Basic Bell-MHD Turbulence

Abstract: Nonresonant current instability was identified by Bell (2004) as an important mechanism for magnetic field amplification in supernova remnants. In this paper we focus on studying the nonlinear stage of this instability using the incompressible MHD formulation. We demonstrate that the evolution of magnetic turbulence driven by the Bell instability resembles turbulence driven on large scales. More importantly, we demonstrate that the energy-containing scale for magnetic fields is proportional to the square root of magnetic energy density. Given the observational constraints of the possible field amplification, this new relation allows us to directly estimate the maximum energy of particles scattered by such fields and this estimate is normally below the average particle energy. This implies that, without taking into account the feedback to cosmic rays, the typical scales of Bell fields, in either linear or nonlinear regime, will be too small to affect high energy particle acceleration. We mention several scenarios of back-reaction to cosmic rays that could be important.