributions in which particles with large pitch angles underwent magnetic mirroring at an interplanetary field enhancement that was too weak to reflect particles with the smallest pitch angles. Unusual oscillations in the width of a beamed distribution at the onset of the 23 July 2012 SEP event were also observed and remain puzzling. We report LET anisotropy observations at both STEREO spacecraft and discuss their implications for SEP transport, focusing exclusively on the extreme event of 23 July 2012 in which a large variety of anisotropies were present at various times during the event.

Additional Information

© 2016 AIP Publishing LLC. This work was supported by the National Aeronautics and Space Administration (NASA) at the California Institute of Technology and the Jet Propulsion Laboratory under sub-contract SA2715-26309 from the University of California at Berkeley under NASA contract NAS5-03131, and by NASA award NNX08AK87G. We thank the STEREO/MAG team for making their data publicly available, and Lan Jian and Ying Liu for helpful discussions.

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