Fibrillation of solar magnetic fields

• Published in: The European physical journal. D, Atomic, molecular, and optical physics Atomic, molecular, and optical physics, 2009-06, Vol.53 (2), p.205-212
• Main Authors: Ashbourn, J. M.A., Woods, L. C.
• Format: Article
• Language: English
• Subjects: Applications of Nonlinear Dynamics and Chaos Theory; Applied classical electromagnetism; Astronomy; Atomic; Earth, ocean, space; Electric and magnetic fields; Electromagnetism; electron and ion optics; Exact sciences and technology; Fundamental areas of phenomenology (including applications); Magnetostatics; magnetic shielding, magnetic induction, boundary-value problems; Molecular; Optical and Plasma Physics; Physical Chemistry; Physics; Physics and Astronomy; Plasma Physics; Quantum Information Technology; Quantum Physics; Solar physics; Solar system; Spectroscopy/Spectrometry; Spintronics
• ISSN: 1434-6060; 1434-6079
• DOI: 10.1140/epjd/e2009-00114-9

Solar magnetic structures are often observed in the form of flux tubes composed of a number of smaller elements called fibres or threads, although theoretically such concentrations should not appear but should be flattened by magnetic diffusivity into a uniform, low intensity field. In this paper we describe a mechanism which may be responsible for the fibrillation and also for the very large diffusivity which dissipates magnetic flux tubes in hours instead of years. Firstly, the electric current associated with magnetic field gradients usually increases the local electron temperature and reduces the resistivity, so that the current becomes concentrated into sheets or streamers. Secondly, the magnetic field gradients continue to increase until the current magnitude reaches its limit, which is determined by the electron-ion streaming instability. Then with appropriate temperature and number densities, the Larmor radius of the ions overlaps the near discontinuity in B z and generates a sharply peaked fluid motion at the edge that is close to the thermal speed. Finally, the resulting vorticity generates an axial magnetic field opposing B z in the term , and if this is sufficient to change the sign of this term, the very unstable backward heat equation results. This instability repeatedly switches on and off and maintains the magnetic structure in the fibrillated form. Such structures are eventually eliminated by magnetic diffusivity in the usual way, but because of the fluctuations in B z , this occurs at a vastly increased rate. We show that this phenomenon increases the magnetic diffusivity, D, by a factor ~ 10 8 in agreement with some observations of plasma loops and supergranules.