The Heliotail

Abstract:

The heliotail is formed when the solar wind (SW) interacts with the local interstellar medium (LISM) and is shaped by the interstellar magnetic field (ISMF). While there are no spacecraft to perform in situ measurements of the SW plasma and heliospheric magnetic field in the heliotail, it is of importance for the interpretation of measurements of energetic neutral atom fluxes performed by Interstellar Boundary Explorer (IBEX). It has been shown recently that the orientation of the heliotail in space and distortions of the unperturbed LISM caused by its presence may be able to explain some features in the anisotropy map of TeV cosmic ray flux detected in air shower experiments. The SW flow in the heliotail is a mystery itself because it is strongly affected by charge exchange between the SW ions and interstellar neutral atoms. If the angle between the Sun's magnetic and rotation axes is constant, the SW in the tail tends to be concentrated inside the heliospheric magnetic field spirals deflected tailward, exhibiting a two-lobe structure similar to Yu (1974) and Opher et al. (2015). However, the twisted field soon becomes unstable and the reason for the SW collimation within a two-lobe structure vanishes. The resulting solution exhibits features typical of magnetic reconnection. We demonstrate that kinetic treatment of the H atom transport becomes essential in this case to explain the lobe absence further along the tail. If the angle between the Sun’s magnetic and rotation axes is not constant, the flow collimation becomes latitudinally dependent. We show that the heliotail flow is strongly affected by the solar cycle that eliminates artifacts typical of solutions based on simplifying assumptions. The heliopause in the tail is subject to Kelvin--Helmholtz instability, but its orientation and shape are determined by the ISMF direction and strength.