Twisted Magnetic Flux Tubes - Effect of Small Twist
Abstract
The shape of a magnetic flux tube is investigated when photospheric motion causes small twist at the magnetic footpoints. Using a Fourier-Bessel series expansion, the previous results of Zweibel and Boozer (1985) and Steinolfson and Tajima (1987), when the twist is small, are substantiated. A twisting motion that is restricted to a finite region is investigated. Inside the twisted region, the tube contracts, but in the outer region the field remains straight, except for a slight expansion at the outside of the loop near the footpoints. The amount of twist depends on the radial position and can in fact be larger in the contracted region with the twist decreasing as the tube expands. An axial boundary-layer region is present, as noted by the above authors, through which the field adjusts to the line-tied magnetic footpoint positions. An analysis of the boundary layer shows that the thickness remains constant as the loop-length is increased with the result that the main part of the loop has constant cross-sectional area and can be described by cylindrically-symmetric fields. This new 1-D model predicts the main behaviour of the loop without the need to solve the more complicated 2-D problem directly. It is speculated that the boundary layers will remain even when the twist becomes large and a simple example is presented. A detailed parametric study of different twist profiles shows how the central part of the loop responds.
- Publication:
-
Solar Physics
- Pub Date:
- September 1989
- DOI:
- 10.1007/BF00912994
- Bibcode:
- 1989SoPh..122..227L
- Keywords:
-
- Coronal Loops;
- Magnetic Flux;
- Solar Magnetic Field;
- Flux Pinning;
- Photosphere;
- Solar Physics;
- Coronal Loop;
- Magnetic Flux Tube;
- Tube Expand;
- Detailed Parametric Study;
- Small Twist