Investigating the expansion of a braided interplanetary magnetic field

Abstract

Plasma motion in the Sun creates a persistent magnetic field that extends outwardly as far as Pluto and beyond. Assuming a steady state system, this so called interplanetary magnetic field takes the shape of an Archimedes spiral, known as the Parker spiral. However, the outer layers of the Sun are evolving continuously creating fluctuations in the magnetic field lines. These braid the magnetic field lines making its representation much more complicated. As the magnetic field lines move outwardly from the Sun, the separation between them increases. This affects the transport properties of fast moving particles, called solar energetic particles, that are emitted from the Sun. Such particles are of interest not only because they are a health hazard in outer space, but also because they can be used to probe the magnetic turbulence of outer space. Thus, it is important to account for the expansion of the magnetic field lines when calculating their transport properties. While an analytical model for the expansion of the Parker spiral in terms of area enclosing a fixed amount of flux is readily derived, that for a braided magnetic field could not be obtained so easily. In this context, the research uses numerical techniques in order to investigate the evolution of a braided field line. It then builds on the result to determine how the area enclosing a fixed amount of flux in such a field changes with radial distance from the Sun.