Alfvén Radius: A Key Parameter for Astrophysical Magnetospheres

Abstract

The Alfvén radius is a distance where the magnetic energy density is equal to the kinetic energy density, or bulk velocity equals the Alfvén velocity. In this paper we discuss a role of Alfvén radius for different types of magnetospheres and magnetosphere-disk systems. Among the astrophysical disks considered here are the magnetic disks surrounding the outer planets in the Solar System (Jupiter and Saturn) and exoplanets, the heliospheric current sheet, accretion disks of neutron stars, pulsars, millisecond X-ray pulsars, white dwarfs and black holes, disks in the X-ray binaries, disks of young stars and active galactic nuclei (AGNs). We note that mainly in the magnetosphere-disk system, the inner edge of astrophysical disk (independently of its origin, direction of motion and material in it) in the presence of a strong magnetic field is located close to the Alfvén radius. For magnetized planets a concept of Alfvén radius is important as for the interaction with the solar/stellar wind, either for the inter-magnetospheric processes.