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Magnetic reconnection is a fundamental process known to play a crucial role in electron acceleration and heating, however, the mechanism of electron energization during reconnection is still not fully understood. This study introduces a novel electron acceleration mechanism in which electrons can be accelerated by secondary reconnection in the separatrix region. The secondary reconnection occurs in a thin current sheet resulted from the shear of the out‐of‐plane Hall magnetic fields of the primary magnetopause reconnection. It results in the intense electron energy fluxes toward the primary X‐line. This mechanism will likely be an important piece in the puzzle of particle acceleration by reconnection. Magnetic reconnection is a fundamental process that plays a critical role in electron acceleration and heating. The separatrix region of magnetic reconnection, which distinguishes the inflow and outflow regions, is a crucial area in the particle acceleration of the reconnection process. However, previous studies mainly focused on the electron acceleration mechanism in the separatrix region under two‐dimensional reconnection, without considering the influence of the three‐dimensional structures. This paper reports, for the first time, the secondary reconnection occurring in the opposite Hall magnetic field in the three‐dimensional separatrix region of magnetopause primary reconnection. This secondary reconnection can accelerate and heat the inflow electrons of the primary reconnection, which provides important clues on how the particles are accelerated and heated by reconnection. Secondary reconnection occurs between Hall magnetic fields in the separatrix region of the primary magnetopause reconnectionIntense electron enthalpy fluxes are injected toward the primary X‐line along the separatrixElectrons are accelerated by the secondary reconnection before being injected into the primary X‐line for further energization Secondary reconnection occurs between Hall magnetic fields in the separatrix region of the primary magnetopause reconnection Intense electron enthalpy fluxes are injected toward the primary X‐line along the separatrix Electrons are accelerated by the secondary reconnection before being injected into the primary X‐line for further energization

Using high‐resolution data of Magnetospheric Multiscale mission, we observed a gradual but periodical dipolarization process (DPs) in the Earth's magnetotail for the first time. These sub‐DPs, characterized by increase of Bzcomponent and magnetic magnitude Bt, have a period of ~16 s. The electrons have a cigar distribution in the first three sub‐DPs but have a pancake distribution in the last sub‐DP, which excites two oppositely propagating whistler waves. At the trailing boundary of the last sub‐DP, Bzdecreases sharply. Meanwhile, strong tailward and duskward plasma flow, intense current, ion and electron demagnetization, and positive energy conversion from the fields to the plasmas are observed at this boundary. By comparing the motion of this boundary with the plasma flow, we infer that the flow rebounds in longer period of the last DP and causes perpendicular electron acceleration. The periodical sub‐DPs and kinetic effects on the DP could improve our understanding of substorm and the magnetotail dynamics. A gradual but periodical dipolarization process (DP) is observed in the magnetotail for the first timeDifferent electron distributions are observed in the different sub‐DPsDynamic process and fine structures in the last sub‐DP are analyzed

Dipolarization fronts (DFs) are important for energy conversion, particle acceleration, and flux transport in the magnetotail. The partition of energy conversion between ions and electrons and the energy dissipation at DFs are not well understood. In this paper, we present a statistical study of energy conversion and dissipation of 122 DFs observed by Magnetospheric Multiscale mission in the magnetotail. Statistically, electromagnetic energy transfers to plasma at DF. The released energy is mainly transferred to ions rather than electrons. On average, ions gain energy across the whole DF, while electrons gain energy at the leading part but lose energy at the trailing part of DFs. Joule dissipation J · (E+ve× B)can be either positive or negative at DFs, and its average value is very small. The kinetic energy dissipation parameter Pi − Ddoes not exhibit clear signatures at the DFs; hence, it is not suitable for quantifying the energy dissipation at DF. Statistical study of the energy conversion and dissipation at DFs observed by MMSElectromagnetic energy released at DF mainly goes to heat and accelerate ionsAverage Joule dissipation across DF is very small, and the parameter Pi − Dis not suitable for quantifying the energy dissipation at DF

Magnetic holes have been widely observed in various space plasma systems. The origin of magnetic holes and their influence to background plasma are under debate. In this paper, we show a kinetic‐scale electron vortex magnetic hole in a nonideal region of an active X line, which was observed by the Magnetospheric Multiscale mission at the dayside magnetopause. Intense current and nonideal electric field in the electron frame were observed within the magnetic hole, which led to a strong energy dissipation. Thus, the electron vortex magnetic hole probably provided an additional energy dissipation channel besides the electron diffusion region adjacent to the hole. We suggest that magnetic reconnection provided favorable conditions for the formation of this kinetic‐scale magnetic hole and is an important source of magnetic holes in space plasma. First evidence for reconnection‐driven kinetic‐scale magnetic holeThe hole was associated with an electron vortex and was observed in the vicinity of an electron diffusion regionThe hole probably provided an additional energy dissipation channel besides the electron diffusion region

An ion‐scale flux rope (FR), embedded in a high‐speed electron flow (possibly an electron vortex), is investigated in the magnetotail using observations from the Magnetospheric Multiscale (MMS) spacecraft. Intense electric field and current and abundant waves are observed in the exterior and interior regions of the FR. Comparable parallel and perpendicular currents in the interior region imply that the FR has a non‐force‐free configuration. Electron demagnetization occurs in some subregions of the FR. It is surprising that strong dissipation (J× E'up to 2,000 pW/m3) occurs in the center of the FR without signatures of secondary reconnection or coalescence of two FRs, implying that FR may provide another important channel for energy dissipation in space plasmas. These features indicate that the observed FR is still highly dynamical, and hosts multiscale coupling processes, even though the FR has a very large scale and is far away from the reconnection site. Flux ropes, 3‐D helical magnetic structures, in which magnetic field lines twist with each other, play an important role in the macroscopic and microscopic physical process during magnetic reconnection. Most of previous studies focused on the flux ropes in the reconnection region. However, some physical process inside macroscopic flux ropes far away from the reconnection site in the magnetotail is still unclear due to the lack of high time resolution data. In this letter, thanks to the unprecedented high time resolution data of the Magnetospheric Multiscale (MMS) mission, we report an ion‐scale flux rope and study its dynamics. Our observations demonstrate that the observed flux rope is still highly dynamical, and hosting multiscale coupling processes and strong energy dissipation, even though the flux rope has very large scale and is far away from the reconnection site. An ion‐scale flux rope, embedded in possible electron vortex, is investigated in the magnetotail in detailsElectron demagnetization occurs in some subregions of the flux rope, and abundant waves are detected in the exterior and interior regionsStrong dissipation occurs in the flux rope, implying that flux rope provides another important channel for energy dissipation in space plasma

It has been proposed that, in the near‐Earth magnetotail, earthward propagating flux ropes can merge with the Earth's dipole magnetic field and dissipate its magnetic energy. However, the reconnection diffusion region related to this process has not been identified. Here we report the first in situ observation of magnetic reconnection between an earthward propagating flux rope and the closed magnetic field lines connecting to Earth. Magnetospheric Multiscale (MMS) spacecraft crossed a vertical current sheet between the leading edge of the flux rope (negative Bz) and the geomagnetic field (positive Bz). The subion‐scale current sheet, super‐Alfvénic electron outflow, Hall magnetic and electric field, conversion of magnetic energy to plasma energy (J·E > 0), and magnetic null were observed during the crossing. All the above signatures indicate that MMS detected the reconnection diffusion region. This result is also relevant to other planets with intrinsic magnetosphere. Magnetic reconnection is an essential source process in space weather. Reconnection produces many magnetic structures, such as the magnetic flux ropes and reconnection fronts, and ejects them away from the reconnection site. These structures interact with the surrounding space environment during its propagation, which may have great geomagnetic effects. A highly asymmetric earthward propagating magnetic flux rope is often observed in the Earth's magnetotail. It has long been suggested that this asymmetrical magnetic flux rope is formed due to the flux erosion of the earthward part of the flux rope by magnetic reconnection between the flux rope and the geomagnetic field. Despite various theoretical and numerical simulation studies, there has been no observational evidence to confirm this scenario. This paper reports the first observation of magnetic reconnection occurring at the earthward front of a flux rope in the Earth's magnetotail, confirming the previous theoretical predictions and explaining the formation of the asymmetric flux rope which is often observed in the near‐Earth magnetotail. First in situ observation of reconnection between an earthward propagating FR and the geomagnetic fieldThis is a highly deformed FR with its leading part being eroded by the reconnectionMMS crossed the diffusion region of this reconnecting current sheet

In this letter, first observations of ion‐scale magnetic island from the Magnetospheric Multiscale mission in the magnetosheath turbulent plasma are presented. The magnetic island is characterized by bipolar variation of magnetic fields with magnetic field compression, strong core field, density depletion, and strong currents dominated by the parallel component to the local magnetic field. The estimated size of magnetic island is about 8 di, where diis the ion inertial length. Distinct particle behaviors and wave activities inside and at the edges of the magnetic island are observed: parallel electron beam accompanied with electrostatic solitary waves and strong electromagnetic lower hybrid drift waves inside the magnetic island and bidirectional electron beams, whistler waves, weak electromagnetic lower hybrid drift waves, and strong broadband electrostatic noise at the edges of the magnetic island. Our observations demonstrate that highly dynamical, strong wave activities and electron‐scale physics occur within ion‐scale magnetic islands in the magnetosheath turbulent plasma. Ion‐scale magnetic island is identified in the magnetosheathElectron beams and intense wave emissions in the magnetic islandMultiscale coupling occurs in magnetic island

Magnetic reconnection is a universal phenomenon existing in the space. Energy conversion is one of the essential parts of reconnection discussed for decades. Positive energy conversion (J·E′> 0) is usually regarded as the sign of electron diffusion region (EDR), while the negative one (J·E′< 0) turns out to gather in the outer EDR. Here we report the negative J·E′appearing in the inflow edge of the inner EDR, based on Magnetospheric Multiscale mission observations and particle‐in‐cell simulations. Both observations and simulations verify that this negative J·E′is mainly contributed from the electron pressure tensor term in generalized Ohm's law. The energy loss of electrons plays the dominant effect in the electron pressure tensor, and this energy decline is caused by the electric field which is induced by the decreasing magnetic field. Our results provide significant insights to expand the understanding of the reconnection regimes. In interplanetary space and other space environments, magnetic reconnection can be detected through spacecraft observations. It affects the energy conversion between the fields and the plasmas. The positive energy conversion in the reconnection usually describes the process in that energy is transformed from the magnetic field to the particles. The negative one, however, represents the inverse form. With the help of satellite data, we find negative energy conversion in the inflow edge of the center site of magnetic reconnection (i.e., the inner electron diffusion region). We also perform kinetic simulations to verify the satellite observation results. These two results have well corresponded to a certain extent. We make further investigation on the simulation data to reveal the physical mechanism. It is found that electrons' energy has decreased in the inflow edge due to the electric field formed by the decreasing magnetic field in the inflow region. Such decline of the electrons' energy causes the change of electron pressure tensor. Our study may promote a more profound understanding of energy conversion during magnetic reconnection. Negative energy conversion (J·E′< 0) can appear in the inflow edge of the inner EDR during tail magnetic reconnectionBoth simulations and observations confirm that the negative energy conversion closely connects with the electron pressure tensor termThe electric field induced by the decreasing magnetic field causes the electron velocity decline and negative energy conversion Negative energy conversion (J·E′< 0) can appear in the inflow edge of the inner EDR during tail magnetic reconnection Both simulations and observations confirm that the negative energy conversion closely connects with the electron pressure tensor term The electric field induced by the decreasing magnetic field causes the electron velocity decline and negative energy conversion

Kinetic‐size magnetic holes (KSMHs) can accelerate and heat the particles, resulting the energy dissipation in space plasmas. KSMHs in the terrestrial foreshock region are statistically investigated based on the Magnetospheric Multiscale data from October 2015 to December 2019. A total of 98 KSMHs are successfully selected, and most of them are accompanied by electron vortices. The fluxes of electrons at ∼90° of pitch angle enhance inside the magnetic holes, where the perpendicular and total electron temperature increase, and the parallel electron temperature slightly decreases inside these magnetic holes. Moreover, we reveal a novel phenomenon that a lot of the observed KSMHs are probably moving toward the Sun in the plasma flow frame. Such sunward propagating KSMHs are detected for the first time in the foreshock region. We propose that these observed KSMHs are probably generated in the foreshock region and then propagate to the Sun. Magnetic hole, characterized by the significant decrease of magnetic strength, is usually observed in the planetary magnetosphere. Kinetic‐size magnetic holes (KSMHs) with the scale below ion gyroradius have attracted much interest in recent years. KSMHs can accelerate and heat the particles, resulting the energy dissipation in space plasmas. Based on the Magnetospheric Multiscale data, KSMHs in the terrestrial foreshock region are statistically investigated for the first time. We reveal some novel phenomenon for the observed KSMHs in the foreshock region. We present the observational evidence of kinetic‐size magnetic holes (KSMHs) in the terrestrial foreshock regionA lot of the observed KSMHs are probably moving toward the Sun in the plasma flow frameThese observed KSMHs are probably generated in the foreshock region and then propagate toward the Sun We present the observational evidence of kinetic‐size magnetic holes (KSMHs) in the terrestrial foreshock region A lot of the observed KSMHs are probably moving toward the Sun in the plasma flow frame These observed KSMHs are probably generated in the foreshock region and then propagate toward the Sun

Despite a growing number of observations of whistler waves in/around the reconnection diffusion region, the origin of the whistler waves in the diffusion region and their role in reconnection remain elusive. This paper investigates the whistler‐mode waves within an electron‐scale reconnecting current sheet observed by the Magnetospheric Multiscale mission on 23 December 2016. These whistlers were observed in both the electron diffusion region (EDR) and the immediate inflow region of the electron‐scale magnetic reconnection site. The dispersion relation of these whistlers is measured and compared to predictions from linear theory for the first time in these regions. The comparison shows that the field‐aligned drifting electron component critically modifies the dispersion relation of whistlers in the EDR due to the Doppler‐shift effects. We demonstrate that these whistlers propagated into the EDR from outside rather than locally excited, however, they did not provide sufficiently large anomalous dissipation in this EDR. Whistler‐mode waves are important plasma waves in the space plasma system, which can effectively interact with electrons to exchange energy. Whistler waves are frequently observed in various areas of magnetic reconnection, for example, outflow region, separatrix region, and diffusion region. However, the nature of whistler waves in the electron diffusion region (EDR), where the magnetic field line is broken and reconnected, is still unclear. Here, we present an observation of whistler waves in the EDR and the inflow region of magnetic reconnection. These whistler waves originate from the outside and propagate into the vicinity of the reconnection site. We measured the dispersion relation of these whistler waves for the first time within these regions and compare it with the dispersion relations predicted by linear theory. The results indicate that the effects from the drift velocity of electrons play a key role in modifying the dispersion relation of these whistlers in the EDR. Additionally, we find that these whistlers did not provide effective anomalous dissipation in this EDR. These results advance our knowledge of the source and features of whistlers in the vicinity of the reconnection site. Observations confirm that whistler waves can propagate into the electron diffusion region (EDR) from outsideThe dispersion relation of the whistler waves is measured for the first time in the EDRThe observed whistler waves contributed to negligible anomalous effects in the EDR Observations confirm that whistler waves can propagate into the electron diffusion region (EDR) from outside The dispersion relation of the whistler waves is measured for the first time in the EDR The observed whistler waves contributed to negligible anomalous effects in the EDR

Utilizing data from the Magnetospheric Multiscale mission, the sub‐structures of the separatrix region during reconnection are investigated. A small current system in the separatrix region is embedded in the large Hall current system, which appears as a quadrupole current. This small current system has an opposite direction to the Hall current and cancels part of the Hall magnetic field. The formation of the quadrupole current is possibly related to the patchy parallel electric field. The electric field in the separatrix region can be divided into three sub‐layers according to the electron frozen‐in condition. Enenhances in two outside sub‐layers, where the strong Enis supported by the Hall term and electron pressure gradient term, and the nearly zero En, separating the enhanced electric fields, is caused by a small flow and electron density gradient. Our observations imply complex dynamics in the separatrix region during magnetic reconnection. Magnetic reconnection is an explosive energy conversion process that converts magnetic energy to particles. Meanwhile, the topology of the magnetic field is changed. Many phenomena in space are related to magnetic reconnection, like solar flares and aurora. The separatrix is the layer separating the plasmas not yet entered the reconnection process and those already processed by reconnection. The characteristics of plasmas are quite different at the two sides of the separatrix, which makes the separatrix dynamic. With high‐resolution data from the Magnetospheric Multiscale mission, we investigate the sub‐structures of the separatrix region in a reconnection event in the magnetotail. The quadrupole current and multi‐layer electric field are observed in the separatrix region. The quadrupole current is possibly related to the patchy parallel electric field. And the multi‐layers electric field are caused by the complex electron flow patterns. Our observations reveal the highly‐structured nature of the separatrix region and may help to improve our understanding of the reconnection process. Quadrupole current and multi‐layer electric field are observed in the separatrix regionQuadrupole current is possibly related to the patchy parallel electric fieldMulti‐layer electric field are caused by complex electron flow patterns Quadrupole current and multi‐layer electric field are observed in the separatrix region Quadrupole current is possibly related to the patchy parallel electric field Multi‐layer electric field are caused by complex electron flow patterns

Dipolarization front (DF), characterized by a sharp increase of the south‐north component of the magnetic field, is suggested to play an important role in transferring plasmas, magnetic fluxes, and energy in the planetary magnetosphere. Using the measurements from the Cassini spacecraft between January 1, 2005 and June 15, 2011, we successfully selected 96 DF events, and obtained the global spatial distribution of DFs in the Saturn's magnetosphere. For the first time, we found that DFs are distributed not only in the nightside magnetotail but also in the dayside magnetosphere. The dayside DF events are mainly located from X= 10 RSto X= 30 RS(RSis the Saturn's radius) while the nightside events have a wide range, up to X∼−50 RS. Moreover, the DFs are observed to be asymmetric in the south‐northern hemisphere: ∼70% of events in the northern hemisphere and ∼30% of events in the southern hemisphere, which is likely to be due to the asymmetric orbit coverage of the Cassini in south‐northern hemisphere. The occurrence of dayside DFs provides a strong evidence that the magnetic reconnection could also occur in the dayside of Saturn's magnetosphere. Thus, our results concerning the location and distribution of DFs are helpful for the study of the site of magnetic reconnection and energy transport/dissipation in Saturn's magnetosphere. Saturn's magnetosphere is influenced by not only the solar wind, but more importantly the internal sources. Therefore, its magnetosphere would show different characteristics from terrestrial planets. Dipolarization front (DF), characterized by a sharp increase of the south‐north component of the magnetic field, is directly and closely related to magnetic reconnection, planetary substorm and planetary aurora, and can be a great medium for studying the magnetosphere's response to the solar wind and internal sources. In this work, using the measurements from the Cassini, we selected 96 DF events and obtained the global spatial distribution of DFs in the Saturn's magnetosphere. For the first time, we found that quite a big portion of DF events appear in the dayside magnetosphere. Therefore, we propose that the dynamics processes that generally occur at the magnetotail can extend to the whole magnetosphere in Saturn. Our work provides further evidence for the previous studies that the magnetic reconnection occurs in Saturn's dayside magnetodisc. A complete global distribution is provided to show the location and local time of dipolarization fronts in Saturn's magnetosphereThere is a surprising large portion of dipolarization fronts observed at the dayside of Saturn's magnetosphereOur results support the existence of dayside magnetodisc reconnection in Saturn A complete global distribution is provided to show the location and local time of dipolarization fronts in Saturn's magnetosphere There is a surprising large portion of dipolarization fronts observed at the dayside of Saturn's magnetosphere Our results support the existence of dayside magnetodisc reconnection in Saturn

We perform a statistical analysis on current density, energy conversion, and electron acceleration in the primary flux ropes (PFRs) and the secondary flux ropes (SFRs) distinguished by out‐of‐plane electron current, respectively. It is found that current filaments are plentiful in both PFRs and SFRs. The closer to the center of the SFRs, the stronger the |J| is. The J•E′is intermittent in the PFRs. However, J•E′almost stays positive in the SFRs. The local adiabatic acceleration of the electrons in the PFRs and the SFRs are explored. Fermi and betatron acceleration are closely related to the adjacent magnetic field and bulk flow which can lead to the contraction/relaxation of the PFRs and SFRs and enhance/reduce the gradient of the magnetic field. The acceleration by the parallel electric field is significant in the SFRs. Our results can help to improve the understanding of the PFRs and SFRs and their roles in the magnetotail dynamics. Flux ropes, characterized by bipolar change of normal component of the magnetic field and enhancement of the core filed, are 3D magnetic structures. It is believed that flux ropes are dynamic and they usually play an important role in magnetic reconnection, such as dissipating energy and accelerating electrons. Flux ropes can be divided into primary flux ropes (PFRs) and secondary flux ropes (SFRs) according to different generation mechanisms. Though flux ropes have been investigated for decades, the statistical properties of the PFRs and the SFRs have not been obtained and compared. The commonalities and differences between PFRs and SFRs are ambiguous. Using the unprecedented high time resolution data of the Magnetospheric Multiscale (MMS) mission, we perform statistical analysis on current density, energy conversion, and electron acceleration in the PFRs and the SFRs, respectively. Our results may help to improve the understanding of the PFRs and SFRs and their roles in magnetotail dynamics. Current filaments are plentiful in both primary flux ropes (PFRs) and secondary flux ropes (SFRs). In the SFRs, |J| gets stronger as close to its centerJ.E is intermittent in PFRs and nearly stays positive in the SFRs, implying that SFRs are important in energy dissipationEgainfand Egainbare strongly associated with the adjacent magnetic field and bulk flow. Egaineparais significant in SFRs Current filaments are plentiful in both primary flux ropes (PFRs) and secondary flux ropes (SFRs). In the SFRs, |J| gets stronger as close to its center J.E is intermittent in PFRs and nearly stays positive in the SFRs, implying that SFRs are important in energy dissipation Egainfand Egainbare strongly associated with the adjacent magnetic field and bulk flow. Egaineparais significant in SFRs

Magnetic reconnection and turbulence are the two most important energy dissipation processes in plasma. These two processes intertwine with each other and play important roles in their respective dynamic evolution. Here, we present the first evidence that secondary reconnections occur in the turbulent outflow driven by a primary reconnection in the Earth's magnetotail. We have identified 14 secondary reconnections in a large number of current filaments in the turbulent outflow, which persisted for about one and half an hour. Most of these secondary reconnections were electron‐only reconnection that has recently been discovered in the magnetosheath. These secondary reconnections entangled the magnetic field lines and dissipated the magnetic energy in the outflow region far away from the primary Xline. Magnetic reconnection usually evolves into turbulent state. The role of turbulence in reconnection is one of the most outstanding questions in reconnection. By using the unprecedented high‐cadence data from the magnetospheric multiscale mission, we present the first evidence that secondary reconnections occur in the turbulent outflow driven by a primary reconnection in the Earth's magnetotail. Most of these secondary reconnections occurred in electron‐scale current sheets and were without ion couplings. These secondary reconnections future released energy in the outflow region besides the primary Xline. These results are important for understanding the turbulent reconnection, which is ubiquitous in the astrophysical system but barely investigated by in situ measurements. First observation of secondary reconnections in the turbulent outflow driven by primary reconnectionThese secondary reconnections occurred in current sheets with broad shear angles and different orientationsMost of these secondary reconnections were without clear ion couplings First observation of secondary reconnections in the turbulent outflow driven by primary reconnection These secondary reconnections occurred in current sheets with broad shear angles and different orientations Most of these secondary reconnections were without clear ion couplings

We investigate whistler‐mode waves and broadband electrostatic waves (EWs) within an ion diffusion region (IDR) at the magnetopause. The quasi‐parallel whistlers are observed in the separatrix regions associated with the electron anisotropy or loss cone, while the oblique whistlers, the Buneman‐type waves, and the oblique EWs are observed in the center of the current sheet associated with the accelerated electron or ion beams. The whistlers are linked with Buneman‐type waves by the electron Pacman distribution rather than the wave‐wave process. The accelerated cold electron beams excite the Buneman‐type instabilities and make the anisotropy or loss cone of hot electrons less apparent, which led to the conversion of the whistlers from quasi‐parallel to oblique. Additionally, the oblique EWs are associated with the ion beams produced by the multiple X‐line reconnection. These results provide a further understanding of the relation between plasma waves and plasma kinetics in the IDR. The whistler waves and the broadband electrostatic waves are two common waves produced by magnetic reconnection in space plasma. Generally, the whistler waves have two modes, the quasi‐parallel and the oblique to the ambient magnetic field associated with different generation mechanisms. Both of them have been observed in the magnetic reconnection, but their relation is still unclear. The broadband electrostatic waves are sometimes simultaneously observed with the oblique whistlers. The relationship between them is also inconclusive. Here, we present a new view of the relation between the quasi‐parallel whistlers, the oblique whistlers, and the broadband electrostatic waves. They are linked by the evolution of electron Pacman distribution, which is composed of the hot magnetospheric population with the loss cone and the cold accelerated magnetosheath beam characteristic for magnetopause reconnection, in the reconnection diffusion region rather than the wave‐wave process. Besides, the oblique electrostatic waves associated with ion beams are observed in the diffusion region. These results advance our understanding of the relation between plasma waves and magnetic reconnection. Large‐amplitude electromagnetic and electrostatic waves are observed in the separatrices and center of the diffusion region, respectivelyQuasi‐parallel whistlers, oblique whistlers, and Buneman‐type waves are linked via the evolution of electron Pacman distributionThe oblique broadband electrostatic waves associated with the ion beams are observed in the diffusion region Large‐amplitude electromagnetic and electrostatic waves are observed in the separatrices and center of the diffusion region, respectively Quasi‐parallel whistlers, oblique whistlers, and Buneman‐type waves are linked via the evolution of electron Pacman distribution The oblique broadband electrostatic waves associated with the ion beams are observed in the diffusion region

Two‐dimensional Hall reconnection model can explain the fast release of magnetic energy and most of its predictions have been demonstrated by in situ satellite observations. However, the three‐dimensional effect to reconnection remains poorly known. Recent numerical simulations have shown that three‐dimensional evolution is more complex than two‐dimensional in that reconnection occurs in multiple sites that are not necessarily in the primary neutral current sheet. Here, we present the first observational evidence for localized secondary reconnection at the separatrix surface of a magnetic flux rope (MFR). This secondary reconnection occurs between the axial magnetic field of the MFR, which points out‐of‐plane of the Earth's magnetopause reconnection, and the magnetospheric field. This three‐dimensional electron‐scale reconnection in the exhaust facilitates the cross‐scale energy conversion from the macro‐scale down to electron‐scale. Magnetic reconnection releases large amounts of magnetic energy in astrophysical, space and laboratory plasmas. For the decades, the reconnection has been investigated largely based on two‐dimensional models while the three‐dimensional effect to reconnection remains poorly known. It has been shown by numerical simulation that three‐dimensional evolution, because of the freedom in the out‐of‐plane direction, is turbulent and distinct from the two‐dimension laminar reconnection in that reconnection occurs in multiple sites that are not necessarily in the primary neutral current sheet. The turbulent reconnection is characterized by the formation and interaction of multiple flux ropes, or chaotic magnetic field lines in the outflow region. Although small‐scale magnetic flux ropes and their interactions have been observed, one important missing piece is whether reconnection occurs at the region away from the neutral current sheet. Here we present the first evidence for localized secondary reconnection at the separatrix surface of the primary reconnection. This secondary reconnection occurs between the axial magnetic field of the flux rope, which points out‐of‐plane of the magnetopause reconnection, and the magnetospheric field. This three‐dimensional patchy reconnection in the exhaust facilitates the cross‐scale energy conversion from the macro‐scale down to electron‐scale and leads to a turbulent evolution of reconnection. Three‐dimensional secondary electron‐scale reconnection was observed at the separatrix of a flux rope in the magnetosphereNew light on the destiny of flux rope from a three‐dimensional perspective: dissipation through the reconnection of the core fieldThe cross‐scale energy conversion is through the formation and dissipation of flux ropes Three‐dimensional secondary electron‐scale reconnection was observed at the separatrix of a flux rope in the magnetosphere New light on the destiny of flux rope from a three‐dimensional perspective: dissipation through the reconnection of the core field The cross‐scale energy conversion is through the formation and dissipation of flux ropes

We report a Magnetospheric Multiscale (MMS) mission observation of magnetic reconnection occurring at the edge of a large‐scale magnetic flux rope (MFR), the cross‐section of which was about 2 RE. The MFR was observed at the duskside in the Earth's magnetotail and was highly oblique with its axis approximately along the XGSMdirection. We find an electron‐scale current sheet near the edge of this MFR. The Hall magnetic and electric field, super‐Alfvénic electron outflow, parallel electric field, and positive energy dissipation were observed associated with the current sheet, which indicates that MMS detected a reconnecting current sheet with a large guide field. Interestingly, ions were not coupled in this reconnection, akin to the electron‐only reconnection observed in the turbulent magnetosheath. We further find that the electron‐only reconnection is commonly associated with a macroscopic MFR. This result will shed new light on understanding the multiscale coupling associated with an MFR in space plasmas. MMS observed an electron‐only reconnection at the edge of a macroscopic magnetic flux rope in the magnetotailElectron‐only reconnection is ubiquitous in space plasma

Using four‐point measurements from the Magnetospheric Multiscale (MMS) mission, we identify whistler waves at the boundary of an ion scale magnetic hole, which should be locally excited rather than propagated from other regions. Based on the measured local plasma parameters, which include those for bidirectional electron beams with electron temperature anisotropy, the frequency range with the growth rate derived from kinetic theory is consistent with the observations, while the growth rate in the absence of such beams is negative; hence, the observed whistler waves are locally excited by the bidirectional electron beams with electron temperature anisotropy. The dispersion relation, derived from one‐dimensional particle‐in‐cell simulations that are conducted using the inputs of the bidirectional electron beams with electron temperature anisotropy, agrees well with the one from kinetic theory, which further supports this excitation mechanism. Our results shed light on a new excitation mechanism for whistler waves. Whistler waves are nearly field‐aligned propagating and right‐hand polarized electromagnetic modes with the frequency between lower hybrid frequency and electron cyclotron frequency. Whistler waves are frequently and widely detected in the planetary magnetosphere and the interplanetary space and play an important role in the particle dynamics therein. For example, the electrons could be effectively accelerated by the electromagnetic whistler waves. Thanks to the unprecedented high time resolution and multipoint data from the Magnetospheric Multiscale (MMS) mission, the whistler waves are identified at the boundary of a magnetic hole in the magnetosheath. Based on the measured local plasma parameters including bidirectional electron beams with electron temperature anisotropy, both the linear theory based on kinetic plasma and one‐dimensional particle‐in‐cell (PIC) simulations confirm that the observed whistler waves are locally excited by the presence of bidirectional field‐aligned electron beams with electron temperature anisotropy for the first time. Our results shed new light on new excitation mechanism for whistler waves. Whistler waves are identified at the boundary of a magnetic holeLinear theory and simulations confirm that whistler waves are excited by bidirectional electron beams with temperature anisotropyOur results shed new light on new excitation mechanism for whistler waves

Dipolarization front (DF), embedded in fast plasma flow, is a sharp structure with the increase of Bzin a short time and plays an important role in the energy dissipation and particle accelerations in the magnetotail. However, detailed physics at the small scales is still unclear in the DF due to the limitation of the previous mission. In the present study, an electron‐scale plateau in Bzis observed at the DF by Magnetospheric Multiscale (MMS) mission. By analyzing the velocity of the electrons, we find that the electron‐scale plateau is surrounded by a sub‐ion‐scale electron vortex, which is the first observation of electron vortex at the DF as far as we know. The magnetic field generated by the electron vortex cancels parts of the increase of Bzand causes the plateau. Strong asymmetric electric field and demagnetization of electrons and ions are observed at the boundary of the electron vortex. These results shed new light on that small‐scale structures can develop within the ion‐scale DF and affect the evolution and kinetic effects at the DF. A sub‐ion‐scale electron vortex is observed at a dipolarization frontThe excited magnetic field of the electron vortex suppresses parts of the increase of Bzand causes the plateau at the dipolarization frontStrong asymmetric electric field and demagnetization of electrons and ions are observed at the boundary of the electron vortex