Direction of interstellar hydrogen flow in the heliosphere: theoretical modelling and comparison with SOHO/SWAN data

Abstract

The analysis of SOlar and Heliospheric Observatory (SOHO) Solar Wind ANisotropies (SWAN) data for backscattered solar Lyman α radiation performed by Lallement et al. showed for the first time that the average direction of interstellar hydrogen flow in the heliosphere is deflected by several degrees relative to the original direction of the interstellar wind outside the heliosphere. This deflection is caused by the indirect influence of the interstellar magnetic field (IsMF) through charge exchange between hydrogen atoms and interstellar protons deflected by the IsMF in the region of interaction between the solar wind and the local interstellar medium (LISM). Thus, measurements of the backscattered Lyman α radiation at the Earth's orbit can be used as a remote IsMF diagnostic. However, the direction of interstellar hydrogen flow in the vicinity of the Sun may be influenced by other effects such as the solar radiation pressure, gravitation and ionization and kinetic non-Maxwellian properties of the hydrogen distribution and also may depend on other LISM parameters besides IsMF. In this work, we perform a theoretical modelling of the backscattered solar Lyman α radiation seen at 1 au from the Sun and analyse the direction of hydrogen flow in the heliosphere, which can be obtained from the spectral properties of the backscattered radiation. The influence of different effects mentioned above is investigated. Also we compare our results obtained by means of a state-of-art 3D time-dependent kinetic model of the hydrogen distribution with the SWAN data of 1996.