Chorus waves are some of the strongest electromagnetic emissions naturally occurring in space and can cause radiation that is hazardous to humans and satellites1-3. Although chorus waves have attracted extreme interest and been intensively studied for decades4-7, their generation and evolution remain highly debated7. Here, in contrast to the conventional expectation that chorus waves are governed by planetary magnetic dipolar fields5,7, we report observations of repetitive, rising-tone chorus waves in the terrestrial neutral sheet, where the effects of the magnetic dipole are absent. Using high-cadence data from NASA's MMS mission, we present ultrafast measurements of the wave fields and three-dimensional electron distributions within the waves, which provides evidence for chorus-electron interactions and the development of electron holes in the wave phase space. We found that the waves are associated with resonant currents antiparallel to the wave magnetic field, as predicted by nonlinear wave theory. We estimated the nonlinear field-particle energy transfer inside the waves, finding that the waves extract energy from local thermal electrons, in line with the positive growth rate of the waves derived from an instability analysis. Our observations may help to resolve long-standing controversies regarding chorus emissions and in gaining an understanding of the energy transport observed in space and astrophysical environments.

High-speed electron flows (HSEFs) play a crucial role in the energy dissipation and conversion processes within the terrestrial magnetosphere and can drive various types of plasma waves and instabilities, affecting the electron-scale dynamics. The existence, spatial distribution, and general properties of HSEFs in the Earth magnetotail are still unknown. In this study, we conduct a comprehensive survey of HSEFs in the Earth magnetotail, utilizing NASA's Magnetospheric Multiscale (MMS) mission observations from 2017 to 2021. A total of 642 events characterized by electron bulk speeds exceeding 5,000 km/s are identified. The main statistical properties are: (a) The duration of almost all HSEFs are less than 4 s, and the average duration is 0.74 s. (b) HSEFs exhibit a strong dawn-dusk (30%–70%) asymmetry. (c) 39.6%, 29.0%, and 31.4% of the events are located in the plasma sheet, plasma sheet boundary layer (PSBL), and lobe region, respectively. (d) In the plasma sheet, HSEFs have arbitrary moving directions regarding the ambient magnetic field, and the events near the neutral line predominantly move along the same direction as the ion outflows, indicating outflow electrons generated by magnetic reconnection. (e) HSEFs in the PSBL and lobe mainly move along the ambient magnetic field, and 70% of HSEFs in the PSBL exhibit features of reconnection inflow. The HSEFs in lobe regions may locate near the reconnection electron edges. Our study reveals that the HSEFs in magnetotail are closely associated with magnetic reconnection, and the statistical results deepen the understanding of HSEF fundamental properties in collisionless plasma.

Turbulent energy dissipation is a fundamental process in plasma physics that has not been settled. It is generally believed that the turbulent energy is dissipated at electron scales leading to electron energization in magnetized plasmas. Here, we propose a micro accelerator which could transform electrons from isotropic distribution to trapped, and then to stream (Strahl) distribution. From the MMS observations of an electron-scale coherent structure in the dayside magnetosheath, we identify an electron flux enhancement region in this structure collocated with an increase of magnetic field strength, which is also closely associated with a non-zero parallel electric field. We propose a trapping model considering a field-aligned electric potential together with the mirror force. The results are consistent with the observed electron fluxes from ~50 eV to ~200 eV. It further demonstrates that bidirectional electron jets can be formed by the hourglass-like magnetic configuration of the structure.

Detailed analysis of a high Mach number quasiperpendicular Earth bow shock crossing by the Magnetospheric Multiscale (MMS) spacecraft fleet reveal that lower-hybrid (LH) whistler waves generated in the shock foot region transport energy predominately along the shock surface and slightly toward the shock ramp in the shock normal incidence frame, where wave energy accumulates and is dissipated into the plasma. This suggests the LH whistlers play an integral role in energy reconfiguration at high Mach number collisionless shocks with ramifications to plasma heating. The multipoint observations are used to quantify the wave characteristic parameters (via interferometry), Poynting fluxes, and energy conversion rates D, and to assess their scale dependencies and spatial and temporal properties. The whistler associated energy transport and conversion are found to depend on scale and location within the layer. High-frequency electrostatic waves yield largest values of D. However, the dominant net energy exchange contribution is from the LH whistlers. In the foot spatially temporally coherent net energy exchange from the plasma to whistlers is observed, whereas deeper in the ramp net wave energy dissipation to the plasma is observed exhibiting significant space-time variability. These results are consistent with the modified two stream instability driven by the relative drift between reflected ions and electrons as the mechanism for wave growth in the foot. Owing to strong electron heating, whistler energy dissipation in the ramp is attributed to Landau damping, which out-competes the destabilizing effect of the reflected ion and electron drift.

Dayside magnetosphere interactions are essential for energy and momentum transport between the solar wind and the magnetosphere. In this study, we investigate a new phenomenon within this regime. Sudden enhancements of ion fluxes followed by repeating dropouts and recoveries were observed by Magnetospheric Multiscale on 5 November 2016, which is the very end of the recovery phase from a moderate geomagnetic storm. These repetitive flux variations display energy-dispersive characteristics with periods relevant to ion bounce motion, suggesting they are corresponding echoes. Alongside the flux variations, bipolar electric field impulses originating from external sources were detected. We traced the source region of the initial injection and found it is located near the spacecraft's position. To elucidate the underlying physics, a test-particle simulation is conducted. The results reveal that radial transport resulting from impulse-induced acceleration can give rise to these echoes. Observations demonstrate dayside magnetosphere interactions are more common than we previously considered, which warrants further research.

The four spacecraft of the NASA Magnetospheric Multiscale (MMS) mission carry instruments to reduce the positive potential by means of indium ion beams. Since the start of the nominal mission in September 2015 and until the end of 2021, the instruments active spacecraft potential control (ASPOC) have been actively operating for more than 16 000 h at a nominal emission current of $20 \mu \text{A}$ per spacecraft. Based on data from more than six years in orbit with more than 50 000 h in regions of scientific interest, statistical results regarding the potential's interdependencies with ambient plasma were obtained. This article reports on the derivation of the photo electron energy spectrum from the correlation between the potential and the plasma data obtained by the fast plasma instrument with and without controlled potential. Finally, the time constants during the relaxation of the controlled potential when the active control instrument is turned off, if measured at high time resolution, allow to estimate the electric capacitance of the spacecraft system.

Pulsating auroras are usually observed with ultralow-frequency (ULF) waves in the Pc 3-5 band (period 10-600 s). These auroras are thought to result from interactions between energetic electrons and chorus waves, but their relationship with ULF waves remains an open question. In this study, we investigated this question by conducting a comparative study on two ULF wave events with pulsating auroras observed near the magnetic footprints. Conjugate observations from the Magnetospheric Multiscale mission and the Chinese Yellow River Station were used. In both events, lower-band chorus waves were observed, which were suggested to be connected with the auroral pulsations by wavelet analysis. The intensity of these waves oscillates at the period of the ULF waves, but the physics laid behind them differs by events. During the event of 22 January 2019, compressional ULF waves changed the threshold for the whistler anisotropy instability periodically, affecting the emission of chorus waves. In the event on 10 January 2016, poloidal ULF waves modulated the chorus wave generation by regulating electron temperature anisotropy through drift resonance. ULF waves in these events may originate from perturbations in the solar wind. We highlight the role of ULF waves in the solar wind-magnetosphere-ionosphere coupling, which requires further study.

In situ measurements from the Magnetospheric Multiscale (MMS) mission are used to estimate electron density from spacecraft potential and investigate compressive turbulence in the Earth's magnetosheath. During the MMS Solar Wind Turbulence Campaign in February 2019, the four MMS spacecraft were arranged in a logarithmic line constellation enabling the study of measurements from multiple spacecraft at varying distances. We estimate the electron density from spacecraft potential for a time interval in which the ion emitters actively control the potential. The derived electron density data product has a higher temporal resolution than the plasma instruments, enabling the examination of fluctuation for scales down to the sub-ion range. The inter-spacecraft separations range from 132 to 916 km; this corresponds to scales of 3.5-24.1 ion inertial lengths. As an example, the derived density and magnetic field data are used to study fluctuations in the magnetosheath through time lags on a single spacecraft and spatial lags between pairs of spacecraft over almost one decade in scale. The results show an increase in anisotropy as the scale decreases, similar for the density and the magnetic field. This suggests different drivers in the strongly compressive magnetosheath and the weakly compressive solar wind. Compressive structures such as magnetic holes, compressive vortices and jets might play key roles.

Turbulence is ubiquitous within space plasmas, where it is associated with numerous nonlinear interactions. Magnetospheric Multiscale (MMS) provides the unique opportunity to decompose the electric field (E) dynamics into contributions from different linear and nonlinear processes via direct measurements of the terms in generalized Ohm's law. Using high-resolution multipoint measurements, we compute the magnetohydrodynamic ( E MHD ), Hall ( E Hall ), electron pressure ( E P e ), and electron inertia ( E inertia ) terms for 60 turbulent magnetosheath intervals, to uncover the varying contributions to the dynamics as a function of scale for different plasma conditions. We identify key spectral characteristics of the Ohm's law terms: the Hall scale, k Hall , where E Hall becomes dominant over E MHD ; the relative amplitude of E P e to E Hall , which is constant in the sub-ion range; and the relative scaling of the nonlinear and linear components of E MHD and of E Hall , which are independent of scale. We find expressions for the characteristics as a function of plasma conditions. The underlying relationship between turbulent fluctuation amplitudes and ambient plasma conditions is discussed. Depending on the interval, we observe that E MHD and E Hall can be dominated by either nonlinear or linear dynamics. We find that E P e is dominated by its linear contributions, with a tendency for electron temperature fluctuations to dominate at small scales. The findings are not consistent with existing linear kinetic Alfvén wave theory for isothermal fluctuations. Our work shows how contributions to turbulent dynamics change in different plasma conditions, which may provide insight into other turbulent plasma environments.

We carried out a statistical study of equatorial dipolarization fronts (DFs) detected by the Magnetospheric Multiscale mission during the full 2017 Earth's magnetotail season. We found that two DF classes are distinguished: class I (74.4%) corresponds to the standard DF properties and energy dissipation and a new class II (25.6%). This new class includes the six DF discussed in Alqeeq et al. (2022, ) and corresponds to a bump of the magnetic field associated with a minimum in the ion and electron pressures and a reversal of the energy conversion process. The possible origin of this second class is discussed. Both DF classes show that the energy conversion process in the spacecraft frame is driven by the diamagnetic current dominated by the ion pressure gradient. In the fluid frame, it is driven by the electron pressure gradient. In addition, we have shown that the energy conversion processes are not homogeneous at the electron scale mostly due to the variations of the electric fields for both DF classes.