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Sub-ion-scale magnetic holes (MHs) are ubiquitous structures in plasmas across a wide range of environments. Despite previous observational and modeling efforts, the three-dimensional (3D) electric field in MHs has yet to be adequately resolved. In this study, utilizing high-resolution measurements of an MH (∼0.08 ρ i × 0.14 ρ i ) from the Magnetospheric Multiscale mission in Earth's turbulent magnetosheath, we report this 3D electric field and unveil its roles and generation mechanism. A model is established to quantify the impacts of E ∥ on increasing the loss cone of trapped electrons. The electric field is attributed to electron convection and pressure gradient terms of generalized Ohm's law. The MH, primarily coupling to the electron, is accompanied by electron jets. These electron jets can be interpreted as different segments of an electron vortex. These electron jets combined with nonideal electric fields not only lead to strong energy conversion ( j · ( E + v e × B ) ∼ 40 nW m−3) from the electromagnetic field to electrons but also enable energy conversion between different electron motion directions. Our study significantly clarifies the physical image of kinetic-scale MHs. [ABSTRACT FROM AUTHOR]

Electromagnetic ion cyclotron (EMIC) waves are believed to play an important role in the dynamics of the inner magnetosphere, including the ring current, the radiation belts and potentially, the cold plasma. In this work, we investigate their occurrence in the magnetosphere and the geomagnetic and solar wind conditions which lead to their excitation. We use an automated detection algorithm of EMIC waves observed by Van Allen Probes over the entire mission duration between 2012 and 2019. Consistent with earlier studies, we find that the H+ band occurrence maximizes in the dayside magnetosphere during enhancements of solar wind dynamic pressure. Both the H+ and He+ band are also generated along the duskside magnetosphere during disturbed geomagnetic conditions. In addition, to H+ and He+ bands commonly surveyed, we investigate the occurrence of H+ waves above and below 0.5 H+ gyrofrequency, as well as wave occurrence in the N+ and O++ bands. Most H+ waves are observed in the band below 0.5fH+. We find several events in the N+ band, indicative of their very low occurrence. The O++ band is observed during disturbed geomagnetic conditions and high solar wind dynamic pressure at low L‐shells. Its radial localization coincides with the O++ torus. This study provides a comprehensive picture of EMIC wave distribution and insight into ion composition in the inner magnetosphere during variable geomagnetic conditions. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are a type of plasma wave that occurs in the Earth's magnetosphere, typically in the frequency range of 0.1–5 Hz. These waves are generated by the interaction of magnetospheric ions and guided along the ambient magnetic field lines. EMIC waves are thought to significantly influence the dynamics of the Earth's most energetic particle populations, such as the ring current and the outer radiation belt, by inducing particle precipitation into the atmosphere. They can appear in multiple spectral bands, which are crucial for determining the energy of resonant particle interactions and identifying the presence of minor ions like He++, O++, and N+ in the magnetosphere. This study analyzes the distribution of EMIC wave activity using several years of data from the Van Allen Probes spacecraft, offering a detailed overview of the EMIC wave environment and providing insights into ion composition under varying geomagnetic conditions, which is essential for global magnetospheric modeling. Key Points: Most H+ waves are observed in the band below 0.5fH+O++ band is observed during disturbed conditions, radially coincident with cold O++ ion density enhancements previously observed by DE 1N+ band occurrence is low, less than 0.01% [ABSTRACT FROM AUTHOR]

Two sounding rockets, designated TRICE‐2, were launched on 8 December 2018 into the northern cusp region. The two rockets were designated the high‐ and low‐flyers, respectively, and launched 2 min apart to investigate cusp structures, specifically their spatial or temporal nature. 2 hr prior to the cusp encounter by the TRICE‐2 rockets, the MMS satellites, located in the magnetopause boundary layer, observed switching ion beams under very similar IMF conditions as later observed by TRICE‐2. The observed ion beam switch in the boundary layer defined the location of the primary dayside X‐line. Both, TRICE‐2 and MMS, also observed the signatures of multiple X‐lines at the magnetopause, overlapping ion‐energy dispersions in the cusp and counterstreaming ion beams in the magnetopause boundary layer, respectively. In addition to the TRICE‐2 cusp observations, ionospheric convection patterns from the SuperDARN radar are used to explain the vastly different cusp ion signatures observed by the TRICE‐2 rockets. While the high‐flyer rocket progressed north through the center of the cusp, the low‐flyer rocket drifted off to the east and crossed into the dusk convection cell, traveling perpendicular to the ionospheric convection direction before reaching the poleward oriented section of the convection cell also observed by the high‐flyer counterpart. Key Points: TRICE‐2 cusp ion dispersions are explained using the different magnetic foot points of the rockets through the ionospheric convection cellsTRICE‐2 cusp crossing occurred 2 hr after an MMS magnetopause crossing during similar IMF conditionsOverlapping cusp ion energy dispersions result from multiple magnetopause reconnection locations in agreement with MMS observations [ABSTRACT FROM AUTHOR]

As the shocked solar wind flows past the flank magnetopause, surface waves can form and roll up into flow vortices. This is known as the Kelvin–Helmholtz Instability, which is thought to be an important mechanism whereby energy and momentum are transferred from the solar wind into the magnetosphere. One mechanism whereby this can occur is magnetic reconnection in the equatorial plane on compressed current sheets between vortices. In 2015, the NASA Magnetospheric Multiscale (MMS) mission observed this form of in‐plane reconnection during a KHI event in the post‐noon sector of the dayside magnetopause. We use data from MMS to investigate 12 KHI events at different positions along the magnetospheric flanks. For each event, we identify periodic compressed current sheets and perform Walén tests for each to identify reconnection jets. We then investigate the fraction of current sheets that exhibit reconnection signatures, and refer to it as the "event ratio." Results show that the event ratio decreases for events further down the magnetospheric flanks. Additionally, we investigate solar wind parameters during each event and find that the event ratio increases with an increasing northward component of the interplanetary magnetic field. Key Points: 12 Kelvin‐Helmholtz events observed by MMS are investigatedWe determine the fraction of current sheets where magnetic reconnection occursThe fraction of currents sheets with reconnection signatures correlates with northward interplanetary magnetic field strength [ABSTRACT FROM AUTHOR]

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, https://doi.org/10.1063/5.0069432) 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. Key Points: We reveal a new class of dipolarization front related to a bump of the magnetic field associated with a minimum in the ion and electron pressuresThe energy conversion process in the S/C frame is driven by the diamagnetic current dominated by the ion pressure gradientThe energy conversion processes are not homogeneous at the electron scale due to the variations of the electric fields [ABSTRACT FROM AUTHOR]

Using Magnetospheric Multiscale mission (MMS) orbits in the Earth's magnetotail from 2017 to 2020, plasma conditions and the 3D spatial structure of inner‐magnetotail plasma environments (with a focus on the plasma sheet (PS)) are studied with different approaches. Threshold conditions for distinguishing the PS, PS boundary layers, and lobes are derived from the statistical properties of background plasma parameters. Our results support previous studies that employed similar methods using Cluster data. However, stronger currents are observed in both the lobes and PS, likely due to the smaller spacecraft separation (≲70 km) that can resolve thin electron‐scale currents. Threshold conditions are used together with magnetic field and electric field measurements to image the spatial structure of the PS. Results are in good agreement with a global neutral sheet model based on solar wind conditions and magnetospheric configurations. Furthermore, the Earth's dipole tilts toward the Sun around June solstice, which warps the magnetotail as much as ∼2–4 RE in Z geocentric solar magnetospheric. This warping effect is relaxed toward September equinox. Consequently, as MMS travels through the magnetotail from dawn to dusk during this period, there is an apparent dawn‐dusk asymmetry in plasma conditions between June and September. Kink‐like flapping waves and interplanetary magnetic field twisting are other mesoscale processes attributed with a few RE of flaring near the flanks. These findings reveal important insights into the mesoscale structure and dynamics of the magnetotail. Plain Language Summary: Data from 4 years of observations by NASA's MMS mission are used to statistically identify distinctive regions within the Earth's magnetospheric tail. This study reveals insights into the spatial structure of this "magnetotail" and seasonal variations attributed with changes in the Earth's magnetic field configurations, particularly those of the orientation of the Earth's dipole. Our results agree with reported findings from ESA's Cluster mission. However, certain aspects unique to MMS lead to some improved measurements and features relating to MMS orbital design. The presented results are highly beneficial to future large statistical studies with MMS data. Key Points: Inner‐magnetotail environments are statistically identified with background plasma conditions and their global 3D structure is studiedWarping effects attributed to changes in the Earth's dipole tilt angle leads to an apparent dawn‐dusk asymmetry during the summer monthsWe utilize a large volume of Magnetospheric Multiscale mission data with partial plasma moments calculated from low‐energy plasma and energetic particle instruments [ABSTRACT FROM AUTHOR]

Large‐amplitude electric fields (>50 mV/m) typical to bursty bulk flow (BBF) braking regions of the Earth's magnetotail can accelerate energetic electrons and ions to many times their initial thermal energies. We follow up on the Usanova and Ergun (2022), https://doi.org/10.1029/2022JA030336, study of electron energization and examine wave and plasma observations from the THEMIS satellites over four tail seasons to investigate the transfer of BBF energy to ions by turbulent electric fields. The results show that the large‐amplitude electric fields are accompanied by an ion temperature increase of ∼50% when compared to times when the turbulence is not observed. Electric field turbulence is also associated with a roughly ten‐fold increase in temperature fluctuations and a five‐fold increase in variations of energetic ion fluxes. We discuss the contribution of this turbulent energy transfer process to the dynamics of energetic ions in the magnetosphere. Plain Language Summary: Bursty bulk flows are high‐speed ion flows propagating toward Earth from the reconnection sites. On their approach to Earth, they decelerate and divert, while generating a turbulent cascade through which their energy dissipates. We use data from NASA's THEMIS satellites to show that high‐amplitude turbulent electric fields are produced through this energy dissipation process, which, in turn, transfer energy to ions. Further, we discuss the contribution of this turbulent energy transfer to the energetic ion dynamics in the inner magnetosphere. Key Points: Large‐amplitude electric fields are linked to a 1,000% increase in ion temperature fluctuations and a 500% increase in ion flux variationsThe effect on ion temperature is smaller than on electron temperature, being 50% versus 300%The accelerated energetic ions may contribute to ring current and plasmasheet energization [ABSTRACT FROM AUTHOR]

Two types of filamentary currents (FCs) were observed inside a magnetic flux rope at the magnetopause by the Magnetospheric Multiscale mission. The first FC is identified as an electron vortex, while the other is a reconnecting current sheet. Stochastic electric fields were generated within the FCs, resulting in electron acceleration up to a few keV, similar to recent simulations of electron acceleration inside vortex, which is a second‐order Fermi acceleration. Furthermore, two FCs propagated at different speeds, causing compression in the region between them. Energetic electrons up to 200 keV were detected in the compressed region and displayed a double power‐law spectrum. Observations suggest that the electrons were mainly accelerated by betatron mechanism in the compressed region. The formation, evolution, and interaction of FCs provide a novel mechanism for electron acceleration. These results clearly show the significance of electron‐scale dynamics within flux rope. Plain Language Summary: Magnetic reconnection is a fundamental plasma process by which magnetic energy is converted into the kinetic energy of charged particles. Understanding the acceleration mechanisms for the energetic electrons during magnetic reconnection is a long‐standing question in the study of space and astrophysical plasmas. Using Magnetospheric Multiscale observations at Earth's magnetopause, we present in situ evidence of electron acceleration up to 200 keV between two consecutive filamentary currents (FCs) inside a magnetic flux rope. Two FCs propagate at different speeds, with the second moving faster, thus causing a compressed region between them. These results provide an important new way for electron acceleration in magnetic reconnection. Key Points: Two types of filamentary currents (FCs) were observed near the center of a magnetic flux ropeStochastic electric fields were generated inside two FCs and accelerated electronsElectrons were accelerated up to 200 keV in the compressed region between two currents by the betatron mechanism [ABSTRACT FROM AUTHOR]

We investigate the detailed properties of electron inflow in an electron-only reconnection event observed by the four Magnetospheric Multiscale (MMS) spacecraft in the Earth's turbulent magnetosheath downstream of the quasi-parallel bow shock. The lack of ion coupling was attributed to the small-scale sizes of the current sheets, and the observed bidirectional super-Alfvénic electron jets indicate that the MMS spacecraft crossed the reconnecting current sheet on both sides of an active X-line. Remarkably, the MMS spacecraft observed the presence of large asymmetries in the two electron inflows, with the inflows (normal to the current sheet) on the two sides of the reconnecting current layer differing by as much as a factor of four. Furthermore, even though the four MMS spacecraft were separated by less than seven electron skin depths, the degree of inflow asymmetry was significantly different at the different spacecraft. The asymmetry in the inflow speeds was larger with increasing distances downstream from the reconnection site, and the asymmetry was opposite on the two sides of the X-line. We compare the MMS observations with a 2D kinetic particle-in-cell (PIC) simulation and find that the asymmetry in the inflow speeds stems from in-plane currents generated via the combination of reconnection-mediated inflows and parallel flows along the magnetic separatrices in the presence of a large guide field. [ABSTRACT FROM AUTHOR]

Energetic electrons have been frequently observed during magnetic reconnection in the magnetotail. The acceleration process of the energetic electrons is not fully understood. In this paper, we select for a detailed study a case of energetic electron acceleration from the earlier reported interval of turbulent magnetic reconnection in Earth's magnetotail observed by the Magnetospheric Multiscale mission. We use the first-order Taylor expansion method to reconstruct the magnetic topology of electron acceleration sites from the data. We find that the energetic electron fluxes increase inside the flux rope forming in front of the magnetic pileup region. We show that the energetic electrons are produced by a two-step process where two different acceleration mechanisms are successively operating outside and inside the flux rope. First, the thermal electrons are energized in the field-aligned direction inside the magnetic pileup region owing to the Fermi mechanism forming a cigar-like distribution. Second, those energized electrons are further accelerated predominately antiparallel to the magnetic field direction by a parallel electric field inside the flux rope. Our findings provide information for a better understanding of the generation of energetic electrons during turbulent reconnection process. [ABSTRACT FROM AUTHOR]

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. Key Points: High time resolution electron density is derived from spacecraft potential on Magnetospheric Multiscale during operation of the ion emittersThe high time resolution allows to study plasma density fluctuations down to the sub‐ion kinetic scaleAn increase of magnetic field and density anisotropy is found that indicates the presence of compressive structures in the magnetosheath [ABSTRACT FROM AUTHOR]

The terrestrial bow shock is the boundary that slows and diverts the supermagnetosonic solar wind around the terrestrial magnetosphere by converting the kinetic energy of the solar wind into thermal and magnetic energy. Shock fronts are an important acceleration site for ions and electrons in collisionless plasmas, and are responsible for much of the particle acceleration in solar, planetary, and astrophysical regions. One of the fundamental outstanding questions of ion acceleration at shocks for which the upstream magnetic field is nearly aligned with the shock normal (i.e., quasi‐parallel shocks) is which portion of the incoming solar wind ion distribution ultimately becomes the seed population that is subsequently accelerated to high energies. This study discusses distribution functions of protons and alpha particles observed by the HPCA and FPI instruments onboard the MMS satellites during a crossing of the quasi‐parallel bow shock. The bow shock transition from the downstream region into the upstream solar wind shows the occasional presence of reflected ions and a population of 90° pitch angle ions in the shock ramp consistent with shock drift accelerated ions. Both populations contribute to the seed population of the shock accelerated ions known as the diffuse ion population. Key Points: Two ion sources for the bow shock accelerated diffuse ion population are identified: Shock drift accelerated and specularly reflected ionsShock drift accelerated ions are directly accelerated out of the solar wind without an intermediate step to a suprathermal distributionThe bow shock magnetic field steepens enough to cause reflected solar wind ions that contribute to the source distribution [ABSTRACT FROM AUTHOR]