No abstract

We investigate the decay of magnetic and kinetic energies behind IP shocks with motivation to find a relaxation time when downstream turbulence reaches a usual solar wind value. We start with a case study that introduces computation techniques and quantifies a contribution of kinetic fluctuations to the general energy balance. This part of the study is based on high-time (31 ms) resolution plasma data provided by the Spektr-R spacecraft. On the other hand, a statistical part is based on 92 s Wind plasma and magnetic data and its results confirm theoretically established decay laws for kinetic and magnetic energies. We observe the power-law behavior of the energy decay profiles and we estimated the power-law exponents of both kinetic and magnetic energy decay rates as −1.2. We found that the decay of MHD turbulence does not start immediately after the IP shock ramp and we suggest that the proper decay of turbulence begins when a contribution of the kinetic processes becomes negligible. We support this suggestion with a detailed analysis of the decay of turbulence at the kinetic scale.

Interplanetary (IP) shocks as typical large-scale disturbances arising fromprocesses such asstream-stream interactions or Interplanetary Coronal Mass Ejection (ICME) launching play a significant role in the energy redistribution, dissipation, particle heating, acceleration, etc. They can change the properties of the turbulent cascade on shorter scales. We focus on changes of the level and spectral properties of ion flux fluctuations upstream and downstream of fastforward oblique shocks. Although the fluctuation level increases by an order of magnitude across the shock, the spectral slope in the magnetohydrodynamic range is conserved. The frequency spectra upstream of IP shocks are the same as those in the solar wind (if not spoiled by foreshock waves). The spectral slopes downstream are roughly proportional to the corresponding slopes upstream, suggesting that the properties of the turbulent cascade are conserved across the shock;thus, the shock does not destroy the shape of the spectrum as turbulence passes through it. Frequency spectra downstream of IP shocks often exhibit "an exponential decay" in the ion kinetic range that was earlier reported at electron scales in the solar wind or at ion scales in the interstellar medium. We suggest that the exponential shape of ion flux spectra in this range is caused by stronger damping of the fluctuations in the downstream region.

Wave packets are frequently observed upstream and/or downstream of shocks in a magnetized plasma. We present a comparison of Wind and Spektr‐R observations of 27 interplanetary low‐Mach number (<5.5) shocks that reveals that (1) the wavelengths of both upstream and downstream waves conserve over the spacecraft separation, (2) in the frequency range of 0.5–5 Hz, their wavelengths are directly proportional to the shock ramp thickness that is controlled by the ion thermal gyroradius, and (3) the phase shift between density and temperature variations within downstream wave packets is about 90°. These results emphasize a role of kinetic processes in the formation of low‐Mach number shocks.

Kinetic-scale fluctuations in magnetized collisionless plasmas, such as a solar wind, attract attention owing to their vital role in the dynamics of the dissipation of free energy to random particle motion. As the free energy cascades in the inertial range of turbulence, fluctuations at ion characteristic scales become more compressible. Measurements show that these fluctuations possess highly oblique propagation angles with respect to the background magnetic field and follow theoretical predictions for kinetic Alfvén waves (KAWs). We performed alarge (465 cases) statistical study of normalized fluctuations of the density, bulk velocity, and magnetic field around ion gyroscale and concentrated on (i) their compressibility, (ii) the ratio of density and magnetic field fluctuations, and (iii) the ratio of density and velocity fluctuations. We find that observed fluctuations follow the two-fluid prediction for KAWs generally, but the spread of measured values around their theoretical predictions is large. The analysis of measurement uncertainties shows that the difference between the observed and predicted levels of fluctuations cannot be fully explained by these uncertainties and that the nature of solar wind fluctuations is more complex.