Kinking flux tubes and global eruptions: The 3D dynamics of strongly twisted magnetar magnetospheres

The origin of the various outbursts of hard X-rays from magnetars, highly magnetized neutron stars,
is still unknown. They range from frequent and faint events to the most energetic Giant Flares, spanning several orders of magnitude in burst energies. Our work explores how different instabilities of the magnetar magnetosphere can inject energy (shining in the X-ray), and even launch waves with feedback on the outer magnetosphere (creating X-ray bursts via other channels, or possibly fast radio bursts via reconnection). Besides globally erupting magnetospheres that release the twist energy of a fluxtube in extended reconnection events, we find a new instability that operates on a more confined scale. Kink-like helical eruptions are capable of dissipating energy without significant rearrangements of the magnetosphere’s topology. While global eruptions eject powerful energy bubbles that propagate to the outer magnetosphere as non-linear structures, kinking fluxtubes seed fast magnetosonic waves. These waves travel across magnetic fieldlines until they eventually become electrically dominated, and a possible source of strong shocks that can drive further X-ray activity.

Research summary

During an eruption, around 25% of the twist energy – injected into the magnetosphere by twisting magnetic fieldline foot points – is dissipated. We suggest that this release of free energy drives X-ray flaring activity.
2D twists extending over the entire stellar surface can easily drive the dissipation of Giant Flare energies and always erupt on large scales. In 3D, the available free energy is generally lower, and we reach dissipation events of 0.1% to 1% of the initial dipole energy. Such 3D events could power a wide range of faint(er) X-ray bursts.
Global eruptions eject energetic non-linear structures that carry energy to the outer magnetosphere in Poynting-flux-loaded blobs (in our models up to 1% of the dipole energy). Confined, kink-like eruptions do not affect the large-scale magnetospheric structure. However, they seed low-amplitude fast magnetosonic waves that become electrically dominated at around 100 stellar radii.

Visualizing science

Axisymmetric magnetospheric twist triggering a large-scale reconnection event and global rearrangement of the magnetosphere. A constant twist is induced in the twisting region centered at θ = 45° with an extension of 0.05π.

3D visualization of field line dynamics in the magnetar magnetosphere. A constant twist is induced in the twisting region centered at θ = 45° with an extension of 0.05π. Subsequent eruptions stay confined in the inner magnetosphere. Volume-filling colors denote the conserved parallel current. Inset plots represent poloidal (field lines) and toroidal (color) magnetic fields in a meridional slice centered on the induced twist.

3D visualization of field line dynamics in the magnetar magnetosphere. A constant twist is induced in the twisting region centered at θ = 55° with an extension of 0.1π. Subsequent eruptions open up the magnetosphere on a global scale. Volume-filling colors denote the conserved parallel current. Inset plots represent poloidal (field lines) and toroidal (color) magnetic fields in a meridional slice centered on the induced twist.

Collaborative results

Mahlmann, J. F., Philippov, A. A., Mewes, V., Ripperda, B., Most, E. R., & Sironi, L. (2023). Three-dimensional dynamics of strongly twisted magnetar magnetospheres: Kinking flux tubes and global eruptions. The Astrophysical Journal Letters, 947, L34, https://doi.org/10.3847/2041-8213/accada.