Collimation of Particle Beams by the Structure of Two-Dimensional Magnetic Turbulence

We computationally examined the motion of energetic charged particles in the interplanetary medium, assuming a radial mean magnetic field and a two-component (2D+slab) model of turbulent transverse magnetic fluctuations. For the 2D component, which varies only in the angular directions, we employed 3 different models: a spherical harmonic series, 2D FFT, and 2D MHD (which we consider to be the most physically accurate). Given a narrow injection region, as expected for solar energetic particles (SEPs) from an impulsive solar flare, all 3 models yield intermittent particle distributions consistent with dropouts, for various particle energies. In addition, we find that relativistic ions are systematically drawn toward potential maxima (minima) of the 2D turbulence structure for a positive (negative) radial field, which can be attributed to guiding center drifts. The effect is strong when the Larmor radius exceeds the perpendicular coherence scale. We show that this effect leads to spatially collimated beams of relativistic ions, even for a wide injection region, as expected for gradual SEP events. Such collimation is relevant to spectral and temporal variability in neutron monitor observations of relativistic ions during ground level enhancements (GLEs), and such variability in the space radiation environment. Partially supported by the Thailand Research Fund, NSF SHINE ATM-0752135, and NASA Heliophysics Theory Program NNX08AI47G.