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In the solar wind, STEREO occasionally observes mirror‐mode storms, periods in which small amplitude waves suddenly appear and persist for hours. Two triggers of these storms are high‐plasma beta and weak shocks, both consistent with conditions for the growth of mirror‐mode waves. The appearance of these waves is quite distinct from the isolated mirror‐mode structure that is frequently seen in the solar wind. They have not been reported previously. Here we show three examples of their occurrence.

With unique simultaneous observations both in the magnetosheath and magnetosphere by the THEMIS probes, Alfvénic variations in the plasma velocity are observed at the inner edge of low‐latitude boundary layer (LLBL) and are induced by the mirror mode waves in the magnetosheath near the subsolar magnetopause on 31 July 2007. These Alfvénic variations appeared as the wavy perturbations in the Vx and Vy components observed by THEMIS C, D, and E, which had the same periodicity as associated magnetic field variations. Simultaneously, THEMIS B observed the mirror mode waves in the magnetosheath. The periodicities of the magnetic and plasma pressure variations of mirror modes in the magnetosheath were consistent with those of the Alfvénic wavy variations in the LLBL. Therefore, the mirror mode waves can induce the magnetopause undulations, launching Alfvén waves, and resultant Alfvénic variations are observed in the LLBL. Also, in the succeeding magnetosheath interval by THEMIS B, we examined whether the mirror mode waves occurred and associated Alfvénic variations were observed in the LLBL. However, no clear evidence for an existence of the mirror mode waves was obtained, and THEMIS C, D, and E do not also observe associated magnetic field and plasma Alfvénic responses in the LLBL. These results suggest that the Alfvénic variations in the LLBL are strongly related to the mirror mode waves in the magnetosheath. On the basis of these results, we emphasize that the magnetosheath energy is transmitted and transported into the magnetosphere via magnetopause surface waves.

Dayside transients, such as hot flow anomalies, foreshock bubbles, magnetosheath jets, flux transfer events, and surface waves, are frequently observed upstream from the bow shock, in the magnetosheath, and at the magnetopause. They play a significant role in the solar wind-magnetosphere-ionosphere coupling. Foreshock transient phenomena, associated with variations in the solar wind dynamic pressure, deform the magnetopause, and in turn generates field-aligned currents (FACs) connected to the auroral ionosphere. Solar wind dynamic pressure variations and transient phenomena at the dayside magnetopause drive magnetospheric ultra low frequency (ULF) waves, which can play an important role in the dynamics of Earth’s radiation belts. These transient phenomena and their geoeffects have been investigated using coordinated in-situ spacecraft observations, spacecraft-borne imagers, ground-based observations, and numerical simulations. Cluster, THEMIS, Geotail, and MMS multi-mission observations allow us to track the motion and time evolution of transient phenomena at different spatial and temporal scales in detail, whereas ground-based experiments can observe the ionospheric projections of transient magnetopause phenomena such as waves on the magnetopause driven by hot flow anomalies or flux transfer events produced by bursty reconnection across their full longitudinal and latitudinal extent. Magnetohydrodynamics (MHD), hybrid, and particle-in-cell (PIC) simulations are powerful tools to simulate the dayside transient phenomena. This paper provides a comprehensive review of the present understanding of dayside transient phenomena at Earth and other planets, their geoeffects, and outstanding questions.

We present a nonlocal gyrokinetic theory for the drift‐mirror mode in high‐β$\\beta $anisotropic plasmas. Here, β$\\beta $represents the ratio of plasma pressure to magnetic pressure. The equilibrium distribution is established self‐consistently via guiding‐center Hamiltonian theory. To keep the nonuniformity and finite Larmor radius (FLR) effects on an equal footing, we derive the three‐field nonlocal eigenmode equations in Fourier space under the assumption of weak nonuniformity. It is found that the drift‐mirror mode is essentially a dissipative instability confined within the potential well due to the FLR effect. Numerical analyses demonstrate that the drift‐mirror mode originates from the coupling between shear Alfvén and mirror branch, where the ion‐sound wave component is negligible. Furthermore, the β$\\beta $threshold of this nonlocal drift‐mirror mode is significantly lower than that of the conventional drift‐mirror mode. Plain Language Summary The drift‐mirror instability has been intensively investigated in space physics. It can be destabilized by pressure anisotropy in magnetized plasmas and result in the formation of the cavity across the field lines where the mode is confined. In this study, we investigate the linear properties of the nonlocal drift‐mirror mode by employing the electromagnetic gyrokinetic theory. The nonuniform equilibrium is self‐consistently established, with both magnetic and plasma inhomogeneities taken into account on the equal footing. An eigenmode system describing the nonlocal drift‐mirror mode is then derived in Fourier space. It is found that the drift‐mirror mode arises from the coupling between the shear Alfvén branch and the mirror branch. The ion‐sound wave branch, however, is negligible. Additionally, further numerical analyses demonstrate that the kinetic drift‐mirror mode is more easily excited than the mirror mode in uniform and fluid limits. These results provide a deeper understanding of the drift‐mirror mode. Key Points By employing the nonlocal electromagnetic gyrokinetic theory, we present a self‐consistent description of the drift‐mirror mode It is showed that the drift‐mirror mode is a dissipative instability confined within the potential well arising from the FLR effect The drift‐mirror mode originates from the coupling between shear Alfvén and mirror branch, where the ion‐sound wave component is subdominant

Electromagnetic ion cyclotron (EMIC) waves and mirror modes (MMs), both driven by ion temperature anisotropy, are commonly observed in planetary magnetosheaths. Conventional explanations for their co‐occurrence are largely based on linear instability theory in proton–electron plasmas, which requires comparable growth rates for the EMIC and MM instabilities. Magnetosheath plasmas, however, contain a fraction of heavy ions, and how such composition affects the coexistence of EMIC waves and MMs has been less explored. Using kinetic hybrid simulations with typical magnetosheath parameters, we show that although the presence of heavy ions suppresses the initial linear EMIC instability, EMIC waves arise as MMs develop. The evolving MMs generate flat‐top proton velocity distributions with enhanced resonant populations, which in turn excite EMIC waves. These results extend the conventional coexistence scenario of MMs and EMIC waves and reveal a new pathway for energy transfer among MMs, EMIC waves, and particles in magnetosheath plasmas.

Magnetic holes at the ion-to-electron kinetic scale (KSMHs) are one of the extremely small intermittent structures generated in turbulent magnetized plasmas. In recent years, the explorations of KSMHs have made substantial strides, driven by the ultra-high-precision observational data gathered from the Magnetospheric Multiscale (MMS) mission. This review paper summarizes the up-to-date characteristics of the KSMHs observed in Earth’s turbulent magnetosheath, as well as their potential impacts on space plasma. This review starts by introducing the fundamental properties of the KSMHs, including observational features, particle behaviors, scales, geometries, and distributions in terrestrial space. Researchers have discovered that KSMHs display a quasi-circular electron vortex-like structure attributed to electron diamagnetic drift. These electrons exhibit noticeable non-gyrotropy and undergo acceleration. The occurrence rate of KSMH in the Earth’s magnetosheath is significantly greater than in the solar wind and magnetotail, suggesting the turbulent magnetosheath is a primary source region. Additionally, KSMHs have also been generated in turbulence simulations and successfully reproduced by the kinetic equilibrium models. Furthermore, KSMHs have demonstrated their ability to accelerate electrons by a novel non-adiabatic electron acceleration mechanism, serve as an additional avenue for energy dissipation during magnetic reconnection, and generate diverse wave phenomena, including whistler waves, electrostatic solitary waves, and electron cyclotron waves in space plasma. These results highlight the magnetic hole’s impact such as wave-particle interaction, energy cascade/dissipation, and particle acceleration/heating in space plasma. We end this paper by summarizing these discoveries, discussing the generation mechanism, similar structures, and observations in the Earth’s magnetotail and solar wind, and presenting a future extension perspective in this active field.

AbstractThe paper is devoted to propagation of time-harmonic mirror wave modes in hemitropic micropolar media. Dynamics equations of a hemitropic micropolar elastic solid are formulated in terms of pseudotensors formalism. Transformation formulae for translational and spinor displacements, differential pseudovector operators and constitutive pseudoscalars for the cases of space inversion and mirror reflection relative to a given plane are obtained. Simultaneous existence of the direct, inverse and mirror reflected wave modes of propagating plane waves is predicted. Plane wave propagating in hemitropic micropolar elastic media comprises the three types of wave modes: direct, inverse and mirror. Algorithm for transformation direct wave modes into inverse and mirror modes of spinor and translational displacements is proposed.

The five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft offer new possibilities to analyze ULF waves in the magnetosphere by means of multipoint measurements. During the coast phase, THEMIS observed many compressional oscillations with periods in the Pc5 range and longer. The observed events occur inside a well‐defined spatial domain in the outer equatorial duskside magnetosphere. We analyze these waves using the unique string‐of‐pearls configuration of the THEMIS constellation to evaluate their phase speed and propagation direction. We find that the waves are propagating sunward (westward) and radially outward, orthogonal to the mean magnetic field, with phase speeds around 30 km/s and higher in the spacecraft frame. In the plasma frame the propagation direction is still sunward, with lower speeds (up to 30 km/s for most events). The oscillations exhibit a strong anticorrelation between the magnetic field and the plasma density. On the basis of this, as well as their low propagation speed, orthogonal to the mean magnetic field propagation direction and almost parallel to the magnetic field maximum variance direction, we conclude that the most likely source of these waves is the drift mirror instability.

We report the observation of mirror mode structures by Cluster spacecraft at around X∼-16 RE in the Earth’s magnetotail. The wavelength of the mirror structure is larger than 7000 km, corresponding to tens of ion gyroradii. Features of the mirror structures are similar to those detected in the magnetosheath: the anti-correlation between the magnetic field strength and plasma density, zero phase velocity in the plasma rest frame and linear polarization. The structures were observed in a region bounded by two dipolarizations during a substorm intensification. Thus, the dipolarization process may provide a plasma condition facilitating the growth of the mirror mode structures. Another interesting feature is the electron dynamics within the mirror structures. Thermal electron energy flux has an enhancement at 0° and 180° pitch angles inside the magnetic dips of the first three mirror structures and an enhancement at 90° pitch angle inside the magnetic dip of the last structure. The different electron distribution inside the mirror structures might be a result of different evolution stages of the mirror wave. The last structure may be in the nonlinear stage of the mirror instability, whereas the three others with quasi-sinusoidal waveforms may be in the linear stage. In addition, we found that intense whistler waves were confined within the magnetic dips. We conjecture that whistler waves observed in the first three dips were generated in a remote region, then they were trapped in the mirror mode troughs and transported toward the spacecraft; while the whistler wave detected in the last dip was excited locally by the electron anisotropy instability.

Magnetic field measurements play a crucial role in deep space exploration, contributing significantly to our understanding of planetary habitability and the space plasma environment. Among the various instruments employed in space exploration missions, the fluxgate magnetometer stands out as a widely used tool. However, its zero offset undergoes gradual changes, necessitating regular in-flight calibration. This article comprehensively reviews in-flight calibration methods for spaceborne magnetometers in deep space exploration, leveraging physical phenomena inherent to the space environment. The methods for calculating the zero offset can be divided into two categories. The first group employs formulas, including the Belcher method, Hedgecock method, Davis-Smith method, and both one-dimensional and three-dimensional mirror mode methods. Notably, the Davis-Smith method emerges as the optimal choice among these approaches. The second group employs probability-based solutions, constituting the in-flight calibrati