Alfvén waves travel along magnetic fields in a plasma as variations of the electromagnetic fields and currents. When two such waves interact, they produce a plasma fluctuation that starts reacting with the primary waves. Mediated by this fluctuation, new plasma waves 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 Alfvén wave interactions distort an ordered magnetic field and form elongated structures of strong currents, called current sheets. As the ongoing cascade stretches them, they break apart into 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 transfer energy into tertiary and higher-order wave modes. Alfvén waves are thus the mediators of turbulence in relativistic plasma.
• The emerging fast waves are astrophysically relevant because they can propagate across field lines. They offer one way for energy to leave strongly magnetized systems in the form of plasma waves. The strength of the coupling to the fast mode depends on the obliqueness of the primary waves.
• Current sheets and reconnection regions emerge naturally during the interaction of Alfvén waves. Higher-order modes shear and stretch field lines, which significantly reshapes field vortices. Current sheets form at the interface of these compressed eddies and tend to tear apart due to their elongated shape. In this nonlinear phase of the interaction, we observe a rapid decay of electromagnetic energy and the emergence of turbulence.

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 the primary waves, which is also the direction of the background magnetic field. While stabilizing, the sheets gradually thin out until they reach a critical scale. The ongoing cascade of energy into higher-order modes then drives them to break up rapidly into turbulence.

2D outline of the currents that emerge during the interaction of continuously overlapping Alfvén waves. Here, streamlines visualize the field vortices. Between extended eddies, the compression of reversed field lines drives the formation of current sheets. After thinning significantly, they tear apart and induce turbulence. At the same time, transient non-ideal electric fields emerge, and the inset plot shows their approximate magnitude.

Current and field structures in the interaction region of counter-propagating Alfvén wave packets. The initial eddies are repeatedly sheared into a spiral-like structure. Reconnection regions form at the interface of the eddies, especially in the central vortex. We identify such regions by the simultaneous appearance of strong currents, field reversals, non-ideal electric fields as approximated in the inset, and magnetic null points. Eventually, these structures become unstable again. They tear apart and the currents become turbulent, while electromagnetic energy dissipates significantly.

Current and field structures in the interaction region of counter-propagating Alfvén wave packets of different obliquity. In contrast to the back-and-forth motion shown above, the vortices are sheared more continuously. This over-shearing stretches out sheet-like structures at the interface of eddies with the same polarization. We again identify these structures as current sheets by the simultaneous appearance of strong currents, field reversals, non-ideal electric fields as approximated in the inset, and magnetic null points.

Collaborative results

J. M. TenBarge, B. Ripperda, A. Chernoglazov, A. Bhattacharjee, J. F. Mahlmann, E. R. Most, J. Juno, Y. Yuan, A. A. Philippov, Weak Alfvénic turbulence in relativistic plasmas. Part 1. Dynamical equations and basic dynamics of interacting resonant triads, Journal of Plasma Physics, Volume 87, Issue 6, 2021, arXiv:2105.01146

B. Ripperda, J. F. Mahlmann, A. Chernoglazov, J. M. TenBarge, E. R. Most, J. Juno, Y. Yuan, A. A. Philippov, A. Bhattacharjee, Weak Alfvénic turbulence in relativistic plasmas. Part 2. Current sheets and dissipation, Journal of Plasma Physics, Volume 87, Issue 5, 2021, arXiv:2105.01145