Lyman-alpha Radiation Pressure in the Heliosphere: Results from a 3D Monte Carlo Radiative Transfer Simulation

Abstract

Global models of the heliosphere and its interaction with the local interstellar medium have evolved to fit spacecraft observations since the beginning of their development. It is now understood that neutral hydrogen plays a very important role in the modification of the shape and location of the heliopause. Comparisons of simulations to observations have often led to the need for a more accurate treatment of a particular process. We bring attention to a long-held assumption that radiation pressure on neutral hydrogen atoms due to Lyman-alpha absorption and emission falls off with heliocentric distance at the same rate as solar gravitation. These have generally been combined into a single term. Through our Monte Carlo simulation of Lyman-alpha radiative transfer in the heliosphere, we show that the radiation pressure 1) falls off at a slower rate than r-2 and 2) is not isotropic. We find that the radiation pressure in the upwind direction falls off with heliocentric distance with a power of approximately -1.80 between ~6 and 75 AU, while the downwind direction falls of with a much slower rate and more variability over heliocentric distance. We discuss our methods and results as well as the implications below.