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In this paper, the particle acceleration processes around magnetotail dipolarization fronts (DFs) were reviewed. We summarize the spacecraft observations (including Cluster, THEMIS, MMS) and numerical simulations (including MHD, test-particle, hybrid, LSK, PIC) of these processes. Specifically, we (1) introduce the properties of DFs at MHD scale, ion scale, and electron scale, (2) review the properties of suprathermal electrons with particular focus on the pitch-angle distributions, (3) define the particle-acceleration process and distinguish it from the particle-heating process, (4) identify the particle-acceleration process from spacecraft measurements of energy fluxes, and (5) quantify the acceleration efficiency and compare it with other processes in the magnetosphere (e.g., magnetic reconnection and radiation-belt acceleration processes). We focus on both the acceleration of electrons and ions (including light ions and heavy ions). Regarding electron acceleration, we introduce Fermi, betatron, and non-adiabatic acceleration mechanisms; regarding ion acceleration, we present Fermi, betatron, reflection, resonance, and non-adiabatic acceleration mechanisms. We also discuss the unsolved problems and open questions relevant to this topic, and suggest directions for future studies.

Dipolarization fronts (DFs) are widely believed to host energy conversion processes. However, which mechanism is responsible for the energy conversion is still obscure. Using data from the Magnetospheric Multiscale mission, a current sheet is observed at a DF. This current sheet is caused by interchange instability bending the edge of the DF. Inside the current sheet, Hall electromagnetic field, super Alfvénic electron jets, demagnetization of ions and electrons, strong energy conversion, and steady ion flow and temperature are observed, indicating an electron‐only reconnection at the DF. The duskward plasma flow, which may be deflected by the DF, compresses the bent edges of the DF. As a result, the width of the current sheet between two adjacent bent edges of the DF reduces, and then reconnection begins. Our observations give direct evidence that magnetic reconnection results in energy conversion at a DF. Plain Language Summary Magnetic reconnection is an explosive phenomenon in space, which can rapidly convert energy from magnetic field to particles. Dipolarization fronts are essential carriers of mass and energy from the magnetotail to the Earth. Strong energy conversion, considered to be even more significant than magnetic reconnection, is usually observed at dipolarization fronts. However, the specific mechanism responsible for the energy conversion at a dipolarization front is elusive. Using data from the Magnetospheric Multiscale mission, we present direct evidence of magnetic reconnection at a dipolarization front, leading to strong energy conversion. The reconnection occurs in the current sheet at the dipolarization front. The duskward diverted flow compresses the bent dipolarization front induced by interchange instability, resulting in reconnection. Key Points A current sheet with strong energy conversion is found at a DF The strong energy conversion at this DF is mainly caused by the magnetic reconnection The duskward diverted flow compresses the bent DF caused by interchange instability, leading to the reconnection

Dipolarization fronts (DFs), characterized by a strong and steep increase of the tail magnetic field component Bz normal to the neutral plane and preceded by a much less negative dip of Bz, are reported in many observations of bursty bulk flows and substorm activations throughout the whole Earth's magnetotail. It is shown that similar structures appear in full‐particle simulations with open boundaries in a transient regime before the steady reconnection in the original Harris current sheet driven out of the equilibrium by the initial X‐line perturbation is established. Being secondary reconnection structures propagating with the Alfvén speed, DFs are different from the magnetic field pileup regions reported in earlier simulations with closed boundaries. They also differ from the secondary plasmoids with bipolar Bz changes reported in earlier fluid simulations and particle simulations with open boundaries. In spite of their transient nature, DFs are found to form when the force balance is already restored in the system, which justifies their interpretation as a nonlinear stage of the tearing instability developing in two magnetotail‐like structures on the left and on the right of the initial central X‐line. Both electrons and ions are magnetized at the front of the dipolarization wave. In contrast, in its trail, ions are unmagnetized and move slower compared to the E × B drift, whereas electrons either follow that drift being completely magnetized or move faster, forming super‐Alfvénic jets. In spite of the different motions of electrons and ions, the growth of the front is not accompanied by the corresponding growth of the electrostatic field and the energy dissipation in fronts is dominated by ions.

We examine the pitch angle distribution (PAD) of suprathermal electrons (>40 keV) inside the flux pileup regions (FPRs) that are located behind the dipolarization fronts (DFs), in order to better understand the particle energization mechanisms operating therein. The 303 earthward‐propagating DFs observed during 9 years (2001–2009) by Cluster 1 have been analyzed and divided into two groups according to the differential fluxes of the >40 keV electrons inside the FPR. One group, characterized by the low flux (F < 500/cm2 · s · sr · keV), consists of 153 events and corresponds to a broad distribution of IMF Bz components. The other group, characterized by the high flux (F ≥ 500/cm2 · s · sr · keV), consists of 150 events and corresponds to southward IMF Bzcomponents. Only the high‐flux group is considered to investigate the PAD of the >40 keV electrons as the low‐flux situation may lead to large uncertainties in computing the anisotropy factor that is defined asA = F⊥/F∥ − 1 for F⊥ > F∥, and A = −F∥/F⊥ + 1 for F⊥ < F∥. We find that, among the 150 events, 46 events have isotropic distribution (|A| ≤ 0.5); 60 events have perpendicular distribution (A > 0.5), and 44 events have field‐aligned distribution inside the FPR (A < −0.5). The perpendicular distribution appears mainly inside the growing FPR, where the flow velocity is increasing and the local flux tube is compressed. The field‐aligned distribution occurs mainly inside the decaying FPR, where the flow velocity is decreasing and the local flux tube is expanding. Inside the steady FPR, we observed primarily the isotropic distribution of suprathermal electrons. This statistical result confirms the previous case study and gives an overview of the PAD of suprathermal electrons behind DFs. Key Points One hundred fifty hot flux pileup regions (FPRs) are examined Perpendicular PAD in growing FPR; field‐aligned PAD in decaying FPR Maximum flux is well correlated with the Bz peak at the DF

Two dipolarization front (DF) structures observed by Cluster in the Earth midtail region (XGSM ≈ −15 RE), showing respectively the feature of Fermi and betatron acceleration of suprathermal electrons, are studied in detail in this paper. Our results show that Fermi acceleration dominates inside a decaying flux pileup region (FPR), while betatron acceleration dominates inside a growing FPR. Both decaying and growing FPRs are associated with the DF and can be distinguished by examining whether the peak of the bursty bulk flow (BBF) is co‐located with the DF (decaying) or is behind the DF (growing). Fermi acceleration is routinely caused by the shrinking length of flux tubes, while betatron acceleration is caused by a local compression of the magnetic field. With a simple model, we reproduce the processes of Fermi and betatron acceleration for the higher‐energy (>40 keV) electrons. For the lower‐energy (<20 keV) electrons, Fermi and betatron acceleration are not the dominant processes. Our observations reveal that betatron acceleration can be prominent in the midtail region even though the magnetic field lines are significantly stretched there. Key Points Fermi acceleration dominates inside a decaying flux pileup region Betatron acceleration dominates inside a growing flux pileup region Betatron acceleration is caused by a local compression of magnetic field

The electron butterfly distribution, characterized by pitch angles (PA) primarily at 45° and 135°, was rarely observed in Earth's magnetotail. Here using the high‐resolution measurements from Magnetospheric Multiscale mission, we present the observation of electron butterfly distribution in a contracting dipolarization front (DF), and propose a new physical mechanism to explain its formation. Specifically, we discover that the electron butterfly distribution only exhibited in the locally contracted DF and was observed above 1.7 keV. We infer that local contraction of the DF transformed its configuration from a magnetic bottle to an hourglass‐shaped magnetic structure, and the butterfly distribution was formed by the magnetic mirror effect of this magnetic hourglass. Additionally, the theoretically estimated loss cone of the magnetic hourglass fits well with the observations of electrons, validating our inference about the formation mechanism. These findings can improve our understanding of electron dynamics in Earth's magnetosphere. Plain Language Summary Examining the pitch‐angle (PA) distribution of electrons can help us understand the electron dynamic process in space. In this paper, we present the observation of electron butterfly distribution, characterized by PA primarily around 45° and 135°, in Earth's magnetotail. We find that the electron butterfly distribution was observed only above 1.7 keV, and exhibited in a locally contracting dipolarization front (DF). We propose a new formation mechanism for this distribution, and perform the theoretical calculations to validate it. Our findings can significantly improve the knowledge of electron dynamics in Earth's magnetosphere. Key Points The electron butterfly distribution was observed above 1.7 keV and only exhibited in the contracted dipolarization front (DF) The local contraction of the DF transformed its configuration from a magnetic bottle to an hourglass‐shaped magnetic structure The butterfly distribution is formed by the magnetic mirror effect of the hourglass‐shaped structure

Modes and manifestations of the explosive activity in the Earth’s magnetotail, as well as its onset mechanisms and key pre-onset conditions are reviewed. Two mechanisms for the generation of the pre-onset current sheet are discussed, namely magnetic flux addition to the tail lobes, or other high-latitude perturbations, and magnetic flux evacuation from the near-Earth tail associated with dayside reconnection. Reconnection onset may require stretching and thinning of the sheet down to electron scales. It may also start in thicker sheets in regions with a tailward gradient of the equatorial magnetic field B z ; in this case it begins as an ideal-MHD instability followed by the generation of bursty bulk flows and dipolarization fronts. Indeed, remote sensing and global MHD modeling show the formation of tail regions with increased B z , prone to magnetic reconnection, ballooning/interchange and flapping instabilities. While interchange instability may also develop in such thicker sheets, it may grow more slowly compared to tearing and cause secondary reconnection locally in the dawn-dusk direction. Post-onset transients include bursty flows and dipolarization fronts, micro-instabilities of lower-hybrid-drift and whistler waves, as well as damped global flux tube oscillations in the near-Earth region. They convert the stretched tail magnetic field energy into bulk plasma acceleration and collisionless heating, excitation of a broad spectrum of plasma waves, and collisional dissipation in the ionosphere. Collisionless heating involves ion reflection from fronts, Fermi, betatron as well as other, non-adiabatic, mechanisms. Ionospheric manifestations of some of these magnetotail phenomena are discussed. Explosive plasma phenomena observed in the laboratory, the solar corona and solar wind are also discussed.

Dipolarization fronts (DFs) host plentiful dynamics in the magnetotail. Small‐scale magnetic structures and waves are frequently found at and around the DFs. Whistler waves are closely related to DFs, and the flux pileup regions (FPRs) behind DFs are thought to be the source regions of the whistler waves around DFs. Statistically, whistler waves are much more frequently observed in FPRs than at DFs. Here, we present an observation of whistler waves in a magnetic hole at a DF. Theoretical calculations show that the observed whistler waves can be trapped in the magnetic hole. Furthermore, theoretical calculations suggest that whistler waves cannot be trapped at a DF if the DF has no sub‐structures. Our results indicate the importance of the magnetic structures on the propagation of whistler waves around the DFs and could explain why whistler waves are rarely observed at the DFs. Plain Language Summary Dipolarization fronts (DFs), as the boundary separating the hot and tenuous high‐speed plasma flow and the cold and dense background plasmas, are important in transporting energy and magnetic fluxes from magnetotail to the Earth. Abundant dynamic processes have been observed related to DFs. Occasionally, magnetic structures can be found at DFs, for example, magnetic holes, current sheets, etc. Various waves, especially whistler waves, have been observed around DFs. The flux pileup regions behind DFs have been thought to be the source regions of whistler waves around DFs. However, observations of whistler waves at DFs are much fewer in contrast to those in flux pileup regions. Using data from the Magnetospheric Multiscale (MMS) mission, we found whistler waves in a magnetic hole at a DF. By calculating the perpendicular refractive index, we find the whistler waves observed here are trapped in the magnetic hole. Besides, we consider a DF without a small‐scale magnetic structure, and the theory predicts that no whistler waves can be trapped at the DF. Our work implies the importance of the magnetic structures on the propagation of the whistler waves around DFs and provides a possible explanation for why so few whistler waves are observed at DFs. Key Points Whistler waves are observed in the magnetic hole at a dipolarization front Theory predicts the observed whistler waves can be trapped in the magnetic hole Theory predicts that whistler waves cannot be trapped at a dipolarization front without a small‐scale magnetic structure

Using Cluster data, we investigate the electric structure of a dipolarization front (DF) – the ion inertial length (c/ωpi) scale boundary in the Earth's magnetotail formed at the front edge of an earthward propagating flow with reconnected magnetic flux. We estimate the current density and the electron pressure gradient throughout the DF by both single‐spacecraft and multi‐spacecraft methods. Comparison of the results from the two methods shows that the single‐spacecraft analysis, which is capable of resolving the detailed structure of the boundary, can be applied for the DF we study. Based on this, we use the current density and the electron pressure gradient from the single‐spacecraft method to investigate which terms in the generalized Ohm's law balance the electric field throughout the DF. We find that there is an electric field at ion inertia scale directed normal to the DF; it has a duskward component at the dusk flank of DF but a dawnward component at the dawn flank of DF. This electric field is balanced by the Hall (j × B/ne) and electron pressure gradient (∇ Pe/ne) terms at the DF, with the Hall term being dominant. Outside the narrow DF region, however, the electric field is balanced by the convection (Vi × B) term, meaning the frozen‐in condition for ions is broken only at the DF itself. In the reference frame moving with the DF the tangential electric field is almost zero, indicating there is no flow of plasma across the DF and that the DF is a tangential discontinuity. The normal electric field at the DF constitutes a potential drop of ∼1 keV, which may reflect and accelerate the surrounding ions. Key Points We calculate E at DF using single‐ and four‐ spacecraft methods Normal E is balanced by the Hall (dominant) and pressure gradient terms At dawn flank, E is dawnward; At dusk flank, E is duskward

Dipolarization fronts (DFs), characterized by sharp increases in the northward magnetic field and usually preceded by magnetic dips, are suggested to play an important role in energy conversion and transport in the magnetotail. It has been documented that strong energy conversion typically develops right at the fronts. Here we present spacecraft observations of electron‐scale energy conversion (EEC) developed inside the dip region ahead of a DF, by using high‐cadence data from the Magnetospheric Multiscale Mission. The EEC, with magnitude comparable to that at the front, is primarily driven by ion current and electron‐scale electric field. The electric field inside the dip is provided by electrostatic waves fed by lower hybrid drift instability, which experiences temporal decaying. Such decaying leads to nonhomogeneity of EEC along the dawn‐dusk direction. These results, uncovering a new channel for DF‐driven energy conversion, can provide important insights into understanding energy transport in the magnetotail. Plain Language Summary Space weather is determined by earthward transport of energy and mass in the magnetosphere. In the magnetotail, such transport is usually associated with dipolarization fronts embedded inside high‐speed plasma jets, which are characterized by a sharp enhancement of the northward component of the magnetic field and serve as the leading boundaries of plasma jets. Statistical studies reveal that a small decrease in the magnetic field often occurs ahead of the fronts, which is dubbed as a magnetic dip. Dipolarization fronts have been suggested to play a key role in the energy conversion chain in the magnetotail, and the energy conversion typically happens right in the front region where strong currents and electric fields usually develop. In this research, we find that in addition to the front region, the dip preceding the fronts can also host strong energy conversion. Our results help further understand energy conversion in the terrestrial magnetotail. Key Points Electron‐scale energy conversion (EEC) is observed for the first time in the magnetic dip ahead of a dipolarization front The EEC, with magnitude comparable to that at the front, is primarily driven by ion current and electron‐scale motional electric field The EEC electric field is induced by a decaying lower hybrid drift instability which may cause temporal damping of the EEC