Modeling magnetic fields in the three-dimensional heliosphere

Abstract

Theoretical aspects are discussed of the solar wind interaction with the local interstellar medium (LISM) for different solar wind (SW) and space environment conditions. It is shown that charge exchange between plasma particles and interstellar neutrals is of major importance not only for geometrical scaling of the termination shock and the heliopause, but also for various asymmetries observed by the Voyager spacecraft and SOHO satellite. In essence, neutral atoms symmetrize the heliosphere, as compared with solutions based on entirely MHD models of the SW‐LISM interface. This is applicable both to east‐west asymmetries of the termination shock that can result in transverse anisotropies in fluxes of energetic charged particles, observed by Voyager 1, and to north‐south asymmetries that may explain similar anisotropies being observed now by Voyager 2 in the southern hemisphere. It is shown by numerical simulation that interplanetary magnetic field lines do intersect the termination shock multiple times for all possible orientations of the interstellar magnetic field with respect to Sun’s magnetic axis and the LISM velocity vector. However, only certain orientations and magnitudes of the interstellar magnetic field (ISMF) vector are suitable for explaining the data observed. Physical reasons are discussed that lead to deflections of the interstellar neutral hydrogen flow from the direction of propagation of neutral helium in the inner heliosheath. On the basis of numerical simulations, possibilities are investigated for deriving the orientation of the interstellar magnetic field as a function of the deflection angle. Parameters are enlisted that affect the divergence between the LISM neutral hydrogen and neutral helium velocity vectors: strength and direction of the interstellar magnetic field, and the density of the neutral hydrogen in the unperturbed LISM. It is shown that the possibility of using the SOHO SWAN experiment as an ISMF compass is not straightforward, since the heliosphere is generically asymmetric. The effects of the slow‐fast solar wind region separation in the context of the Sun’s 11‐year activity cycle are investigated. The consequences of a tilt between the Sun’s magnetic and rotational axes are analyzed. The importance of imaging the heliosheath in fluxes of energetic neutral atoms is briefly addressed.