Energetic Particle Anisotropies Observed By Stereo/LET in the 23 July 2012 Solar Particle Event

Abstract:

Solar energetic particle (SEP) pitch angle distributions are shaped by the transport of these particles through interplanetary space as they undergo focusing and scattering. Studies of the anisotropies can therefore probe interplanetary conditions far from the observer. The Low Energy Telescopes onboard the twin STEREO spacecraft measure SEP pitch angle distributions for protons, helium, and heavier ions with energies of about 2-12 MeV/nucleon. Among the more interesting periods observed to date is the very large SEP event of 23 July 2012. At the Ahead spacecraft, the solar source of the activity was near central meridian and the pitch angle distribution was initially an outward-flowing beam, but it exhibited peculiar oscillations in beam width on a timescale of several minutes. After a shock passed, particle flow became bidirectional while inside a magnetic cloud. At the Behind spacecraft, more than 120 degrees of heliolongitude away, particles flowed in towards the Sun once the spacecraft was magnetically connected to the back side of the shock after it reached about 2 AU. These particles then experienced partial mirroring closer to the Sun, forming a distinctive loss-cone distribution indicating that the magnetic field strength at the mirror point was too small to turn around particles with the smallest pitch angles. We present the observations of this rich variety of anisotropies within a single event, compare with observations in other events, and discuss the implications for SEP transport in the inner heliosphere.