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Standard magnetic reconnection couples with both ions and electrons on different scales. Recently, a new type of magnetic reconnection, electron‐only reconnection without the coupling of ions, has been observed in various plasma environments. Standard reconnection typically has a reconnection outflow velocity of about one Alfvén speed. According to the scaling analysis, the electron outflow velocity is expected to be about one electron Alfvén speed in electron‐only reconnection. However, observations and simulations both find that the electron outflows are much slower than the electron Alfvén speed. In this letter, by performing two‐dimensional particle‐in‐cell simulations, we show that this is because ions play a role in electron‐only reconnection. The ions move slower than the electrons in the outflow direction, and such charge separation forms an in‐plane Hall electric field, which prevents the electrons from being accelerated to the electron Alfvén speed in electron‐only reconnection. Plain Language Summary: Magnetic reconnection is a fundamental plasma process during which magnetic field line topologies change and magnetic energy transfers to plasma kinetic and thermal energy. In standard collisionless magnetic reconnection, ions and electrons are both heated and accelerated to form high‐speed outflow. Theoretical analyses have been performed to successfully explain the fast outflow speed in standard magnetic reconnection. Recently, a new type of magnetic reconnection, electron‐only reconnection (in which there is no obvious ion heating and acceleration) has been reported to occur in various plasma environments. However, the same scaling analyses performed in electron‐only reconnection overestimate the outflow speed in the electron‐only reconnection by a factor of ∼10. Here, by performing two‐dimensional (2‐D) particle‐in‐cell (PIC) simulations, we show that the overestimation is due to the neglect of the role of ions. Because of the in‐plane Hall electric field caused by charge separation, electron outflow in electron‐only reconnection cannot be accelerated to the expected value. Key Points: Electrons are not accelerated to electron Alfvén speed in electron‐only magnetic reconnectionDeceleration of electron outflow caused by in‐plane Hall electric field should not be ignored [ABSTRACT FROM AUTHOR]

Magnetosheath high‐speed jets (HSJs), localized impulses of dynamic pressure, are attracting growing attention due to their geoeffectiveness. However, how HSJs modulate chorus waves in the magnetosphere still remains unclear. Utilizing combined observations of the Time History of Events and Macroscale Interactions during Substorms satellites A and E, we report, for the first time, the prompt disappearance of the magnetospheric chorus waves caused by a HSJ. Such wave disappearance is directly due to the flux drop of energetic electrons (∼10–100 keV), leading to the cessation of wave generation, which is supported by the linear theoretical analysis. We propose that the flux drop results from the local indentation of magnetopause after the HSJ impact, where two new smaller magnetic mirrors are formed off the equator and part of electrons are then expelled by the mirror force. The HSJs should be an important factor in modulating chorus waves because of their high occurrence rate. Plain Language Summary: The solar wind with enhanced dynamic pressure can globally compress the dayside magnetosphere, causing the excitation and intensification of chorus waves. Besides the enhancement of solar wind RAM pressure, there also exists the ion‐scale dynamic pressure impulse in the magnetosheath, known as a high‐speed jet (HSJ), even during quiet solar wind conditions. Previous studies have shown that the HSJs have a high occurrence rate and they are geoeffective (causing disturbances in the Earth's magnetosphere). However, it is still unknown whether HSJs can modulate chorus waves in the magnetosphere. In this study, based on joint observations of THEMIS‐A and E probes, we report the prompt disappearance of chorus waves in the magnetosphere caused by the HSJs. We propose that a HSJ causes the local indentation of the dayside geomagnetic fields at the magnetic equator, and creates two small magnetic mirrors. Consequently, the poleward mirror force will expel some of the electrons from the equator, and lead to a significant decrease of the electron flux. Finally, the generation of chorus waves will stop due to the lack of energy sources. This study reveals that HSJs are a new and important factor in modulating chorus waves in the magnetosphere. Key Points: With joint observations of THMIS‐A and THMIS‐E, we first report the prompt disappearance of chorus waves due to the high‐speed jets (HSJs)HSJs cause the local indentation of the magnetopause, creating two new magnetic mirrors, and electrons are expelled by the mirror forceThe electron flux drop leads to the cessation of the chorus wave generation, that is, the disappearance of waves [ABSTRACT FROM AUTHOR]

Thin current sheets (TCS) formed in Earth's magnetotail and hosting magnetic field line reconnection are important element of magnetospheric dynamics. In this paper, a statistical survey of 163 TCS crossing events in Earth's magnetotail using observations by MMS during 2017–2020 is performed. We find that magnetotail TCSs typically have a very strong current density primarily carried by electrons, with a thickness of a few ion inertial lengths. Our statistical results reveal different characteristics between TCSs located on the duskside and dawnside. Specifically, we find that TCSs on the duskside have a smaller thickness, higher current density, and stronger Hall electric field than on the dawnside. Our results also show that there is a correlation between the TCS thickness and the Hall electric field magnitude, with a stronger Hall electric field present in thinner TCSs. Our statistics results confirm previously reported key TCS dawn‐dusk asymmetries, with the main advance of directly measurable plasma currents. TCS characteristics of the presented data set will be useful for initialization of realistic numerical simulations and theoretical investigations of TCS instabilities resulted in various dynamical models of the magnetotail. Key Points: We collect a data set of 163 thin current sheet crossing events in Earth's magnetotail using MMS data during 2017–2020The thin current sheet has a smaller thickness, a higher current density and a stronger Hall electric field on the duskside than the dawnsideThe Hall electric field in the magnetotail thin current sheet is stronger when the current sheet is thinner [ABSTRACT FROM AUTHOR]

High‐speed jets (HSJs) are commonly observed in the Earth's magnetosheath. The HSJs can drive shock‐like bow waves when compressing the ambient plasma, which are important for the HSJ's evolution and the energization of charged particles. Here we present the first two‐dimensional hybrid simulation of the formation and evolution of jet‐driven bow waves. The simulated bow waves exhibit localized enhanced magnetic field and ion density, with their peaks separated by the order of ion inertial length. The bow waves are formed when a super‐magnetosonic HSJ encounters a magnetic structure with the magnetic field nearly perpendicular to the HSJ's velocity. The magnetic field structure acts as an obstacle to deflect and decelerate the jet, causing the pile up of ions on the jet side and the compression of the magnetic structure on the downstream side. Our study explains the observed properties of bow waves, and helps to better understand the evolution of HSJs. Plain Language Summary: When the solar wind interacts with the Earth's magnetic field, it creates a bow shock in front of the magnetosphere. The bow shock slows down and heats up the solar wind plasma, creating a turbulent area known as the magnetosheath. Sometimes, high‐speed jets (HSJ), observed as high velocity and density pulses, are detected in the magnetosheath. The HSJs can transfer the solar wind plasma through the magnetosheath and potentially affect the near‐Earth space where satellites orbit. Recent studies have found that the HSJs can drive shock‐like bow waves at their front. The bow waves can accelerate and heat up plasma, similar to the primary shock wave, and they play an important role in the evolution of the HSJs. In this study, we present the first simulation result concerning the formation and evolution of the bow waves, which will help us to better understand the HSJs and the magnetosheath dynamics. Key Points: We study the formation and evolution of jet‐driven bow waves with the two‐dimensional hybrid model for the first timePlasma piles up at the ramp of compressed magnetic field, causing separated peaks of density and magnetic field at the bow waveThe magnetic field structure acts as an obstacle to decelerate jet, causing the pile up of jet ions at the leading edge [ABSTRACT FROM AUTHOR]

Whistler mode waves are commonly observed inside mirror mode structures (MM‐Ss) in the magnetosheath. Although MM‐Ss have also been detected in the magnetosphere, there is no statistical study investigating the properties of whistler mode waves inside MM‐Ss. Using Magnetospheric Multiscale (MMS) satellites, our present study identifies and statistically analyzes whistler mode waves detected inside MM‐Ss in the Earth's magnetosphere. Both the observational evidence (bidirectional propagation) and theoretical analyses suggest that whistler mode waves are excited in the low‐magnetic‐field regions of MM‐Ss. Statistical results indicate that whistler mode waves are frequently observed inside the MM‐Ss in the dusk sector. Most of these waves (>70%) are in the frequency range of 0.1–0.4 fce and have amplitudes less than ∼50 pT. Moreover, most of them are observed to propagate in the direction both parallel and antiparallel to the background magnetic field, and their wave normal angles are typically less than ∼40°. Our study analyzes the properties and generation of whistler mode waves inside MM‐Ss and reveals that MM‐Ss are a possible source region in the Earth's magnetosphere. Therefore, our study provides new insight into the properties of whistler mode waves inside MM‐Ss which are pervasive in space plasmas. Key Points: We studied whistler mode waves inside the mirror mode structures in the magnetosphere using ∼3 years of MMS satellite dataWhistler mode waves are frequently observed inside the mirror mode structures which preferentially occur in the dusk sectorBoth theoretical analysis and observations suggest that whistler mode waves are locally excited in the mirror mode structures [ABSTRACT FROM AUTHOR]

Recent spacecraft observations have shown that magnetic reconnection occurs commonly in turbulent environments at shocks. At quasi-perpendicular shocks, magnetic field lines are bent by the back-streaming reflected ions, which form a current sheet in the foot region, and then electron-scale reconnection occurs when the current sheet is fragmented at the shock front. Here we study magnetic reconnection at a quasi-perpendicular shock by using a two-dimensional particle-in-cell simulation. Collective properties of the reconnection sites from the shock transition to the downstream region are analyzed by adopting a statistical approach to the simulation data. Reconnecting current sheets are found to be densely distributed near the shock front, with a reconnection electric field larger than those in the downstream region. By tracing a reconnection site from its formation until it is convected downstream, we show the reconnection proceeds intermittently after an active stage near the shock front. Our tracing further shows that, in addition to being originated from the shock front, reconnection in the downstream region can also occur locally, driven by turbulent flows therein. The results help us better understand the evolution of electron-scale reconnection at a perpendicular shock. [ABSTRACT FROM AUTHOR]

Magnetosonic (MS) waves are common plasma waves in the Earth's magnetosphere. The self‐consistent excitation of MS waves has been studied by 2D particle‐in‐cell simulations in the meridian and equatorial planes of a dipole magnetic field. However, the direction of wave propagation is artificially limited in the previous 2D simulations. Therefore, the 3D simulation of MS waves needs to be investigated. In this paper, we investigate the excitation and evolution of MS waves in the Earth's dipole magnetic field based on a 3D general curvilinear particle‐in‐cell simulation. We find that the MS waves are excited primarily within 3° of the equator when the thermal velocity of the ring distribution is much less than the ring velocity of the ring distribution. These waves propagate along both the radial and azimuthal directions nearly perpendicular to the background magnetic field. In the linear stage, the growth rates of MS waves are almost equal in the radial and azimuthal directions. Compared with the waves propagating along the radial direction, the waves propagating along the azimuthal direction can grow for a longer time, resulting in a larger wave amplification in this direction after saturation. The simulation results provide a valuable insight to understand the self‐consistent evolution of MS waves in the Earth's dipole magnetic field, and the findings are useful for understanding the plasma wave‐particle interaction in the Earth's radiation belts. Plain Language Summary: The Earth's magnetosphere is a natural plasma laboratory. Since the plasma in the magnetosphere is collisionless, electromagnetic waves are significant agents for the transport of energy and momentum among different particles. The MS waves are important electromagnetic waves in the magnetosphere, which have a frequency range from several Hertz to several hundred Hertz. Recently, these waves have been receiving an increasing interest due to their potential importance in accelerating radiation belt electrons and heating plasmaspheric ions. The 2D particle‐in‐cell simulations have been used to investigate MS waves in the meridian and equatorial planes of a dipole magnetic field. However, the direction of wave propagation is artificially limited in the previous 2D simulations. Thus, the 3D simulation of MS waves still needs to be studied. In this paper, we perform a 3D particle‐in‐cell simulation to investigate the excitation of the MS waves in a dipole magnetic field. A larger wave amplification is found in the azimuthal direction because they can grow for a longer time in this direction. Our results can contribute to the understanding of the spatial distribution of MS waves in the Earth's magnetosphere. Key Points: Three‐dimensional particle‐in‐cell simulation of magnetosonic (MS) wave excitation in a dipole magnetic field has been studied for the first timeThe linear growth rates of MS waves are almost equal in the radial and azimuthal directionsA larger wave amplification is observed in the azimuthal direction since the waves can grow for a longer time in this direction [ABSTRACT FROM AUTHOR]

Collisionless shocks with low Alfvénic Mach numbers are expected to accelerate electrons, but the underlying physics are still unsolved. Two-dimensional particle-in-cell simulation of low-Mach-number quasi-perpendicular shock in low-β is performed to study the physics of formation of beam-like components with respect to background magnetic fields. The incoming electrons can be trapped and scattered to have velocities along the shock surface by the electrostatic wave in the foot region owing to the free energy in the relative drift between shock reflected ions and upstream electrons. Then fractional electrons can be reflected by the mirror force at the shock overshoot when escaping from the loss cone. The reflection by the mirror force makes the electrons gain quasi-parallel velocities, and the electrons are accelerated in the quasi-parallel direction during trapping in the immediate downstream, forming a beam-like component with respect to magnetic fields. Our results shown in this paper explain the physics of beam formation and could be helpful for accounting for type II radio bursts. [ABSTRACT FROM AUTHOR]

High‐speed jets (HSJs) are frequently observed downstream of a quasi‐parallel bow shock, and they are considered to play important roles in the coupling of the solar wind, the magnetosheath, and the magnetosphere. Using two‐dimensional hybrid simulations, we study the formation of HSJs in quasi‐parallel shocks with different shock angles (θBn). The interaction of the upstream compressive structures that are inhomogeneous in the direction perpendicular to the background magnetic field and the shock front leads to the shock ripples, and then the downstream HSJs. In a parallel shock with the shock angle θBn = 0°, the interaction regions of the upstream compressive structures with the shock front don't change with time. The shock ripples remain in the same regions of the shock front, and then the HSJs can develop into large‐scale sizes, especially under low plasma β or high Mach number (MA). While in a quasi‐parallel shock with a non‐zero shock angle, the interaction regions of the upstream compressive structures with the shock front move along the shock front. The shock ripples change with time, and the scale size of the downstream HSJs becomes smaller with the increase of the shock angle. Key Points: 2‐D hybrid simulations are performed to study the formation of downstream high‐speed jets (HSJs) and ripples in quasi‐parallel shocks due to the interaction between upstream structures and the shock frontIn a parallel shock, the shock ripples remain in the same regions of the shock front, and large‐scale HSJs can form downstream of the ripplesIn a quasi‐parallel shock, the shock ripples move along the shock front and the downstream HSJs become smaller as the shock angle increases [ABSTRACT FROM AUTHOR]

The frequency chirping of chorus waves is commonly observed in the Earth's inner magnetosphere, but its generation remains an open question. Recently, Liu et al. (2021), https://doi.org/10.1029/2021JA029258 reported two unusual rising‐tone (upward chirping) chorus elements. Although the central frequency of constituent subpackets rises, the frequency of a single subpacket is surprisingly downward chirping. With a gcPIC‐δf $\delta f$ simulation in the dipole field, we successfully reproduce this kind of substructure, which contains alternating signs of chirping. Interestingly, both hole and hill structures are formed around the theoretical resonant velocities in the electron phase space, no matter whether the chirping is upward or downward. However, during each chirping interval, only one structure (either a hole or a hill) is associated with wave excitation: the upward chirping is related to the hole, while the hill contributes to the downward chirping. Our study provides a fresh perspective on the theory of frequency chirping in chorus waves. Plain Language Summary: The frequency chirping is a typical feature of chorus waves in the Earth's inner magnetosphere, which generally contain either rising‐tone (upward chirping) elements or falling‐tone (downward chirping) elements. Previous theory has suggested that the chirping is due to the nonlinear wave‐particle interaction, where the hole or hill structure is formed in the electron phase space. Recently, Liu et al. (2021), https://doi.org/10.1029/2021JA029258 have observed the upward chirping elements with their subpackets of downward chirping. What electron structure is associated with these elements becomes a puzzle. With a one‐dimensional (1D) general curvilinear particle‐in‐cell (gcPIC) δf simulation in the dipole magnetic field, we successfully reproduce this kind of chorus element, whose frequency contains alternating upward and downward chirping. Interestingly, both the hole and hill structures are formed during a chirping interval, but only one of the two structures is responsible for wave excitation and frequency chirping. The structure of hole‐hill combination provides an important clue into the theory of the frequency chirping in chorus waves. Key Points: With a gcPIC‐δf $\delta f$ simulation in the dipole field, we reproduce the upward chirping chorus element, whose subpackets are downward chirpingBoth hole and hill structures can be formed in the ζ−v‖ $\zeta -{v}_{\Vert }$ phase space, no matter whether the frequency is upward or downward chirpingThe time evolution of the hole and hill structures in the phase space leads to the alternating frequency chirping [ABSTRACT FROM AUTHOR]

Asymmetric magnetic reconnection usually occurs at the Earth's magnetopause, where the magnetic field strength and plasma density are different between the magnetosheath and magnetosphere. In this paper, a two-dimensional particle-in-cell simulation model is used to study the energy conversion during asymmetric magnetic reconnection. Energy conversion can occur in the vicinity of the X-line, magnetosphere separatrix region, and reconnection fronts. In the vicinity of the X-line and magnetosphere separatrix region, the electromagnetic field energy is mainly transferred to electrons, while at the reconnection fronts, the electromagnetic field energy is mainly transferred to ions. For the case with weak magnetic field asymmetry, the reconnection fronts dominate the energy conversion, which is related to the inflowing Poynting flux S z at the fronts. For the case with strong magnetic field asymmetry, the energy conversion occurs around the X-line and magnetosphere separatrix region, but no longer at the reconnection fronts. This is because the inflowing Poynting flux S x near the magnetosphere separatrices provides electromagnetic energy for energy conversion. The density asymmetry has no significant effect on the spatial distribution of the energy conversion. [ABSTRACT FROM AUTHOR]

Using high‐resolution data from Magnetospheric Multiscale (MMS) mission, we report an intense current layer at the center of a flux rope (FR) in the magnetotail. The intense current layer is caused by the compression of the ion bulk flows at the center of a FR rather than two interlaced flux tubes reported at the magnetopause previously. The intense current layer has been identified as an electron diffusion region of the magnetic reconnection, and the hall magnetic field generated by magnetic reconnection makes the FR show crater‐shaped. The reconnecting current layer is supported by the poloidal magnetic field of the FR, and it is dividing the FR into two secondary FRs. The observations suggest that magnetic FR can be compressed easily to excite instability inside it as it is propagating in the magnetotail current sheet, thus changing its magnetic topology. Plain Language Summary: Magnetic flux ropes (FRs) consist of a poloidal magnetic field and an axial magnetic field component and are always described as helical magnetic field structures. The FRs play an important role in particle acceleration, magnetic flux transportation, and the evolution of reconnection. Recently, with high‐resolution magnetic field and plasma moments measurements, it is found that the interior of the FR is active, and a lot of processes could occur therein. In this work, we report a FR embedded in an unstable tailward plasma flow in the magnetotail. An ongoing reconnection current layer is observed at the center of the FR. The reconnecting current layer could change the magnetic field topology of the initial FR and divide the initial FR into two secondary FRs. Key Points: An electron diffusion region of reconnection is observed at the center of the flux rope (FR) in the magnetotailThe reconnecting current layer can change the magnetic field topology inside the FR and divide it into two secondary FRsThe Hall magnetic field inside the reconnecting current layer leads to the crater‐shaped magnetic field magnitude within the FR [ABSTRACT FROM AUTHOR]

The properties of chorus waves have been extensively studied, which are important to understand chorus generation and wave‐particle interactions between chorus and energetic electrons. Observations reveal that there exists a distinct day‐night asymmetry for the properties of chorus waves, such as the sweep rate and duration, which is predicted to be relevant to the asymmetric configuration of Earth's background magnetic field. In this paper, using the one‐dimensional (1‐D) particle‐in‐cell (PIC) simulation model with a compressed/stretched dipole magnetic field, we study the dependences of the properties of rising‐tone chorus waves on the background magnetic field. It is demonstrated that the saturated amplitude and the chorus sweep rate increase while the chorus duration decreases with an increase of the compression factor ξ, which represents the compression/stretch degree of the field line. Moreover, the threshold of exciting chorus waves in the compressed dipole field is generally lower than that in the stretched dipole field. These results are useful to understand the chorus excitation and the day‐night asymmetry of chorus properties. Key Points: A one‐dimensional particle‐in‐cell model is used to study how the configuration of the magnetic field controls the properties of chorus wavesThe amplitude and frequency sweep rate increase with increasing compression factor ξ, while the duration varies oppositelyThe threshold of exciting chorus waves is lowered by a large compression factor or a large field curvature [ABSTRACT FROM AUTHOR]

This paper investigates energy dissipation during electron-scale magnetic reconnection with laboratory experiments. Magnetic fields with opposite directions are generated by two parallel identical pulsed currents in our Keda linear magnetized plasma device. Magnetic reconnection is realized in the rising phase of the pulsed currents. The ramp-up rate of the pulsed current is found to be proportional to the inflow speed, providing a method to modify the reconnection drive. The incoming magnetic energy and its dissipation into plasma energy have been estimated in the vicinity of the X line. It is found that the plasma energy converted from the incoming electromagnetic energy increases with the increasing reconnection drive, while the conversion ratio remains almost unchanged, which is about 10%. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is a fundamental energy converting process changing the topology of the magnetic field in space. We investigate how the guide field and the background plasma density and temperature influence the energy converting process during magnetic reconnection using a two‐dimensional particle‐in‐cell simulation code. It is shown that the guide field reduces both the reconnection rate and the energy conversion rate. Although the downstream Poynting flux from the reconnection site is enhanced during guide field magnetic reconnection, it does not participate in energy conversion as the inflow and outflow of Poynting flux from the left and right boundaries of the reconnection front cancel each other out. The major input of the Poynting flux participating in energy conversion comes from the upper and lower boundaries of the reconnection front. An electrostatic perturbating structure arises in cases with a strong guide field. Apart from the reconnection rate and the energy conversion rate, the energy outflow is altered by changing background plasma density and temperature. Our research shows that the energy input at a well‐developed reconnection front in the case of the guide field does not come from the reconnection site, which implies the reconnection front can be an effective energy converter in the magnetosphere independent of the reconnection site once formed from the transient magnetic reconnection. These simulation results show that the guide field and background density and temperature quantitatively alter the energy conversion at the magnetic reconnection front, but the energy conversion pattern remains unchanged. Key Points: Guide field reduces energy conversion during magnetic reconnectionAn increase in background density and temperature leads to a decrease in energy conversion during magnetic reconnectionThough guide field and background density and temperature affect energy conversion during magnetic reconnection, it still occurs mainly at reconnection front [ABSTRACT FROM AUTHOR]

Standard magnetic reconnection couples with both electron‐ and ion dynamics. Recently, a new type of magnetic reconnection, electron‐only reconnection without the coupling of ion dynamics, has been observed in space. Using a two‐dimensional particle‐in‐cell simulation, we show that in the externally‐driven magnetotail, electron‐only reconnection is a transition from quiet current sheet to standard reconnection. We find that (a) energy conversion j ⋅ E′ is around zero in quiet current sheet but nonzero in electron‐only reconnection, and (b) ion temperature does not change across electron‐only reconnection but peaks at the center of standard reconnection. These two differences can be used as criteria to distinguish magnetotail electron‐only reconnection from quiet current sheet and standard reconnection, respectively. Based on the two criteria, we justify that the MMS 17 June 2017 event is a magnetotail electron‐only reconnection event. Plain Language Summary: The Earth has a long stretched magnetic tail, that is, magnetotail, formed by the interaction between the geomagnetic field and the plasma (ions and electrons) wind originated from the Sun. At the center of the magnetotail, there is a current sheet flanked by two different topologies of magnetic field lines. Magnetic reconnection, a fundamental plasma process during which magnetic field line topologies change, can occur in this magnetotail current sheet. Based on computer simulations in the past 20 years, a standard reconnection model with both ion and electron dynamics has been established, which can well describe magnetic reconnection observed in the magnetotail. Recently, a new type of reconnection with solely electron dynamics has been observed to occur in the magnetotail. Here using computer simulations, we show that electron‐only reconnection in the magnetotail is a transition from magnetotail current sheet to standard reconnection. Based on the simulation results, we give two criteria for identification of electron‐only reconnection in spacecraft observations in the magnetotail, using which we justify that the MMS 17 June 2017 event is a magnetotail electron‐only reconnection event. Key Points: Energy conversion j ⋅ E′ is around zero in quiet current sheet but nonzero in electron‐only reconnectionIon temperature does not change in electron‐only reconnection but peaks in standard reconnectionWe justify that the MMS 17 June 2017 event is a magnetotail electron‐only reconnection event [ABSTRACT FROM AUTHOR]

The power gap around 0.5Ωe (where Ωe is the equatorial electron gyrofrequency) of whistler‐mode waves is commonly observed in the Earth's inner magnetosphere, but its generation mechanism is still under debate. By performing two‐dimensional particle‐in‐cell simulations in a uniform background magnetic field, we investigate the spectral properties of whistler‐mode waves excited by temperature anisotropic electrons. The waves have positive growth rates in a wide range of normal angles (θ ≈ 0°–35°), resulting in the generation of both parallel and nonparallel waves. Although the nonparallel wave modes are weaker than the parallel ones, they can cause the plateau‐like shape around 0.5 VAe (where VAe represent the electron Alfven speed) in the parallel direction of electron velocity distribution. The plateau‐like electron component can then lead to severe damping in the waves around 0.5Ωe via the cyclotron resonance, and the power gap is formed. This mechanism is called as "spectrum bite". Our study sheds fresh light on the well‐known gap formation at ∼0.5Ωe in the whistler‐mode waves, which is ubiquitously detected near the equator in the inner magnetosphere. Key Points: The whistler‐mode waves with power gaps around 0.5Ωe in the Earth's magnetosphere are investigated by 2‐D PIC simulationsThe power gap is formed due to the severe damping by the plateau‐like electron component in the parallel velocity distribution at ∼0.5VAeThe plateau‐like distribution in the parallel direction is caused by the nonparallel wave modes through Landau resonance [ABSTRACT FROM AUTHOR]

Chorus waves are coherent electromagnetic emissions in the inner magnetosphere, generally composed of a series of discrete and repetitive elements. These elements usually have rising frequency chirpings, which are occasionally mingled with one or several elements with a hooked spectrogram. In this letter, we perform a one‐dimensional general curvilinear particle‐in‐cell (gcPIC)‐δf simulation of chorus waves excited by temperature anisotropic electrons in a dipole magnetic field, and identify one chorus element with a hooked spectrogram, which is embedded in several chorus elements with a rising frequency chirping. In the hooked chorus element, the frequency increases first, and then decreases. Interestingly, we find that there are clear electron holes in the electron velocity space, no matter whether the chorus element has a rising‐tone or hooked‐tone spectrogram. Our study presents an important clue for the theoretical analysis of the frequency chirping in the chorus waves. Plain Language Summary: Chorus waves are intense electromagnetic emissions that occur naturally in the Earth's inner magnetosphere, which usually contain a series of discrete chirping elements. Via a one‐dimensional (1‐D) general curvilinear particle‐in‐cell (gcPIC)‐δf simulation in a dipole field, we have investigated the frequency chirping of chorus waves, excited by anisotropic electrons. After the waves leave away from the equator, their frequencies chirp, exhibiting as quasi‐monochromatic emissions. Together with two rising‐tone elements, there is one element with a hooked spectrogram, whose frequency first increases, and then decreases. This is consistent with the satellite observation of hooked‐tone elements. In the v||‐ζ plane, there are clear electron holes along with both rising‐tone and hooked‐tone chorus elements. Our study provides a novel idea for the theoretical work of chorus frequency chirping. Key Points: Via a 1‐D gcPIC‐δf simulation in a dipole field, we have investigated the repetitive chorus emissions excited by anisotropic electronsTwo of the chorus elements have rising frequency chirpings, mingled with one element with a hooked‐tone structureAlong with the generation of both rising‐tone and hooked‐tone chorus elements, there are clear electron holes in the v||‐ζ ${v}_{\vert \vert }\mbox{-}\zeta $ plane [ABSTRACT FROM AUTHOR]

Recent satellite observations, combined with instability analyses, have shown that the background plasma density variation can modulate the magnetosonic (MS) waves by controlling the wave excitation. However, the detailed modulation process needs to be identified since the MS waves propagate nearly perpendicular to the background magnetic field. In this study, we investigate the MS wave modulation by background plasma density using a 2‐D general curvilinear particle‐in‐cell simulation in the meridian plane of a dipole magnetic field. The simulation model consists of three plasma components: the background cold electrons and protons and ring distribution protons. We find that MS waves can be locally generated by ring distribution protons in the low plasma density region, while no MS wave is generated in the high density region. These MS waves are confined near the local source region since they are damped by the background cold plasma. The background protons gain more energy than background electrons, implying the plasmaspheric protons may dominate the modulation of MS waves. Moreover, we have also investigated the generation and propagation of MS waves in the plasmapause density structure. Our simulation results demonstrate that the background plasma density variation can modulate the MS waves and may play an important role in the spatial distribution of MS waves. Key Points: We have investigated the magnetosonic (MS) wave modulation by background plasma density using a 2‐D general curvilinear particle‐in‐cell simulationMS waves can be locally excited by proton ring distribution in the low plasma density region, while no MS wave is generated in the high density regionThese MS waves are confined near the local source region since they are damped by the background cold plasma [ABSTRACT FROM AUTHOR]

In this paper, by performing a two-dimensional particle-in-cell simulation, we investigate magnetic reconnection in the downstream of a quasi-perpendicular shock. The shock is nonstationary, and experiences cyclic reformation. At the beginning of the reformation process, the shock front is relatively flat, and part of the upstream ions are reflected by the shock front. The reflected ions move upward in the action of the Lorentz force, which leads to the upward bending of the magnetic field lines at the foot of the shock front, and then a current sheet is formed due to the squeezing of the bending magnetic field lines. The formed current sheet is brought toward the shock front by the solar wind, and the shock front becomes irregular after interacting with the current sheet. Both the current sheet carried by the solar wind and the current sheet associated with the shock front are then fragmented into many small filamentary current sheets. Electron-scale magnetic reconnection may occur in several of these filamentary current sheets when they are convected into the downstream, and magnetic islands are generated. A strong reconnection electric field and energy dissipation are also generated around the X line, and a high-speed electron outflow is also formed. [ABSTRACT FROM AUTHOR]

During magnetic reconnection, such as in the magnetopause of magnetized planets, the upstream plasma conditions between the two inflow regions are usually different. In this paper, we demonstrate that such a kind of asymmetric reconnection can be studied in the laboratory using the recently proposed experimental scheme where reconnection is driven by laser-powered capacitor coils. Two-dimensional particle-in-cell simulations on the plane in a cylindrical coordinate are conducted to study magnetic reconnection with the inflow along the direction. Magnetic reconnection is found to be asymmetric with a stronger magnetic field in the inner (small) inflow region and a weaker magnetic field in the outer (large) inflow region due to the cylindrical symmetric geometry. Electron crescent velocity distributions are observed near the flow stagnation point while ion crescent velocity distributions are observed in the region with Larmor electric field. The out-of-plane Hall magnetic field is asymmetric between the two inflow regions with a larger spatial scale in the outer inflow region. This asymmetric Hall magnetic field configuration is different from that in previous studies. The typical reconnection rate increases with a stronger driver and the highest rate is around 0.2 , where is the typical value of the magnetic field and is the Alfven speed. This study provides a new method to experimentally study asymmetric reconnection in the laboratory and has potential applications regarding magnetic reconnection in the magnetopause of magnetized planets. [ABSTRACT FROM AUTHOR]

By comparing an electron‐only reconnection event with a traditional reconnection event observed in the magnetotail, we illustrate the differences between and similarities of the two events. The electron behaviors are very similar in both events, but intensities of the electron flows and temperature in the traditional reconnection are much stronger than those in the electron‐only reconnection. The Hall electric field in the traditional reconnection occurs on the ion‐scale and is deflected from the normal direction by the significant magnetic field reconnected, while this field varies on the electron‐scale and points to the middle plane in the electron‐only reconnection. The comparison indicates that the electrons are undergoing the same process in both events, and the electron‐only reconnection was prior to the traditional reconnection. The Hall electric field could control the form of reconnection: producing either electron‐only reconnection or traditional reconnection. Key Points: We illustrate the differences between and similarities of the electron‐only reconnection and the normal reconnectionThe electron reconnection was prior to the normal reconnectionThe Hall electric field and the reconnected magnetic field intensity could control the form of reconnection [ABSTRACT FROM AUTHOR]

Magnetic fields are transported and tangled by turbulence, even as they lose identity due to nonideal or resistive effects. In a balanced field, lines undergo stretch-twist-fold processes. The curvature field, a scalar that measures the tangling of the magnetic field lines, is studied in detail here, in the context of magnetohydrodynamic turbulence. A central finding is that the magnitudes of curvature and magnetic field are anticorrelated. A high curvature colocates with a low magnetic field, which gives rise to power-law tails of the probability density function of the curvature field. The curvature drift term that converts magnetic energy into flow and thermal energy largely depends on the curvature field behavior, a relationship that helps to explain particle acceleration due to the curvature drift. This adds as well to evidence that turbulent effects most likely play an essential role in particle energization since turbulence drives stronger tangled field configurations, and therefore curvature. [ABSTRACT FROM AUTHOR]

Rising‐tone chorus waves have already been successfully produced in a mirror magnetic field with the use of one‐ and two‐dimensional particle‐in‐cell (PIC) simulations. However, in reality, the background magnetic field in the inner Earth's magnetosphere is a dipole magnetic field, unlike symmetric mirror fields. In this paper, with the two‐dimensional (2‐D) general curvilinear PIC (gcPIC) code, we investigate the generation of rising‐tone chorus waves in the dipole magnetic field configuration. The plasma consists of three components: immobile ions, cold background, and hot electrons. In order to save computational resource, the topology of the magnetic field is roughly equal to that at L = 0.6 RE, although the plasma parameters corresponding to those at L = 6 RE (RE is the Earth's radius) are used. Whistler mode waves are first excited around the magnetic equator by the hot electrons with a temperature anisotropy. The excited whistler mode waves propagate almost parallel and antiparallel to the background magnetic field in their source region, which is limited at ∣λ ∣  ≤ 3° (where λ is the magnetic latitude). When the waves leave from the source region and propagate toward high latitudes, both their amplitude and wave normal angle become larger. However, the group velocity of the waves is directed toward high latitudes almost along the magnetic field. During such a process, the waves have a frequency chirping, as shown by a rising tone in the frequency‐time spectrogram. To our best knowledge, it is for the first time that rising‐tone chorus are generated in a dipole magnetic field with a PIC simulation. Key Points: The rising‐tone chorus waves are generated in a dipole magnetic field with a 2‐D gcPIC simulation for the first timeWhistler waves are excited around the magnetic equator and propagate almost along the background magnetic fieldWhen whistler waves propagate toward high latitudes, their wave normal angles increase with the growth of the latitude [ABSTRACT FROM AUTHOR]

A novel magnetically driven reconnection platform was created by a pair of U-shaped Cu coils that connect two parallel Cu plates irradiated at a focused laser intensity of ∼3 × 1016 W/cm2 and characterized using ultrafast proton radiography. The proton data show two prolate voids, each corresponding to the coil current, with an inferred maximum magnitude of 57 ± 4 kA. A center "flasklike" feature was also observed in the proton radiographs. By prescribing electromagnetic fields associated with magnetic reconnection in proton ray tracing simulations, characteristics of this center feature were reproduced. These results demonstrate the robustness of the laser-driven capacitor coils for generating strong magnetic fields and provide promise of using such coils as a viable platform for studying magnetically driven reconnection in the laboratory. [ABSTRACT FROM AUTHOR]

The primary power gap of chorus waves is statistically investigated using 7‐year waveform data from THEMIS probes for the first time. Overall, ~2/3 of chorus events have a power gap between the lower and upper bands. Both the gap frequency and the frequency bandwidth of the gap have a broad distribution, which peaks at frequencies of ~0.49fce and ~0.07fce, respectively. The gap frequency tends to increase with increasing |MLAT| (magnetic latitude), while the frequency width gradually increases with L‐shell. In most of banded events, the peak frequency of upper band is roughly twice that of lower band. Two types of events are studied. For Type I events, one population is located around Pup/Plow = 10−1, and the other is around Pup/Plow = 10−3. However, Type II events are roughly concentrated around Pup/Plow = 10−3. The gap frequency positively correlates with the frequency of upper band. Our study provides detailed observational constraints on potential mechanisms of the power gap formation. Key Points: We provide the first statistical study to comprehensively describe this mysterious power gap of chorus waves in the Earth's magnetosphereOverall, there are about 2/3 of events identified to have a power gap between two bands, and 1/3 of events are observed without a power gapWe have studied the distribution of both the gap frequency and frequency width of the power gap and their dependencies on two bands [ABSTRACT FROM AUTHOR]

With the measurements of the Magnetospheric Multiscale mission at the magnetopause, we investigated the electron distribution and the whistler waves associated with a series of six ion‐scale flux transfer events (FTEs). Based on the magnetic field signature, each FTE can be divided into the core region and the draping region. In the draping regions of the most FTEs, the low‐energy electrons displayed a bidirectional field‐aligned distribution. The medium‐energy electrons showed a field‐aligned or beam distribution in the leading part, while a pancake distribution was presented for the electrons in the trailing part of the draping region, which has never been reported previously. The close correlation between the pancake distribution and the compression of the localized magnetic field suggests that the pancake distribution may be due to the betatron acceleration. The whistler waves associated with the FTEs were observed and categorized into the lower and upper bands according to the frequency range. The lower band whistler waves propagated in variable directions and therefore could be generated locally. The trailing part of the draping region with the electron pancake distribution was considered to be one possible source region. On the contrary, the upper band whistler waves were all found in the core region and propagated antiparallel to the magnetic field and therefore originated from the same source region. The observations confirmed that the FTEs are important channels for the mass and wave transport between the magnetosheath and the inner magnetosphere, and the electron dynamics can be modified during the FTE evolution. Key Points: The electron distribution and whistler wave characteristics are significantly different in the core and draping regions of the FTEsThe close correlation between the pancake distribution and compression of the localized magnetic field is observed in the trailing partThe whistler waves associated with the FTEs are categorized into the lower and upper bands according to the frequency range [ABSTRACT FROM AUTHOR]

Abstract: The excitation of magnetosonic waves in the meridian plane of a rescaled dipole magnetic field is investigated, for the first time, using a general curvilinear particle‐in‐cell simulation. Our simulation demonstrates that the magnetosonic waves are excited near the equatorial plane by tenuous ring distribution protons. The waves propagate nearly perpendicularly to the background magnetic field along both radially inward and outward directions. Different speeds of inward and outward propagation result in the asymmetrical distribution about the source region. The waves are accompanied by energization of both cool protons and electrons near the wave source region. The cool protons are heated perpendicularly, while the cool electrons can be heated in the parallel direction and also experience enhanced perpendicular drift at the presence of intense wave power. The implications of simulation results to the observations of magnetosonic waves and related particle heating in the inner magnetosphere are also discussed. [ABSTRACT FROM AUTHOR]

In this paper, with a two-dimensional particle-in-cell simulation model, we study the formation of power-law spectra of energetic electrons in multiple X line magnetic reconnection with a strong guide field. The processes of both magnetic reconnection and electron acceleration can be separated into two stages. In the first stage, two X lines appear at the border and center of the simulation domain, and then, two magnetic islands are formed. In this stage, electrons are accelerated mainly by parallel electric fields, and a power-law spectrum of energetic electrons is generated with the appearance of the second X line. In the second stage, the two magnetic islands are merged into one big island. Besides parallel electric fields, the Fermi mechanism also plays an important role in the production of energetic electrons, and its contribution is comparable to that of parallel electric fields when the electron energy is sufficiently large. In this stage, the generated power-law spectrum of energetic electrons becomes hard. In general, the acceleration efficiencies by both the parallel electric fields and Fermi mechanism become higher with the increase in electron energy, and the tendency is more obvious for the Fermi mechanism. Therefore, both the parallel electric fields and Fermi mechanism are important in the formation of power-law spectra of energetic electrons during multiple X line reconnection. We also investigate the influences of the ion-electron temperature ratio, guide field, and initial flux perturbation on the formed power-law spectra of energetic electrons. [ABSTRACT FROM AUTHOR]

In this paper, by taking advantage of a two-dimensional particle-in-cell simulation model, we study the structure of the out-of-plane magnetic field (Hall magnetic field) during asymmetric magnetic reconnection without a guide field, and the associated in-plane current system is also analyzed. The evolution of asymmetric reconnection has two stages. At the first stage, the electrons move toward the X line along the separatrix in the magnetosheath side, and depart from the X line along the separatrix in the magnetosphere side. Another electron flow toward the X line exists above the separatrix in the magnetosphere side. The resulted in-plane current system, which is mainly determined by electron dynamics, generates the quadrupolar structure of the Hall magnetic field, where the two quadrants in the magnetosheath side are much stronger than those in the magnetosphere side. At the second stage, besides these electron flows, an additional electron flow away from the X line is formed along the magnetic field below the separatrix in the magnetosheath side. A hexapolar structure of the Hall magnetic field is then generated by such a current system. [ABSTRACT FROM AUTHOR]

With the measurements of magnetic field of Venus Express (VEX), magnetic coplanarity and minimum variance analysis (MVA) methods are analyzed and their validity is tested to determine the normal of Venusian bow shocks. It is found that MVA method is the better than magnetic coplanarity, and 95% shock crossings can be accurately determined by the method. However, the occurrence of the shock normal which is not determined accurately by magnetic coplanarity increases with the decrease of the solar zenith angle (SZA). At the same time, compared with quasi-parallel shocks, there is more occurrence of the shock normal which cannot be determined accurately by magnetic coplanarity for quasi-perpendicular shocks. [ABSTRACT FROM AUTHOR]

It is well known that the parallel cuts of the parallel and perpendicular electric field in electron phase-space holes (electron holes) have bipolar and unipolar structures, respectively. Recently, electron holes in the Earth's plasma sheet have been observed by THEMIS satellites to have detectable fluctuating magnetic field with regular structures. Du et al. () investigated the evolution of a one-dimensional (1D) electron hole with two-dimensional (2D) electromagnetic particle-in-cell (PIC) simulations in weakly magnetized plasma (Ω< ω, where Ω and ω are the electron gyrofrequency and electron plasma frequency, respectively), which initially exists in the simulation domain. The electron hole is unstable to the transverse instability and broken into several 2D electron holes. They successfully explained the observations by THEMIS satellites based on the generated magnetic structures associated with these 2D electron holes. In this paper, 2D electromagnetic particle-in-cell (PIC) simulations are performed in the x- y plane to investigate the nonlinear evolution of the electron two-stream instability in weakly magnetized plasma, where the background magnetic field $(\mathbf{B}_{0} =B_{0}\vec{\mathbf{e}} _{x})$ is along the x direction. Several 2D electron holes are formed during the nonlinear evolution, where the parallel cuts of E and E have bipolar and unipolar structures, respectively. Consistent with the results of Du et al. (), we found that the current along the z direction is generated by the electric field drift motion of the trapped electrons in the electron holes due to the existence of E, which produces the fluctuating magnetic field δB and δB in the electron holes. The parallel cuts of δB and δB in the electron holes have unipolar and bipolar structures, respectively. [ABSTRACT FROM AUTHOR]

A test particle code is employed to explore the dynamics of charged particles and perpendicular diffusion in turbulent magnetic field, where a three-dimensional (3D) isotropic turbulence model is used in this paper. The obtained perpendicular diffusion at different particle energies is compared with that of the nonlinear guiding center (NLGC) theory. It is found that the NLGC theory is consistent with test particle simulations when the particle energies are small. However, the difference between the NLGC theory and test particle simulations tends to increase when the particle energy is sufficiently large, and the threshold is related to the turbulence bend-over length. In the NLGC theory, the gyrocenter of a charged particle is assumed to follow the magnetic field line. Therefore, when the particle has sufficiently large energy, its gyroradius will be larger than the turbulence bend-over length. Then the particle can cross the magnetic field lines, and the difference between the test particle simulations and NLGC theory occurs. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) particle-in-cell simulations are performed to investigate the structures of the out-of-plane magnetic field in magnetic island, which is produced during anti-parallel collisionless magnetic reconnection. Regular structures with alternate positive and negative values of the out-of-plane magnetic field along the x direction are formed in magnetic island. The generation mechanism of such structures is also proposed in this paper, which is due to the Weibel instability excited by the temperature anisotropy in magnetic island. [ABSTRACT FROM AUTHOR]

Electron phase space holes (electron holes) are found to be unstable to the transverse instability. Two-dimensional (2D) electromagnetic particle-in-cell simulations are performed to investigate the structures of the fluctuating magnetic field associated with electron holes. The combined actions between the transverse instability and the stabilization by the background magnetic field (B0=B0evector x) lead a one-dimensional electron hole into several 2D electron holes which are isolated in both the x and y directions. The electrons trapped in these 2D electron holes suffer the electric field drift vE=E×B0/B02 due to the existence of the perpendicular electric field Ey, which generates the current along the z direction. Then, the unipolar and bipolar structures are formed for the parallel cut of the fluctuating magnetic field along the x and y directions, respectively. At the same time, these 2D electron holes move along the x direction, and the unipolar structures are formed for the parallel cut of the fluctuating magnetic field along the z direction. [ABSTRACT FROM AUTHOR]

Two-dimensional particle-in-cell simulations are performed to investigate the formation of electron density depletions in collisionless magnetic reconnection. In anti-parallel reconnection, the quadrupole structures of the out-of-plane magnetic field are formed, and four symmetric electron density depletion layers can be found along the separatrices due to the effects of magetic mirror. With the increase of the initial guide field, the symmetry of both the out-of-plane magnetic field and electron density depletion layers is distorted. When the initial guide field is sufficiently large, the electron density depletion layers along the lower left and upper right separatrices disappear. The parallel electric field in guide field reconnection is found to play an important role in forming such structures of the electron density depletion layers. The structures of the out-of-plane magnetic field By and electron depletion layers in anti-parallel and guide field reconnection are found to be related to electron flow or in-plane currents in the separatrix regions. In anti-parallel reconnection, electrons flow towards the X line along the separatrices, and are directed away from the X line along the magnetic field lines just inside the separatrices. In guide field reconnection, electrons can only flow towards the X line along the upper left and lower right separatrices due to the existence of the parallel electric field in these regions. [ABSTRACT FROM AUTHOR]

In collisionless reconnection, the magnetic field near the separatrix is stronger than that around the X-line, so an electron-beam can be formed and flows toward the X-line, which leads to a decrease of the electron density near the separatrix. Having been accelerated around the X-line, the electrons flow out along the magnetic field lines in the inner side of the separatrix. A quadruple structure of the Hall magnetic field By is formed by such a current system. A 2D particle-in-cell (PIC) simulation code is used in this paper to study the collisionless magnetic reconnection without an initial guide field. The current system described above is proved by the simulations. Furthermore, the position of the peak of the Hall magnetic field By is found to be between the separatrix and the center of the current sheet, which is verified by Cluster observations. [ABSTRACT FROM AUTHOR]

Two-dimensional hybrid simulations are performed to investigate the parametric decay of a monochromatic Alfven wave in low beta plasma. Both the linearly and left-hand polarized pump Alfven waves are considered in the paper. For the linearly polarized pump Alfven wave, either a parallel or obliquely propagating wave can lead to the decay along the perpendicular direction. Initially, the parametric decay takes place along the propagating direction of the pump wave, and then the decay occurs in the perpendicular direction. With the increase of the amplitude and the propagating angle of the pump wave (the angle between the propagating direction of the pump wave and the ambient magnetic field), the spectral range of the excited waves becomes broad in the perpendicular direction. But the effects of the plasma beta on the spectral range of the excited waves in perpendicular direction are negligible. However, for the left-hand polarized pump Alfven wave, when the pump wave propagates along the ambient magnetic field, the parametric decay occurs nearly along the ambient magnetic field, and there is no obvious decay in the perpendicular direction. Significant decay in the perpendicular direction can only be found when the pump wave propagates obliquely. [ABSTRACT FROM AUTHOR]