Magnetic Helicity Conservation and Inverse Energy Cascade in Electron Magnetohydrodynamic Wave Packets

Abstract

Electron magnetohydrodynamics (EMHD) provides a fluid-like description of small-scale magnetized plasmas. An EMHD wave (also known as whistler wave) propagates along magnetic field lines. The direction of propagation can be either parallel or anti-parallel to the magnetic field lines. We numerically study propagation of 3-dimensional (3D) EMHD wave packets moving in one direction. We obtain two major results: 1. Unlike its magnetohydrodynamic (MHD) counterpart, an EMHD wave packet is dispersive. Because of this, EMHD wave packets traveling in one direction create opposite traveling wave packets via self-interaction and cascade energy to smaller scales. 2. EMHD wave packets traveling in one direction clearly exhibit inverse energy cascade. We find that the latter is due to conservation of magnetic helicity. We compare inverse energy cascade in 3D EMHD turbulence and 2-dimensional (2D) hydrodynamic turbulence.