Longitudinal gradients of the distribution of anomalous cosmic rays in the outer heliosphere

Abstract

Recent results, both from observation and theory, suggest that the termination shock of the solar wind is not spherical. First, observations made with the Ulysses spacecraft have confirmed that the solar wind momentum flux increases with latitude, implying an elongation of the shock above the sun's poles. Second, results of modeling the global structure of the heliosphere indicate that a so‐called upwind‐downwind asymmetry of the shock is probable. In view of these findings as well as the observed upwind‐downwind asymmetry in pick‐up ion fluxes, one might suspect the existence of longitudinal gradients in the distributions of anomalous cosmic rays, which are supposedly identical with the pick‐up ions accelerated at the termination shock. We demonstrate that such longitudinal gradients should indeed be expected in the outer heliosphere and estimate their magnitude using a three‐dimensional modulation model.