Role of lower hybrid turbulence in surfing acceleration at perpendicular shocks

Abstract

The mechanism for excitation of lower hybrid waves by shock surfing pickup ions is proposed. It is based on the modified two‐stream instability. The stationary distribution function of the surfing particles is calculated, which can serve as a driver for the excitation of the lower hybrid waves. The instability growth rate is found, as well as conditions under which waves have enough time to grow while being convected to the shock front together with the solar wind flow. The excited waves are responsible for the velocity diffusion of the surfing particles in the direction opposite to the acceleration. These particles are detrapped from the shock front and escape from the shock surfing acceleration. The flux of escaping particles is calculated in the quasi‐linear approximation, and it is shown to decrease quickly with the mass of the accelerated particles. This result can provide a possible explanation of Voyager observations, which indicate that the injection efficiency of interstellar pickup into the acceleration process at the solar wind termination shock increases significantly for heavy ions in comparison with hydrogen.