Observations of Loss-Cone Pitch Angle Distributions of Solar Energetic Particles

Year: 2014

Pitch angle distributions of solar energetic particles (SEPs) in interplanetary space sometimes exhibit a loss cone in which an incident beam of particles is partially mirrored; particles with large pitch angles are reflected while those with smaller pitch angles are not. Mirroring requires a magnetic field enhancement, but if the field strength is not large enough to turn around particles with the smallest pitch angles or if these particles scatter before reaching their more distant mirror points, a loss cone forms. Such distributions therefore provide information on the interplanetary environment far from the spacecraft. The Low Energy Telescopes onboard the twin STEREO spacecraft have detected loss-cone distributions in several SEP events. We present some of these and other interesting anisotropy observations, and discuss their implications for SEP transport. In particular, we find that the shapes of the pitch angle distributions generally vary with energy and species, with lower energy particles usually more anisotropic than higher energy particles. Comparison with theory may be used to determine the energy and rigidity dependences of the pitch angle diffusion coefficient.