Energetic Particle Pitch Angle Distributions Observed At Widely-Spaced Longitudes in the 23 July 2012 and Other Large Solar Particle Events

Abstract:

Solar energetic particle (SEP) pitch angle distributions arise from the competing effects of magnetic focusing and scattering as the particles travel through the interplanetary medium, and can therefore indicate interplanetary conditions far from the observer. The STEREO Low Energy Telescopes measure SEP pitch angle distributions for protons, helium, and heavier ions with energies of about 2-12 MeV/nucleon. A wide variety of particle anisotropies was observed in the extreme SEP event of 23 July 2012. At the STEREO-Ahead spacecraft, the solar source of the activity was near central meridian and the pitch angle distribution was initially an outward-flowing beam. High time resolution (1-minute) observations revealed peculiar oscillations in beam width on a timescale of several minutes; such behavior does not seem to have been previously reported in other events. Particle flow became bidirectional while inside a magnetic cloud following a tremendous shock. Particle intensities at the Behind spacecraft, from which the event occurred over the east limb of the Sun, were about 1000 times lower than at Ahead. Pitch angle distributions during the peak of the event show inward-flowing particles that underwent partial mirroring closer to the Sun and formed 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.