Cosmic ray modulation in a non-spherical heliosphere during solar minimum conditions

Abstract

It is expected that the position of the modulation boundary in the equatorial and polar regions of the heliosphere depends on solar activity and possibly also on the polarity of the heliospheric magnetic field. In general, it can be assumed that the changing outward solar wind pressure causes a dynamic boundary position. Since the solar wind plasma quantities are not isotropic, as was confirmed by recent Ulysses spacecraft observations, the geometry of the modulation boundary may significantly deviate from the assumed sphere used in modulation models. We have studied the effect of non-spherical boundaries on the modulation of cosmic rays in the heliosphere using a drift model. In particular a pole-ward elongated heliosphere and the consequences of this approach on the generally too large cosmic-ray latitudinal gradients predicted by drift models are investigated for solar minimum conditions.