Alfvén waves travel along magnetic fields in a plasma as variations in the electromagnetic fields and currents. Their interaction produces a plasma fluctuation that starts reacting with the primary waves. Mediated by this fluctuation, new plasma waves can emerge: Alfvén waves with shorter oscillations than the initial waves; and fast waves that can travel in arbitrary directions. Like a cascade, this process transfers energy from the primary large-scale waves to waves with ever smaller oscillation periods. In this research project, we observe how distortions to an ordered magnetic field by Alfvén wave interactions form elongated structures of large currents. These structures are called current sheets. As they are stretched by the ongoing cascade process, they break apart and form turbulent field fluctuations. This turbulence rapidly dissipates (electromagnetic) energy as it runs away into smaller and smaller structures.
Research summary
- Counter-propagating Alfvén waves interact and grow secondary modes. These secondary modes interact with the primary waves and mediate energy into tertiary (and higher order) wave modes. Alfvén waves are the mediators of turbulence in relativistic plasma.
- The emerging fast waves are astrophysically relevant, as they can propagate across field lines. They are, thus, one possibility for energy to leave strongly magnetized systems in the form of plasma waves. The strength of coupling to the fast mode depends on the obliqueness of the primary waves.
- Current sheets and reconnection regions emerge quite naturally during the interaction of Alfvén waves. The shearing and stretching of field lines by higher order modes reshapes field vortices significantly. At the interface of these compressed eddies, current sheets form. They are prone to tear apart due to their elongated aspect ratio. We observe a rapid decay of electromagnetic energy and the emergence of turbulence in this non-linear phase of Alfvén wave interaction.
Visualizing science
Strong currents emerge during the interaction of continuously overlapping Alfvén waves. These structures stabilize over time and extend across the full length of the domain along the propagation direction of primary waves – and the magnetic background field. During their stabilization, the sheet structures gradually thin out until they reach a critical scale. The ongoing cascade of energy into higher order modes drives them to rapidly break up into turbulence.
2D outline of the currents emerging during the interaction of continuously overlapping Alfvén wave. This time, we visualize the field vortices by streamlines. Between extended eddies, the compression of reversed field lines drives the development of current sheets. After thinning significantly, they tear apart and induce turbulence. At the same time, transient non-ideal electric field emerge, and we show their approximate magnitude in the inset plot.
Current and field structures in the interaction region of counter-propagating Alfvén wave packets. The initial eddies are repeatedly sheared in a spiral-like structure. At the interface of the eddies, especially in the central vortex, reconnection regions form. We identify such regions by the simultaneous appearance of strong currents, field reversals, non-ideal electric field (as approximated in the inset), and magnetic null points. Eventually, these structures become, again, unstable. They tear apart and the currents become turbulent, at the same time as we observe significant dissipation of electromagnetic energy.
Current and field structure in the interaction region of counter-propagating Alfvén wave packets of different obliquity. The shearing of vortices proceeds in a more continuous way, as opposed to the back-and-forth motion presented above. This over-shearing stretches out sheet like structures at the interface of eddies of the same polarization. We identify these structures as current sheets by, once more, checking for the simultaneous appearance of strong currents, field reversals, non-ideal electric field (as approximated in the inset), and magnetic null points.
Collaborative results
J. M. TenBarge and B. Ripperda and A. Chernoglazov and A. Bhattacharjee and J. F. Mahlmann and E. R. Most and J. Juno and Y. Yuan and A. A. Philippov, Weak Alfvénic turbulence in relativistic plasmas. Part 1. Dynamical equations and basic dynamics of interacting resonant triads, arxiv:2105.01146, Journal of Plasma Physics (Volume 87 Issue 6)
B. Ripperda and J. F. Mahlmann and A. Chernoglazov and J. M. TenBarge and E. R. Most and J. Juno and Y. Yuan and A. A. Philippov and A. Bhattacharjee, Weak Alfvénic turbulence in relativistic plasmas. Part 2. Current sheets and dissipation, arxiv:2105.01145, Journal of Plasma Physics (Volume 87 Issue 5)
Jens Mahlmann 