Safety first: Stability and dissipation of line-tied force-free flux tubes in magnetized coronae

, , ,

Columns and other structures of twisted magnetic field lines with footpoints anchored to a surface at both ends exist in many astrophysical settings. Current loops shining in X-rays during the growth of plasma instabilities are observed in the corona of the Sun. They probably also exist around other compact astrophysical objects, such as in highly magnetized neutron star magnetospheres and accretion disk coronae. Solar flux tubes have been studied for many years, with many theories on their stability and dynamics. Compact object magnetospheres have different conditions, with strong magnetic fields, low plasma densities, and often rigid surfaces that anchor and displace magnetic field lines. We study the stability of simplified flux tube geometries under these conditions and estimate the electromagnetic energy dissipated during their instability.

Research summary

  • The well-established plasma safety factor q can serve as an instability criterion for the so-called kink instability in force-free flux tubes. It measures the inverse of the number of magnetic field line windings per flux tube length. Instabilities occur for q<1, that is, when the field lines wind sufficiently often.
  • For a given magnetic twist, the flux tube length determines the instability. Long enough flux tubes with q<1 always become unstable. Even in shorter systems with q>1, so-called fluting instabilities can develop. The kink instability dissipates significantly more energy than the fluting instability.
  • For compact object magnetospheres, these findings can be combined with information about the surface motions that build up the twist. For example, deformations of the magnetar crust can shear a bundle of magnetic flux. The shear velocity determines whether the bundle becomes kink unstable under fast shear or dissipates energy by fluting before reaching the critical winding under slow shear. Less dissipation may leave more free energy in the system, which could be released during powerful magnetar X-ray bursts.

Visualizing science

Currents during the evolution of a selected flux tube with α = 1, p0 = 0.75 and L/r0 = 8. The animation shows structures characteristic of the m = 2 fluting mode. Significant currents remain in the relaxed state.
Currents during the evolution of a selected flux tube with α = 1, p0 = 1.25 and L/r0 = 10. The animation shows structures characteristic of the asymmetric m = 1 kink mode. Some currents remain in the relaxed state.
Currents during the evolution of a selected flux tube with α = 1, p0 = 0.75 and L/r0 = 14. Structures characteristic of both the asymmetric m = 1 kink mode and the m = 2 fluting mode arise during the evolution. The m = 2 mode has shorter wavelengths than the m = 1 mode. Some currents remain in the relaxed state.

Collaborative results

Rugg, N., Mahlmann, J. F., & Spitkovsky, A. (2024). Safety first: Stability and dissipation of line-tied force-free flux tubes in magnetized coronae. The Astrophysical Journal, 966(2), 173, https://doi.org/10.3847/1538-4357/ad3206