Shock ripples and shock–discontinuity interactions (SDIs) have long been proposed to explain the frequent occurrence of high‐speed jets (HSJs) in the magnetosheath; however, there is no direct observational evidence for either of them occurring near a shock. Herein, we report a large‐scale, long‐duration undulated quasi‐perpendicular shock surface that is capable of generating HSJs. Based on the curvatures estimated for the shock undulation and for a nearby hot flow anomaly (HFA), we suggest that the shock ripple mechanism and the SDI mechanism combine to generate HSJs, that is, during an SDI, shock undulations and an upstream HFA form simultaneously, and the solar wind between them is deflected by the undulation into jets. The HSJs, discontinuity, and HFA are then convected downstream. An HSJ consistent with our mechanism in another event is presented.

Geometrical invariants of magnetic field gradient tensors are used to classify the topological structures of a magnetic field. This study presents a statistical analysis on the geometrical invariants of magnetic field gradient based on high-quality data measured by magnetospheric multiscale mission in turbulent magnetosheath. The method for the classification of velocity field topologies cannot be applied to magnetic field with strong intensity directly because the magnetic field cannot be transformed to zero by selecting a co-moving reference frame in which the velocity is zero. During a strong magnetic field, flux ropes and tubes are the most possible magnetic structures. Statistics in the plane formed by geometrical invariants show that about 23% are force-free structures comprised of 20.5% flux tubes and 79.5% flux ropes. The remaining actively evolved structures are comprised of 30% flux tubes and 70% flux ropes. Moreover, the conditional average of current density and Lorentz force decomposition in geometrical invariants plane are investigated. The results show that flux ropes carried more current density than flux tubes for the same geometrical invariants, and flux ropes tend to associate with magnetic pressure force and flux tubes tend to associate with magnetic tension.

Predicting SYM‐H index is significant in space weather because it quantifies the degree of perturbation of the geomagnetic field during storm. This study presents a composite model to predict SYM‐H index based on solar wind parameters by combining the empirical magnetospheric dynamical equation with neural networks. The formula for predicted SYM‐H originates from the well‐known empirical relationship between interplanetary conditions and the Dst index. In particular, the coefficients in the empirical relationship are determined by using neural networks that excel at approaching the function linking the coefficients and the solar wind parameters. The 1‐ and 2‐hr forecasts of SYM‐H during storm time are reliable, and the precision of some cases is even better than the latest models solely using deep neural networks. Based on the composite model, the dependence of loss time and injection rates of ring current energy on the solar wind parameters and SYM‐H are investigated.

The drag reduction efficacy of a large-scale flow control over a rough surface is studied via direct numerical simulations of turbulent channels (at friction Reynolds numbers Reτ=180) by combining together wall riblets and streamwise counter-rotating swirls. In particular, the height of triangular riblets is h+≈10 (+indicating wall units), while the number of riblets (NRib in the range 1–56) along the periodic spanwise direction is varied to find the optimum. The swirls are generated by the spanwise opposed wall-jet forcing (SOJF) in the Navier–Stokes equation, whose controlling parameters follow the optimal ones as for the smooth wall. In total, 12 cases of combined SOJF and riblets are performed to investigate the coupling effects between the two methods. We find a range of NRib=7–14 (with the spanwise width z+≈140−280) yields the largest drag reduction (up to 20%) for Reτ=180, much higher than riblets control only (about 3%). Compared to SOJF control only, riblets suppress the secondary swirls of SOJF hence decreasing drag, while the lateral and down washing motions of SOJF impinging on riblets would increase drag—the opposite two effects thus giving rise to an optimal. Through examinations on coherent structures, we elucidate that the attenuation of both large-scale coherent motions and small-scale random fluctuations leads to the net drag reduction. We conclude that large-scale control is a robust approach in the cases of rough surfaces, and the parameters can be selected for maximum drag reduction in each particular situation.

This study presents statistical features of magnetic field curvature in the magnetosheath region. Two sets of high-quality field and plasma data measured by the Magnetospheric Multiscale mission are analyzed by the multiple-point analysis method. The results include the following: (a) The probability distribution function (PDF) of the curvature exhibits two different power laws consistent with previous studies; the PDF of small curvatures depends on the plasma condition and the PDF of large curvatures shows better agreement. (b) The data validate the derived relation between the current density and the guiding center current as well as the diamagnetic current. (c) The acceleration due to curvature drifts in the perpendicular direction occurs when κ/κ _rms is larger than 1, which is a potential mechanism for anisotropic distribution of plasma pressure at large curvatures.

We studied the thermal features of magnetized plasmas in the Martian subsolar magnetosheath using MAVEN’s observations from 2014 to 2019. Statistical analyses show that the average ion and electron temperature in the Martian subsolar magnetosheath are 210 and 31 eV, respectively, which are significantly lower than their counterparts in the subsolar magnetosheaths of Earth and Saturn which both have an inherent magnetosphere. However, the ratio ${\overline{T}}_i/{\overline{T}}_e$ in the Martian subsolar magnetosheath is about 6.8, which is very close to that of Earth and Saturn. We further investigated the relationship between $T_i/T_e$ and the bulk ion flow velocity $V_i$, as well as the relationship between the total plasma beta $\beta$ and $V_i$. Results show that the average value of $T_i/T_e$ when $V_i<300\mathrm{km}/\mathrm{s}$ is considerably higher than when $V_i>300\mathrm{km}/\mathrm{s}$. A value of $V_i$ closer to 250–300 km/s leads to a higher average value of the total plasma beta $\beta$. These results confirm the prediction of previous researchers, that there is not enough room for solar wind thermalization as the distance between the Martian bow shock and the so-called obstacle is of the order of a solar wind proton gyroradius.