Modeling of heliospheric modulation of cosmic-ray positrons in a very quiet heliosphere

Abstract

Heliospheric modulation conditions were unusually quiet during the last solar minimum activity between solar cycles 23 and 24. Fortunately, the PAMELA space experiment measured 6 month-averaged Galactic positron spectra for the period 2006 July to 2009 December over an energy range of 80 MeV to 30 GeV, which is important for solar modulation. The highest level of Galactic positrons was observed at Earth during the 2009 July-December period. A well-established, comprehensive three-dimensional (3D) numerical model is applied to study the modulation of the observed positron spectra. This model had been used previously to understand the modulation of Galactic protons and electrons also measured by PAMELA for the same period. First, a new, very local interstellar spectrum for positrons is constructed using the well-known GALPROP code, together with the mentioned PAMELA observations. The 3D model is used to distinguish between the dominant mechanisms responsible for the heliospheric modulation of Galactic positrons and understand the effect of particle drift during this unusual minimum in particular, which is considered diffusion-dominant, even though particle drift still had a significant role in modulating positrons. Lastly, the expected intensity of Galactic positrons during an A > 0 polarity minimum, with similar heliospheric conditions than for 2006-2009, is predicted to be higher than what was observed by PAMELA for the 2006-2009 unusual minimum