We investigate the effect of magnetic helicity for the stability of buoyant magnetic cavities as found in the intergalactic medium. In these cavities we insert helical magnetic fields and test whether or not helicity can increase their stability to shredding through the Kelvin-Helmholtz instability and with that their life time. This is compared to the case of an external magnetic field which is known to reduce the growth rate of the Kelvin-Helmholtz instability. By comparing a low and high helicity configuration with the same magnetic energy we find that an internal helical magnetic field stabilizes the cavity. This effect increases as we increase the helicity content. Stabilizing the cavity with an external magnetic field requires a significantly stronger field.

1. Stabilizing Effect of Magnetic Helicity
on Magnetic Cavities
in the Intergalactic Medium
Simon Candelaresi, Fabio Del Sordo

2. (Fabian et al. 2000)
galactic disc
Intergalactic Bubbles
hot, under-dense bubble
stratified medium
2
Bubbles’ age is several tens of
millions of years.
Bubbles rise buoyantly through
density difference.

3. Kelvin-Helmholtz Instability
3
Bubbles should get disrupted.
What is the reason for their stability?
(GRAHAMUK/Wikimedia Commons)

4. Magnetic Helicity
Conservation of magnetic helicity:
magnetic resistivity
Realizability condition:
Magnetic energy is bound from
below by magnetic helicity.
4
Can magnetic helicity stabilize intergalactic cavities?

5. Numerical Experiments
5
Full resistive magnetohydrodynamics simulations
with the PencilCode.
stratified medium
hot, under-dense bubble

6. Initial Condition: Beltrami Field
6
smoothing function:
inside bubble:
Fix magnetic energy, vary magnetic helicity.

7. Thermal Emission
7

8. Temperature Iso-Surfaces
8
hydro low helicity high helicity

9. Bubble Coherence
9
Helical magnetic fields can stabilize the bubbles.

10. Conclusions
simon.candelaresi@gmail.com
●
Magnetic helicity as constraint on plasma dynamics.
●
Magnetic helicity leads to stability at small magnetic energy.
●
Possible mechanism to stabilize intergalactic bubbles.
●
Outlook: Test with geometrically different field (spheromak).
(arXiv:1912.12723)
simon.candelaresi@gmail.com