Insights Into Particle Transport Obtained from Solar Energetic Particle Anisotropies
Solar energetic particle (SEP) pitch-angle distributions are shaped by the competing effects of magnetic focusing and scattering as the particles travel through interplanetary space. Therefore, measurements of SEP anisotropies provide insight into particle transport and can probe interplanetary conditions at remote locations from the observer. The Low Energy Telescopes (LETs) onboard the twin STEREO spacecraft measure pitch-angle distributions for protons and heavier ions at energies of about 2-12 MeV/nucleon. Using these instruments, we have observed a wide variety of SEP anisotropies, such as bidirectional flows within interplanetary coronal mass ejections, sunward-flowing particles when the spacecraft was magnetically connected to the back side of a distant shock, and loss-cone distributions in which particles with large pitch angles magnetically mirror at an interplanetary field enhancement that is too weak to reflect particles with the smallest pitch angles. One of the more puzzling observations is unusual oscillations on a timescale of several minutes in the width of a beamed distribution at the onset of the very large 23 July 2012 SEP event. We report LET anisotropy observations at both STEREO spacecraft during the extreme event of 23 July 2012, in which a large range of anisotropies were observed at various times during the event, and discuss their implications for SEP transport.
Additional InformationCopyright owned by the author(s) under the term of the Creative Commons Attribution-NonCommercial-ShareAlike. 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/PLASTIC (NASA contract NAS5-00132), MAG, and SWEA teams for making their data publicly available, and Lan Jian and Ying Liu for helpful discussions.
Published - ICRC2015_072.pdf