'Torus' distribution of interstellar helium pickup ions: Direct observation

[1] A 'torus'-like velocity distribution of the helium pickup ions has been detected in interplanetary space by GEOTAIL. A statistical survey shows that the observation probability of the 'torus' distribution amounts to nearly 20% and that the distribution pattern depends on the local magnetic field directions and turbulence levels. These results support the recent finding of a low pitch angle scattering rate and a large mean free path.