Two-jet astrosphere model: effect of azimuthal magnetic field

TL;DR

This paper models the formation of two-jet structures in astrospheres influenced by azimuthal magnetic fields, revealing how stellar magnetic fields shape the plasma flow and topology of the astrosphere.

Contribution

It introduces a simplified MHD model showing how azimuthal magnetic fields create jet-like structures in astrospheres, with solutions depending on a key dimensionless parameter.

Findings

The problem depends on a single parameter related to magnetic field strength.

Relations between shock, astropause, and jet size are established.

The model allows estimating stellar magnetic fields from jet geometry.

Abstract

Opher et al. (2015), Drake et al. (2015) have shown that the heliospheric magnetic field results in formation of two-jet structure of the solar wind flow in the inner heliosheath, i.e. in the subsonic region between the heliospheric termination shock and the heliopause. In this scenario the heliopause has a tube-like topology as compared with a sheet-like topology in the most models of the global heliosphere (e.g. Izmodenov and Alexashov, 2015). In this paper we explore the two-jet scenario for a simplified astrosphere in which 1) the star is at rest with respect to the circumstellar medium, 2) radial magnetic field is neglected as compared with azimuthal component, 3) the stellar wind outflow is assumed to be hypersonic (both the Mach number and the Alfv\'enic Mach number are much greater than unity at the inflow boundary). We have shown that the problem can be formulated in dimensionless form, in which the solution depends only on one dimensionless parameter epsilon that is reciprocal of the Alfv\'enic Mach number at the inflow boundary. This parameter is proportional to stellar magnetic field. We present the numerical solution of the problem for various values of epsilon. Three first integrals of the governing ideal MHD equations are presented, and we make use of them in order to get the plasma distribution in the jets. Simple relations between distances to the termination shock, astropause and the size of the jet are established. These relations allow us to determine the stellar magnetic field from the geometrical pattern of the jet-like astrosphere.