A Quantitative Measure of Pitch-angle Anisotropy

D. K. Haggerty (1), E. C. Roelof (1)
(1) The Johns Hopkins University Applied Physics Laboratory
Energetic particle time histories and angular distributions are used to probe both local and distant structures
and acceleration mechanisms throughout the heliosphere. Anisotropies observed in near-relativistic solar
electron events can indicate the duration of particle acceleration and give insight to the acceleration
mechanism. The observation of unidirectional or bidirectional anisotropies in magnetic clouds at 1 AU can
give insight into the global structure of the internal flux rope through which the energetic particles
propagate. Here we detail a quantitative method to measure the strong anisotropy of energetic particle
distributions. The pitch-angle anisotropy is characterized in the frame moving with the solar wind by a
piecewise exponential function j = j0exp() where  is the anisotropy measure and  is the pitch cosine.
Plasma flow velocities are required for ion anisotropy calculations (because of connection effects), but not
for near relativistic electrons. The simplicity of implementing this method, coupled with error estimates for
both Poisson counting fluctuations and root-mean-square deviations of the sector intensities from the
optimal fit values, makes this a robust and effective tool for the analysis of energetic particle observations.