Crossing the Earth’s bow shock is known to crucially affect solar wind plasma including changes in turbulent cascade. The present review summarizes results of more than 15 years of experimental exploration into magnetosheath turbulence. Great contributions to understanding turbulence development inside the magnetosheath was made by means of recent multi-spacecraft missions. We introduce the main results provided by them together with first observations of the turbulent cascade based on direct plasma measurements by the Spektr-R spacecraft in the magnetosheath. Recent results on solar wind effects on turbulence in the magnetosheath are also discussed.

Variations in the solar wind (SW) parameters with scales of several years are an important characteristic of solar activity and the basis for a long‐term space weather forecast. We examine the behavior of interplanetary parameters over 21–24 solar cycles (SCs) on the basis of the OMNI database (https://spdf.gsfc.nasa.gov/pub/data/omni). Since changes in the parameters can be associated with both changes in the number of different large‐scale types of SW and with variations in the values of these parameters at different phases of the solar cycle and during the transition from one cycle to another, we select the entire study period in accordance with the Catalog of large‐scale SW types for 1976–2019 (see the site http://www.iki.rssi.ru/pub/omni, [Yermolaev, Nikolaeva, et al., 2009, https://doi.org/10.1134/s0010952509020014], which covers the period from 21 to 24 SCs) and in accordance with the phases of the cycles, and average the parameters at selected intervals. In addition to a sharp drop in the number of interplanetary coronal mass ejections and associated sheath types, there is a noticeable drop in the value (by 20%–40%) of plasma parameters and magnetic field in different types of solar wind at the end of the 20th century and a continuation of the fall or persistence at a low level in the 23–24 cycles. Such a drop in the solar wind is apparently associated with a decrease in solar activity and manifests itself in a noticeable decrease in space weather factors.

Properties of kinetic‐scale plasma turbulence are examined observationally with the help of Spektr‐R measurements in the Earth's magnetosheath. The statistics covers 6 years of ion flux measurements during years 2011–2017 and includes a few dozens of the magnetosheath crossings with total duration of 250 hr. Several types of Fourier spectra are distinguished in the magnetosheath according to a spectral shape at the scales of transition from magnetohydrodynamic to kinetic regimes. Spectral indices and occurrence rate of different spectral shapes are examined at different locations with respect to the magnetosheath boundaries—the magnetopause and the bow shock. Magnetosheath plasma behind the quasi‐parallel and quasi‐perpendicular bow shock is explored separately. Clear dynamics of spectral shapes across the magnetosheath is shown: spectra with broad bump at the transition frequencies occur more frequently in the vicinity of the bow shock, while spectra with plateau at the same frequencies are usually observed closer to the magnetopause. Spectral indices such as spectral slope at the kinetic scales and break frequency are shown to be weakly affected by the boundaries behind the quasi‐perpendicular bow shock. The steepest spectra are observed close to the quasi‐parallel bow shock.

The magnetosheath is a natural laboratory for the study of plasma turbulence. The magnetopause and the bow shock prevent freely development of turbulence and modify turbulent cascade. In this...

We present a statistical study of the magnetosheath plasma fluctuation spectra at a high-frequency range (with frequencies from 0.01 to 10 Hz). Variations of ion flux value and its direction are considered. The direction of ion flux is characterized by a polar angle – the deviation of the ion flux vector from the Sun–Earth line. We consider 290 Fourier's spectra that can be described by two power laws with a break, i.e., a change of slope. The ion flux fluctuation spectra are shown to have breaks at higher frequencies compared to the polar angle spectra. We compare the frequency of the break with the gyrostructure frequency for a number of cases. We show the polar angle break frequency to usually be smaller than the gyrostructure frequency. The dependencies of spectrum parameters such as the slopes and the break frequency on plasma parameters are also considered.

This work is a continuation of our previous articles (Yermolaev et al., 2015, https://doi.org/10.1002/2015JA021274; 2017, https://doi.org/10.1007/s11207-017-1205-1; 2018, https://doi.org/10.1007/s11207-018-1310-9), which describe the average temporal profiles of interplanetary plasma and field parameters in large‐scale solar‐wind (SW) streams: corotating interaction regions (CIRs), interplanetary coronal mass ejections (ICMEs including both magnetic clouds (MCs), and ejecta), and sheaths as well as interplanetary shocks (ISs). In this work, we analyze the average profile of helium abundance, N_α/N_p, for 1976–2016. Our results confirm the main results that were obtained in earlier studies: N_α/N_p is higher in quasi‐stationary fast streams than in slow ones; it slowly changes in compression regions CIRs and sheaths from values in undisturbed SW to values in the corresponding fast stream type pushing like a piston, in high‐speed stream (HSS) flow for CIRs, or in ICME for sheaths; in ejecta, it is close to the abundance observed in undisturbed streams, and it is maximal in MCs. For the first time, the results show that N_α/N_p correlates with the proton β‐parameter in compression regions CIRs and sheath and anticorrelates in ICMEs. The N_α/N_p versus β dependence is stronger in MCs than in ejecta and may be used as an indicator of conditions at the place on the Sun where CMEs are formed.