dc-Magnetic-Field Generation in Unmagnetized Shear Flows

Abstract

The generation of dc magnetic fields in unmagnetized electron-ion shear flows is shown to be associated to either initial thermal effects or the onset of electron-scale shear instabilities, in particular the cold Kelvin-Helmholtz instability. This mechanism, intrinsic to shear gradients on the electron scale, is described through a kinetic model that predicts the growth and the saturation of the dc field in both scenarios. The theoretical results are confirmed by multidimensional particle-in-cell simulations, demonstrating the formation of long-lived magnetic fields (t similar to 100's w(pi)(-1)) along the full longitudinal extent of the shear layer, with a typical transverse width root gamma(0)c/w(pe), reaching magnitudes eB(dc)/m(e)cw(pe) similar to beta(0) root gamma(0) for an initial sharp shear. The case of an initial smooth shear is also discussed.