Abstract
Abstract. Collisionless shock waves in space and astrophysical plasmas can accelerate electrons along the shock layer by an electrostatic potential, and scatter or reflect electrons back to the upstream region by the amplified magnetic field or turbulent fluctuations. The notion of the critical pitch angle is introduced for non-adiabatic electron acceleration by balancing the two timescales under a quasi-perpendicular shock wave geometry in which the upstream magnetic field is nearly perpendicular to the shock layer normal direction. An analytic expression of the critical pitch angle is obtained as a function of the electron velocity parallel to the magnetic field, the ratio of the electron gyro- to plasma frequency, the cross-shock potential, the width of the shock transition layer, and the shock angle (which is the angle between the upstream magnetic field and the shock normal direction). For typical non-relativistic solar system applications, the critical pitch angle is predicted to be about 10°. An efficient acceleration is expected below the critical pitch angle.
Highlights
The notion of the critical pitch angle is introduced for non-adiabatic electron acceleration by balancing the two timescales under a quasi-perpendicular shock wave geometry in which the upstream magnetic field is nearly perpendicular to the shock layer normal direction
An analytic expression of the critical pitch angle is obtained as a function of the electron velocity parallel to the magnetic field, the ratio of the electron gyro- to plasma frequency, the cross-shock potential, the width of the shock transition layer, and the shock angle
Collisionless shocks in space and astrophysical plasmas are unique in that electrons are efficiently accelerated there
Summary
Collisionless shocks in space and astrophysical plasmas are unique in that electrons are efficiently accelerated there. For a quasi-parallel shock at which the upstream magnetic field is nearly aligned with the normal direction of the shock layer, the electrons can be efficiently trapped by turbulent fluctuations on the both sides of the shock. For a quasi-perpendicular shock, the electrons are accelerated within the shock transition layer by the electrostatic, cross-shock potential which is sustained by different bulk motions of the ions to the electrons (Goodrich and Scudder, 1984). The electrons can be scattered or reflected away from the shock layer by a sudden increase of the magnetic field or turbulent fluctuations at the shock ramp. One may formulate that the electrons at the quasi-perpendicular shock undergo two competing effects: acceleration or scattering. The purpose of this article is to estimate the critical pitch angle for the electron acceleration at the quasi-perpendicular shock
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