Comment on “Electron demagnetization and heating in quasi‐perpendicular shocks” by Mozer and Sundkvist

Abstract

In their paper, Mozer and Sundkvist (2013) present important observations of large‐amplitude high‐frequency electric field fluctuations in the vicinity of the ramp region of the Earth's bow shock. In common with other recent work, they emphasize the role of electron scattering by such fluctuations in the problem of electron heating at collisionless shocks, relegating the DC fields to providing a reservoir of energy which can be tapped apparently to whatever degree is required. While the historical approach of attributing the zeroth‐order electron heating to adiabatic motion in those DC fields has always required some unspecified scattering process, key elements of those DC processes have been lost or misinterpreted. In particular, particle motion parallel and perpendicular to the magnetic field are coupled to one another. More importantly, an O(1/2) fraction of the electron population traverses the shock from downstream to upstream or are trapped downstream. Thus, the concept of irreversible heating proceeding from the solar wind through the shock into the magnetosheath, implicit in Mozer and Sundkvist (2013), needs to be reconsidered. A highly simplified calculation illustrates these points.