We perform a statistical study of flux ropes and reconnection fronts based on MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) magnetic field and plasma observations to study the implications for the spatial distribution of reconnection sites in Mercury's near magnetotail. The results show important differences of temporal and spatial distributions as compared to Earth. We have surveyed the plasma sheet crossings between −2 RM and −3 RM downtail from the planet, i.e., the location of Near‐Mercury Neutral Line (NMNL). Plasma sheets were defined to be regions with β ≥ 0.5. Using this definition, 39 flux ropes and 86 reconnection fronts were identified in the plasma sheet. At Mercury, the distributions of flux ropes and reconnection fronts show clear dawn‐dusk asymmetry with much higher occurrence rate on the dawnside plasma sheet than on the duskside. This suggests that magnetic reconnection in Mercury's magnetotail occurs more frequently in the dawnside than in the duskside plasma sheet, which is different than the observations in Earth's magnetotail showing more reconnection signatures in the duskside plasma sheet. The distribution of plasma sheet thickness shows that plasma sheet near the midnight is the thinnest part and does not show obvious asymmetry. Thus, the reasons that cause magnetic reconnection to preferentially occur on the dawnside of the magnetotail at Mercury may not be the plasma sheet thickness and require further study. The peak occurrence rates of flux ropes and reconnection fronts in Mercury's plasma sheet are ~ 60 times higher than that of Earth's values, which we interpret to be due to the highly variable magnetospheric conditions at Mercury. Such higher occurrence rate of magnetic reconnection would generate more plasma flows in the dawnside plasma sheet than in the duskside. These plasma flows would mostly brake and initiate the substorm dipolarization on the postmidnight sector at Mercury rather than the premidnight susbtorm onset location at Earth.

Magnetic holes (MHs), with a scale much greater than ρi (proton gyroradius), have been widely reported in various regions of space plasmas. On the other hand, kinetic‐size magnetic holes (KSMHs), previously called small‐size magnetic holes, with a scale of the order of magnitude of or less than ρi have only been reported in the Earth's magnetospheric plasma sheet. In this study, we report such KSMHs in the magnetosheath whereby we use measurements from the Magnetospheric Multiscale mission, which provides three‐dimensional (3‐D) particle distribution measurements with a resolution much higher than previous missions. The MHs have been observed in a scale of 10–20 ρe (electron gyroradii) and lasted 0.1–0.3 s. Distinctive electron dynamics features are observed, while no substantial deviations in ion data are seen. It is found that at the 90° pitch angle, the flux of electrons with energy 34–66 eV decreased, while for electrons of energy 109–1024 eV increased inside the MHs. We also find the electron flow vortex perpendicular to the magnetic field, a feature self‐consistent with the magnetic depression. Moreover, the calculated current density is mainly contributed by the electron diamagnetic drift, and the electron vortex flow is the diamagnetic drift flow. The electron magnetohydrodynamics soliton is considered as a possible generation mechanism for the KSMHs with the scale size of 10–20 ρe.

Magnetic cavities (sometimes referred to as magnetic holes) at electron kinetic scale are thought to be one of the extremely small intermittent structures formed in magnetized turbulent plasmas, where the turbulence energy cascaded down to electron scale may finally be dissipated and consequently energize the electrons. However, the geometry and formation of these structures remain not definitively resolved. Here we discuss an electron scale magnetic cavity embedded in a proton scale magnetic cavity observed by the MMS spacecraft in the magnetosheath. By applying an innovative particle sounding technique, we directly depict the boundary of the electron scale magnetic cavity and uncover the geometry. We find that this structure is nearly circular with a radius of 10.0 km and its formation is due to the diamagnetic current. Investigation of the electron scale structure is only recently made possible by the high spatial and temporal resolution provided by MMS observations.

Small‐scale magnetic holes (SSMHs) in the magnetosphere plasma sheet are investigated in this paper. A developed electron magnetohydrodynamics (EMHD) soliton model is proposed as a new approach to SSMHs formation. The Biermann battery effect is taken into account in resolving the magnetic evolution equation with a slow‐mode solution in the weak nonlinear regime. Statistical investigation of SSMH observation data in the plasma sheet by Cluster is carried out in comparison with the theory. We apply multispacecraft data for distinguishing sheet‐like or cylindrical SSMHs observed and clarified by the solitary wave in the EMHD model. Furthermore, the major parameters, such as amplitude, width, maximum magnetic field perturbation, and perpendicular temperature variation of the SSMHs, are found consistent with the theoretical analysis.

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Mirror‐mode structures are widely observed in space plasma environments. Although plasma features within the structures have been extensively investigated in theoretical models and numerical simulations, relatively few observational studies have been made, due to a lack of high‐cadence measurements of particle distributions in previous space missions. In this work, electron dynamics associated with mirror‐mode structures are studied based on Magnetospheric Multiscale observations of electron pitch angle distributions. We define mirror‐mode peaks/troughs as the region where the magnetic field strength is greater/smaller than the mean field. The observations show that most electrons are trapped inside the mirror‐mode troughs and display a donut‐like pitch angle distribution configuration. Besides the trapped electrons in mirror‐mode troughs, we find that electrons are also trapped between ambient mirror‐mode peaks and coexisting untrapped electrons within the mirror‐mode structure. Analysis shows that the observed donut‐like electron distributions are the result of betatron cooling and the spatial dependence of electron pitch angles within the structure.

In this paper, we consider the global robust output regulation problem for a class of uncertain nonlinear systems with nonlinear exosystems. By employing the internal model approach, we show that this problem boils down to a global robust stabilization problem of a time-varying nonlinear system in lower triangular form, the solution of which will lead to the solution of the global robust output regulation problem. An example shows the effectiveness of the proposed approach.nonlinear systems, adaptive control, output regulation

We investigate the plasma sheet pressure variations in the near‐Earth magnetotail (radius distance, R, from 7.5 RE to 12 RE and magnetic local time, MLT, from 18:00 to 06:00) during substorm growth phase with Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations. It is found that, during the substorm growth phase, about 39.4% (76/193) of the selected events display a phenomenon of equatorial plasma pressure (Peq) decrease. The occurrence rates of Peq decrease cases are higher in the dawn (04:00 to 06:00) and dusk (18:00 to 20:00) flanks (> 50%) than in the midnight region (20:00 to 04:00, < 40%). The mean values of the maximum percentages of Peq decrease during the substorm growth phases are larger in the dawn and dusk flanks (~ −20%) than in the midnight region (~ > −16%). The mean value of Peq increase percentages at the end of substorm growth phase is the highest (~ 40%) in the premidnight MLT bin (22:00 to 00:00) and is almost unchanged in the dawn and dusk flanks. Further investigations show that 13.0% of the events have more than 10% of Peq decrease at the end of substorm growth phase comparing to the value before the growth phase, and ~ 28.0% of the events have small changes (< 10%), and ~ 59.0% events have a more than 10% increase. This study also reveals the importance of electron pressure (Pe) in the variation of Peq in the substorm growth phase. The Pe variations often account for more than 50% of the Peq changes, and the ratios of Pe to ion pressure often display large variations (~ 50%). Among the investigated events, during the growth phase, an enhanced equatorial plasma convection flow is observed, which diverges in the midnight tail region and propagates azimuthally toward the dayside magnetosphere with velocity of ~ 20 km/s. It is proposed that the Peq decreases in the near‐Earth plasma sheet during the substorm growth phase may be due to the transport of closed magnetic flux toward the dayside magnetosphere driven by dayside magnetopause reconnection. Both solar wind and ionospheric conductivity effects may influence the distributions of occurrence rates for Peq decrease events and the Peq increase percentages in the investigated region.