Fast Dissipation of Colliding Alfven Waves in a Magnetically Dominated Plasma

Magnetic energy around compact objects often dominates over plasma rest mass, and its dissipation can power the object's luminosity. We describe a dissipation mechanism that works faster than magnetic reconnection. The mechanism involves two strong Alfven waves with anti-aligned magnetic fields B (1) and B (2) that propagate in opposite directions along the background magnetic field B (0) and collide. The collision forms a thin current sheet perpendicular to B (0), which absorbs the incoming waves. The current sheet is sustained by an electric field E breaking the magnetohydrodynamic condition E < B and accelerating particles to high energies. We demonstrate this mechanism with kinetic plasma simulations using a simple setup of two symmetric plane waves with amplitude A = B (1)/B (0) = B (2)/B (0) propagating in a uniform B (0). The mechanism is activated when A > 1/2. It dissipates a large fraction of the wave energy, f = (2A - 1)/A (2), reaching 100% when A = 1. The plane geometry allows one to see the dissipation process in a one-dimensional simulation. We also perform two-dimensional simulations, enabling spontaneous breaking of the plane symmetry by the tearing instability of the current sheet. At moderate A of main interest, the tearing instability is suppressed. Dissipation transitions to normal, slower, magnetic reconnection at A >> 1. The fast dissipation described in this paper may occur in various objects with perturbed magnetic fields, including magnetars, jets from accreting black holes, and pulsar wind nebulae.

Automated summary

The dissipation mechanism involving the collision of two anti-aligned strong Alfven waves form a thin current sheet perpendicular to the background magnetic field, accelerating particles to high energies and dissipation a large fraction of wave energy. This fast dissipation process is activated when the amplitude ratio A > 1/2 and may occur in various objects with perturbed magnetic fields. The mechanism transitions to normal, slower magnetic reconnection at higher amplitude ratios A >> 1.