Observational evidences of wave excitation and inverse cascade in a distant Earth foreshock region

Abstract

The foreshock with nascent plasma turbulence is regarded as a fascinating region to understand basic plasma physical processes, e.g., wave-particle interactions as well as wave-wave couplings. Although there have been plenty of intensive studies on this topic, some key clues about the physical processes still lack observations. A relatively comprehensive case study with some new observations is presented in this work based on the WIND spacecraft observations. In this case, upstream energetic protons were drifting at tens of Alfven speed with respect to the background plasma protons. When looking at the magnetic wave activities, we find the co-existence of high-frequency (0.1−0.5 Hz) large-amplitude right-hand polarized (RHP) waves and low-frequency (0.02−0.1 Hz) small-amplitude left-hand polarized (LHP) waves in the spacecraft (SC) frame. The observed anticorrelation between magnetic and velocity fluctuations along with the sunward magnetic field direction indicates that the low-frequency LHP waves in the SC frame are in fact the sunward upstream RHP Alfvenic waves in the solar wind frame. This new observation corroborates the applicability of theories about plasma non-resonance instability and inverse cascade to the foreshock region, where the downstream high-frequency RHP parent waves are excited by the upstream energetic protons through non-resonance instability and the low-frequency RHP daughter waves are generated by the parent waves due to nonlinear parametric instability. Furthermore, enhanced downstream energetic proton fluxes are inferred to result from scattering of the upstream protons by the nascent turbulent fluctuations. Therefore, some critical clues about the newborn turbulence in the foreshock are provided in this work.