Collisions of relativistic Alfvén waves as sources of weak turbulence, current sheets, and efficient mode conversion

Alfvén waves are the primary building blocks of turbulence in magnetized plasmas. We extend the validity of this statement to relativistic astrophysical plasmas beyond the heliosphere. Such environments, commonly found in black hole accretion disks coronae and neutron star magnetospheres, are highly magnetized and relativistic. Starting from counter-propagating Alfvén waves, we unveil nonlinear dynamics of Alfvén waves by drawing on theory and three-dimensional numerical simulations in both relativistic magnetohydrodynamics (MHD) and force-free MHD.

A large reservoir of magnetic energy is available in the magnetospheres of compact astrophysical objects, such that plasma can be heated significantly even in the weak turbulence regime. We show that an E_B,⊥(k_⊥) ∼ [k_⊥]^-2 energy spectrum develops as a result of the weak turbulence cascade in Alfvén wave interactions. The plasma turbulence ubiquitously generates current sheets, locations where magnetic energy dissipates. These current sheets form due to the compression of elongated eddies, driven by the shear that growing higher-order modes induce. Current layers undergo a thinning process until they eventually break up into small-scale turbulent structures. The interaction of localized relativistic Alfvén waves produces both Alfvén waves and fast waves. It efficiently mediates the conversion and dissipation of electromagnetic energy in astrophysical systems.