http://arxiv.org/abs/2305.12507
In this letter, we provide the first observational evidence of substantial collisionless damping (CD) modulation in the magnetohydrodynamic (MHD) turbulence cascade in Earth’s magnetosheath using four Cluster spacecraft. Plasma turbulence is primarily shaped by the forcing on large scales and damping on small scales. Based on an improved compressible MHD decomposition algorithm, our observations demonstrate that CD enhances the anisotropy of compressible MHD modes due to their strong pitch angle dependence. The wavenumber distributions of slow modes are more stretched perpendicular to the background magnetic field ($\mathbf{B_0}$) under CD modulation compared to Alfv\’en modes. In contrast, fast modes are subject to a more significant CD modulation. Fast modes exhibit a scale-independent, slight anisotropy above the CD truncation scales, and their anisotropy increases as the wavenumbers fall below the CD truncation scales. As a result, CD affects the relative energy fractions in total compressible modes. Our findings take a significant step forward in comprehending the functions of CD in truncating the compressible MHD turbulence cascade and the consequential energy anisotropy in the wavevector space.
S. Zhao, H. Yan, T. Liu, et. al.
Tue, 23 May 23
77/77
Comments: Main text: 5 pages, 4 figures. Submitted to PRL on May 11, 2023