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Subauroral red (SAR) arcs are commonly observed ionospheric red line emissions. They are usually attributed to subauroral electron heating by inner magnetospheric heat flux (IMHF). However, the role of IMHF in changing the ionosphere‐thermosphere (IT) still remains elusive. We conduct controlled numerical experiments with the Thermosphere‐Ionosphere Electrodynamic General Circulation Model (TIEGCM). Coulomb collisional heat flux derived with the Comprehensive Inner Magnetosphere Ionosphere (CIMI) model and empirical subauroral polarization streams (SAPS) are implemented in TIEGCM. The heat flux causes electron temperature enhancement, electron density depletion, and consequently SAR arcs formed in the dusk‐to‐midnight subauroral ionosphere region. SAPS cause more substantial plasma and neutral heating and plasma density variations in a broader region. The maximum enhancement of subauroral red line emission rate is comparable to that caused by the heat flux. However, the visibility of SAR arcs also depends on the relative enhancement to the background brightness. Plain Language Summary The Earth's topside atmosphere is subject to energy inputs from the magnetosphere and solar wind. In addition to the Joule heating generated by high latitude plasma convection and energy flux carried by precipitating magnetospheric particles, magnetospheric energy can be also deposited in the ionosphere‐thermosphere via heat flux, that is, energy flows carried by low‐energy thermal electrons. When hot ions in the ring current collide with the cold plasma in the plasmasphere, heat conduction occurs and the resultant heat flux is transported along geomagnetic field lines to the footprint ionosphere. The additional heating raises the electron temperature in the subauroral ionosphere and modifies the ionosphere‐thermosphere states. This study uses first‐principles inner magnetosphere model and ionosphere‐thermosphere model to illustrate the thermodynamic coupling effects between the topside ionosphere and the magnetosphere, and compare the relative significance between the heat flux and plasma convection due to electrodynamic coupling. The numerical experiments show that the heat flux primarily increases electron temperature while subauroral plasma flow heats up both plasma and neutrals. Despite different physical mechanisms, the heat flux and subauroral plasma convection make comparable contributions to red line emission rates in the subauroral region. Key Points Inner magnetospheric heat flux increases subauroral ionospheric electron temperature and depletes the density to form subauroral red arcs Compared to subauroral polarization streams, the heat flux heating effects are only confined to electrons in the subauroral region The heat flux produces negligible impacts on ions and neutrals compared to subauroral polarization streams

Experimental measurements using the OMEGA EP laser facility demonstrated direct laser acceleration (DLA) of electron beams to (505 ± 75) MeV with (140 ± 30) nC of charge from a low-density plasma target using a 400 J, picosecond duration pulse. Similar trends of electron energy with target density are also observed in self-consistent two-dimensional particle-in-cell simulations. The intensity of the laser pulse is sufficiently large that the electrons are rapidly expelled along the laser pulse propagation axis to form a channel. The dominant acceleration mechanism is confirmed to be DLA and the effect of quasi-static channel fields on energetic electron dynamics is examined. A strong channel magnetic field, self-generated by the accelerated electrons, is found to play a comparable role to the transverse electric channel field in defining the boundary of electron motion.

Electron heating at collisionless shocks in space is a combination of adiabatic heating due to large‐scale electric and magnetic fields and non‐adiabatic scattering by high‐frequency fluctuations. The scales at which heating happens hints to what physical processes are taking place. In this letter, we study electron heating scales with data from the Magnetospheric Multiscale (MMS) spacecraft at Earth's quasi‐perpendicular bow shock. We utilize the tight tetrahedron formation and high‐resolution plasma measurements of MMS to directly measure the electron temperature gradient. From this, we reconstruct the electron temperature profile inside the shock ramp and find that the electron temperature increase takes place on ion or sub‐ion scales. Further, we use Liouville mapping to investigate the electron distributions through the ramp to estimate the deHoffmann‐Teller potential and electric field. We find that electron heating is highly non‐adiabatic at the high‐Mach number shocks studied here. Plain Language Summary Shock waves appear whenever a supersonic medium, such as a plasma, encounters an obstacle. The plasma, which consists of charged ions and free electrons, is heated by the shock wave through interactions with the electromagnetic fields. In this work, we investigate how electrons are heated at plasma shocks. A key parameter to electron heating is the thickness of the layer where the heating takes place. Here, we use observations from the four Magnetospheric Multiscale spacecraft that regularly cross the standing bow shock that forms when the supersonic plasma, known as the solar wind, encounters Earth's magnetic field. We find that the thickness of the shock is larger than previously reported and is on the scales where ion physics dominate. We also find that the electron heating deviates significantly from simple adiabatic heating. Key Points Using multipoint data from Magnetospheric Multiscale, we find that electron heating takes place on ion scales in the quasi‐perpendicular shock ramp We show that the time series of the temperature does not represent the spatial profile due to varying shock ramp speed Electron distributions in the ramp and downstream of the shock show that electrons are heated non‐adiabatically

The relative contribution of adiabatic and non‐adiabatic processes to electron heating across collisionless shocks remains an open question. We analyze the evolution of suprathermal electrons across 310 quasi‐perpendicular shocks with Alfvénic Mach numbers in the normal‐incidence frame MA−NIF$\\left({M}_{A-NIF}\\right)$ranging from 1.7 to 48, using in situ measurements of Earth's bow shock by the Magnetospheric Multiscale (MMS) spacecraft. We introduce a novel non‐adiabaticity measure derived from the electron distribution function and based on Liouville's theorem. Our results reveal, for the first time, that the electron heating mechanism is governed by the Alfvénic Mach number in the de Hoffman‐Teller frame MA−HT$\\left({M}_{A-HT}\\right)$ , with a transition from predominantly adiabatic to non‐adiabatic heating occurring at MA−HT≳30${M}_{A-HT}\\gtrsim 30$ . Furthermore, by examining the spectral index of the suprathermal electron distribution, we find that for shocks exhibiting dominant non‐adiabatic electron dynamics, the observed electron heating is consistent with the predictions of the stochastic shock drift acceleration (SSDA) mechanism. Plain Language Summary Understanding how electrons get heated across shock waves in space is a challenging scientific question. These shocks can heat electrons through different processes: some involve smooth, gradual changes (adiabatic), while others involve more chaotic interactions (non‐adiabatic). In this study, we looked at data from 310 shock events near Earth using the Magnetospheric Multiscale (MMS) spacecraft, focusing on shocks with a normal vector almost perpendicular to the direction of the magnetic field. We developed a new way to measure how much of the heating is due to non‐adiabatic processes by studying the patterns in how the electrons are distributed in energy. Our findings show that the way electrons are heated is mainly controlled by a dimensionless parameter called the Alfvénic Mach number, which describes how fast the shock is moving compared to a specific speed in the plasma, in a particular frame of reference (the de Hoffman‐Teller frame). We discovered that when this Mach number is above about 30, the heating changes from being mostly adiabatic to mostly non‐adiabatic. Additionally, we found that when non‐adiabatic heating is dominant, it matches well with a known process called stochastic shock drift acceleration (SSDA). Key Points We analyze electron heating across 310 quasi‐perpendicular shocks observed by MMS We use a Liouville mapping technique to show the electron heating mechanism is controlled by the Mach number in the de Hoffmann‐Teller frame We find that electron heating at shocks with dominant nonadiabatic dynamics aligns with the stochastic shock drift acceleration mechanism

In Saturn's magnetosphere, the plasma temperature increases with radial distance, requiring a heating mechanism to counteract the adiabatic cooling effect of expanding plasma. To explore potential heating source, we perform a statistical study about intermittent structures and intermittent heating in Saturn's magnetosphere based on the observations from the Cassini spacecraft. Partial Variance of Increments (PVI) technique is used to measure the intermittency of magnetic field and identify the intermittent structures. It is found that the electron temperature has a rising trend as the increase of magnetic field intermittency, implying the occurrence of electron heating in the intermittent structures. Additionally, the turbulence heating rate also exhibits an increasing trend as the increase of probability density of intermittent structures. Our results evidence the effect of intermittent heating in the Saturn's magnetosphere, and suggest that intermittent structures with high magnetic field intermittency play an important role in turbulence heating. Plain Language Summary In Saturn's magnetosphere, the plasma temperature increases with radial distance, requiring a heating mechanism to counteract the adiabatic cooling effect of expanding plasma. Intermittent structure, which has been widely investigated in solar wind turbulence to explain the solar wind plasma heating, have never been considered in Saturn's magnetosphere. In this work, we investigate the intermittent structures and intermittent heating in Saturn's magnetosphere based on the observations from the Cassini spacecraft. We identify a positive correlation between electron temperature and magnetic field intermittency, as well as a positive correlation between turbulence heating rate and the probability density of intermittent structures. These results suggest that intermittent structures contribute to turbulent heating in Saturn's magnetosphere. Key Points Intermittent structures are identified in Saturn's magnetosphere based on partial variance of increment technique Electron temperature has a rising trend as the intermittency increases, indicating the occurrence of intermittent electron heating Turbulence heating rate increases as intermittent density rises, implying a contribution of intermittent structure to turbulence heating

Understanding electron kinetic processes is crucial for elucidating the energy conversion mechanisms in magnetic reconnection. Non‐Maxwellian electron distributions are strong indicators of kinetic‐scale processes near the electron diffusion region, yet they remain incompletely understood. Using in‐situ spacecraft data from 29 magnetopause reconnection events, we unambiguously identify a non‐Maxwellian capsule electron distribution near the electron diffusion region. This distribution comprises an elongated component parallel with the magnetic field at lower energies and a butterfly component (with peaks at pitch angles near 45°$45{}^{\\circ}$and 135°$135{}^{\\circ}$ ) at higher energies. We provide evidence that these distributions are partly linked to electron trapping and preferential heating along the direction of magnetic fields. The parallel electric potentials needed for the parallel heating may be linked to kinetic Alfvén waves. These capsule‐like electron distributions are also found to generate whistler emissions. Our results suggest that these kinetic processes are prevalent in magnetic reconnection. Plain Language Summary Understanding how electrons behave in space plasmas, especially during magnetic reconnection, is crucial. Using data from spacecraft, we studied electron behavior near this reconnection region and identified a unique pattern called a capsule distribution. This distribution combines two shapes: an elongated component at lower energies and a butterfly like component at higher energies. These distributions are closely linked to whistler wave emissions. Our research provides evidence that these capsule‐like distributions indicate electron trapping and heating by parallel electric fields. The parallel electric field and electric potentials may be related to kinetic Alfven waves. These kinetic processes, captured by emitted whistler waves, are prevalent near the diffusion region, offering deeper insights into electron heating during magnetic reconnection. Key Points Non‐Maxwellian capsule electron distributions near the electron diffusion region comprises an elongated component and a butterfly component Capsule‐like electron distributions generate whistler emissions in the presence of unusual temperature anisotropy T‖/T⊥<1$\\left({T}_{\\Vert }/{T}_{\\perp }< 1\\right)$Capsule electron distributions suggest electron trapping and preferential heating by parallel electric fields

How particles are being energized by turbulent electromagnetic fields is an outstanding question in plasma physics and astrophysics. This paper investigates the electron acceleration mechanism in strong turbulence (δB/B0 ∼ 1) in the Earth's magnetosheath based on the novel observations of the Magnetospheric Multiscale mission. We find that electrons are magnetized in turbulent fields for the majority of the time. By directly calculating the electron acceleration rate from Fermi, betatron mechanism, and parallel electric field, it is found that electrons are primarily accelerated by the parallel electric field within coherent structures. Moreover, the acceleration rate by parallel electric fields increases as the spatial scale reduces, with the most intense acceleration occurring over about one ion inertial length. This study is an important step toward fully understanding the turbulent energy dissipation in weakly collisional plasmas. Plain Language Summary The magnetosheath is one of the most turbulent environments in near‐Earth space, which is very beneficial to the study of collisionless turbulent plasma. The mechanism of turbulent energy dissipation and the consequent plasma heating is not fully understood. The Magnetosphere Multiscale mission provides high‐time cadence data and simultaneous multi‐spacecraft measurements at very small inter‐spacecraft separations, that can measure important quantities related to dissipation and heating at kinetic scales. This paper investigates how electrons are being accelerated through the dissipation of magnetic energy in nonlinear turbulence in the Earth's magnetosheath. We classify the acceleration mechanisms into three types: Fermi mechanism, betatron mechanism, and E|| acceleration. By directly calculating and comparing these mechanisms, we find electrons are predominantly accelerated by parallel electric fields within coherent structures. The E|| acceleration is the most effective around the ion inertial length. Key Points Electrons are primarily accelerated by the parallel electric field in the magnetosheath turbulence The E|| acceleration mostly occurs within the coherent structures through Joule‐type dissipation The average E|| acceleration rate increases with the decreasing local spatial scale

From 10 to 12 May 2024, a series of coronal mass ejections led to one of the strongest geomagnetic storms of the century, referred to as the Mother's Day or Gannon Storm. MMS's position on the dayside magnetosphere on 11 May provided observations of a strongly driven and compressed ∼7RE $\\left(\\sim 7\\ {R}_{E}\\right)$ reconnecting magnetopause. Because of the driving conditions, the magnetopause became saturated with O+ ${O}^{+}$ outflows that dominated the mass density of the plasma environment. In the reconnecting magnetopause, MMS observes signatures of parallel electron heating on the magnetopause's magnetosheath side, but anomalous and significant electron cooling, especially from the perpendicular electron temperature on the magnetosphere side, possibly driven by additional mechanisms besides reconnection. Even with the strong driving and O+ ${O}^{+}$ outflows, we find an expected (0.19±0.04) $(0.19\\pm 0.04)$ normalized reconnection rate for the primary exhaust, indicating insensitivity to these conditions. The unnormalized rate, however, is atypically large and scales with the driving conditions.

Magnetic flux ropes (MFRs) are significant regions for the production of energetic electrons in space and astrophysical plasmas. However, the research on electron heating and acceleration driven by turbulence in MFRs is still quite rare. Utilizing in‐situ measurements from MMS satellite, we study electron heating and associated electrostatic waves in an ion‐scale MFR within terrestrial super‐Alfvén plasma flow. Lower‐hybrid drift waves, generated locally in this MFR, can contribute to the perpendicular heating through their electrostatic potential accelerating electrons. The parallel heating is attributed to antiparallel propagating electron beams. These beams excite the broadband electrostatic waves that can interact with electrons and thermalize electrons. Our study promotes understanding of electron energization driven by plasma waves and wave‐particle interaction in MFRs. Plain Language Summary Magnetic flux ropes (MFRs) are ubiquitous magnetic structures existing in space and astrophysical plasmas. They are produced by magnetic reconnection, and can in turn trigger small‐scale reconnection and modulate reconnection process. In addition, they are widely used to explain electron heating and acceleration, which is a long‐standing problem in space and astrophysical plasmas. Thus, they receive significant attention nowadays. However, there has been still lack of research on electron heating and acceleration driven by turbulence in MFRs. In our study, we find perpendicular electron heating is partly attributed to lower‐hybrid drift waves and parallel electron heating is closely related to antiparallel propagating electron beams, which excites strong broadband electrostatic waves that can result in thermalization of electrons. Key Points Electron heating is detected inside an ion‐scale magnetic flux rope Lower‐hybrid drift waves can contribute to the perpendicular heating through their electrostatic potential accelerating electrons The parallel heating is caused by electron beams, which excite the broadband electrostatic waves that can thermalize electrons

Electrostatic Cyclotron Harmonic (ECH) waves have been considered a potential cause of pitch angle scattering of electrons in the energy range from a few hundred eV to tens of keV. Theoretical studies have suggested that scattering by ECH waves is enhanced at lower pitch angles near the loss cone. Due to the insufficient angular resolution of particle detectors, it has been a great challenge to reveal ECH‐driven scattering based on electron measurements. This study reports on variations in electron pitch angle distributions associated with ECH wave activity observed by the Arase satellite. The variation is characterized by a decrease in fluxes near the loss cone, and energy and pitch angle dependence of the flux decrease is consistent with the region of enhanced pitch angle scattering rates predicted by the quasi‐linear diffusion theory. This study provides direct evidence for energy‐pitch angle dependence of pitch angle scattering driven by ECH waves. Plain Language Summary Near‐Earth space is surrounded by the ionized gas so‐called “plasma,” which is trapped by the Earth's intrinsic magnetic field, forming the magnetosphere of the Earth. A variety of waves are present in the plasma, and interaction between plasma and waves results in acceleration, transport, and loss of plasma in the Earth's magnetosphere. Electrostatic Cyclotron Harmonic (ECH) waves are one of the intense waves present in the magnetosphere. The waves are one candidate causing electron heating and loss of electrons in that region. Theoretical studies suggest that ECH waves contribute to a change in electron pitch angle, the angle between the velocity vector of electrons and the magnetic field, in the energy range from a few hundred electron volts to several tens of kilo‐electron‐volts. This process causes electron precipitation into the Earth's atmosphere, resulting in permanent loss of electrons from the magnetosphere. Here, based on the observation by the Arase satellite together with a theoretical framework, we show the observational evidence that ECH waves can cause significant pitch angle changes of electrons in the energy range of 7–20 keV. This study strongly suggests that ECH waves contribute to electron dynamics in the inner magnetosphere through wave‐particle interactions. Key Points A decrease in electron fluxes is observed near the loss cone associated with an enhanced Electrostatic Cyclotron Harmonic (ECH) wave activity Energy‐pitch angle dependent flux decrease is consistent with the prediction by the quasi‐linear diffusion theory The first direct observation showing that ECH waves are capable of energy‐dependent scattering of electrons near the loss cone