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In situ produced photoelectrons and precipitating solar wind electrons are two distinct hot electron populations in the dayside Martian upper atmosphere. While each population has been known for decades, its relative contribution to the measured hot electron flux has not been adequately characterized up to now. In this study, we implement a two‐stream kinetic model to compute the hot electron flux for a open magnetic field configuration. By comparing model results to realistic data acquired by the Mars Atmosphere and Volatile Evolution mission, we show that the electron fluxes predicted by the pure photoelectron model are enormously underestimated, especially in the direction toward Mars, but the measurements can be adequately reproduced once solar wind electron precipitation is taken into account. Such a precipitation plays a crucial role above 180 km, not only for hot electrons at all energies that move toward Mars but also for electrons above 1,000 eV that move away from Mars due to the backscattering of precipitating electrons. In contrast, the hot electron population is predicted to be mostly composed of photoelectrons below 180 km. The significance of precipitating solar wind electrons is also evaluated, revealing an appreciable impact on the structure of the dayside Martian upper atmosphere at high altitudes, where electron impact ionization is enhanced by a factor of 7 and cold electron heating enhanced by a factor of 4. Plain Language Summary: This study examines two hot electron populations in the dayside Martian upper atmosphere: locally produced photoelectrons and precipitating solar wind electrons. We intend to figure out how much each population contributes to the total hot electron flux. For this purpose, we construct a kinetic model and compare the model results to the realistic data collected by the Mars Atmosphere and Volatile Evolution mission. We find that the measurements are seriously underestimated by the model with pure photoelectrons, but adequately reproduced when both electron populations are included. In particular, solar wind electrons account for all hot electrons moving toward Mars and part of them moving away. Meanwhile, solar wind electrons likely exert an appreciable impact on the structure of the dayside Martian upper atmosphere, enhancing both ionization and cold electron heating. Key Points: A two‐stream kinetic model is employed to provide a preliminary distinction between photoelectrons and solar wind electrons at MarsPhotoelectrons are found to dominate the hot electron population in all directions below 180 km in the dayside Martian ionospherePrecipitating solar wind electrons are vital above 180 km, for all hot electrons moving toward Mars and part of them moving away from Mars [ABSTRACT FROM AUTHOR]

We report the properties of low frequency (LF) electromagnetic fluctuations (≤11 Hz) in relation to input electron and ion outflow response observed by Cluster in the mid‐altitude cusp during high solar wind dynamic pressure PSW and active magnetospheric conditions (Kp = 4+). The multipoint observations reveal the dynamic interplay between the spatial‐temporal properties of the wave and electron inputs and the ion outflow response enabling an assessment of causal connections. The LF waves are identified as ingoing traveling Alfvén waves that become dispersive at the ion gyroradius scale (i.e., kinetic Alfvén waves KAWs). The KAWs are collocated with ingoing field‐aligned electrons and ion outflow at keV energies and below. The KAWs are associated with earthward directed Poynting fluxes and energy densities with peak amplitudes occurring at multiple energy enhancements ("step‐ups") exhibited in the electrons and ions, attributed to IMF BZ and/or PSW variations. Indicative of a causal connection, KAW Poynting fluxes and energy densities are strongly correlated with precipitating electron energy fluxes and outflowing O+ energies and energy fluxes. These yield mid‐altitude cusp empirical relationships that can be incorporated into or constrain cusp transport models. These results demonstrate the important role played by KAWs in enhancing electron precipitation into the cusp ionosphere and subsequent O+ energization upward along the magnetic field into the mid‐altitude cusp and beyond. The results also suggest the importance of PSW and/or IMF BZ variations in driving/controlling the reconnection source Alfvén wave and electron inputs into the cusp that significantly impact the ion outflow process. Key Points: Impulsive traveling kinetic Alfven waves are observed in the mid‐altitude cusp associated with solar wind pressure and IMF BZ variationsAlfven wave energy densities and Poynting fluxes in the mid‐altitude cusp are strongly correlated with ingoing soft electron energy fluxesSpace‐time variations in cusp O+ outflow energies and energy fluxes are correlated with kinetic Alfven waves and ingoing soft electrons [ABSTRACT FROM AUTHOR]

On 7 January 2014, a solar storm erupted, which eventually compressed the Earth's magnetosphere leading to the generation of chorus waves. These waves enhanced local wave‐particle interactions and led to the precipitation of electrons from 10 s eV to 100 s keV. This paper shows observations of a low energy cutoff in the precipitation spectrum from Van Allen Probe B Helium Oxygen Proton Electron measurements. This low energy cutoff is well replicated by the predicted loss calculated from pitch angle diffusion coefficients from wave and plasma observations on Probe B. To our knowledge, this is the first time a single spacecraft has been used to demonstrate an accurate theoretical prediction for chorus wave‐induced precipitation and its low energy cutoff. The specific properties of the precipitating soft electron spectrum have implications for ionospheric activity, with the lowest energies mainly contributing to thermospheric and ionospheric upwelling, which influences satellite drag and ionospheric outflow. Plain Language Summary: On 7 January 2014, a large storm erupted from the Sun. This storm encountered the Earth and compressed the magnetosphere a few days later. The compression of the magnetosphere led to the creation of chorus waves, a wave‐type known to interact only with electrons with specific energies. In this case, the waves interacted with electrons in the magnetosphere's outer radiation belt. They caused the loss of electrons from 10 s eV to 100 s keV into the ionosphere and upper atmosphere. This paper uses theory to determine which energies we expect will interact with the observed chorus wave. We use the Helium Oxygen Proton Electron instrument from the Van Allen Probes to see if our predictions are correct. We care about these processes because the loss of these electrons can affect ionospheric activity. Key Points: Upper band chorus waves can have a minimum resonant energy in the 10 s eV energy rangeChanges in the minimum resonant energy can change the cut off for what lower energy particles will be lostThe lower energy cut off can be observed in the Van Allen Probes Helium Oxygen Proton Electron data [ABSTRACT FROM AUTHOR]

Field‐aligned distributions of H+ with energy less than 100 eV in the magnetosphere originate from the ionosphere through ionospheric outflow. Recently, a statistical interhemispheric asymmetry in this ion outflow was reported. In this study, we investigate a case study of asymmetric field‐aligned distributions (E < 100 eV) resulting from asymmetric outflow observed on 13 August 2019 by the Hot Plasma Composition Analyzer onboard the Magnetospheric Multiscale mission. During the reported event, the average phase space density in the parallel direction (pitch angle < 45°) is about 4.7 times higher than in the anti‐parallel direction (pitch angle > 135°), indicating greater outflow from the southern hemisphere. In this event, electron precipitation fluxes are also asymmetric between hemispheres, with more electrons traveling to the southern hemisphere than to the northern hemisphere. Global MHD simulation results (SWMF/BAT‐S‐RUS available from the CCMC) confirm that several ionospheric parameters (joule heating, convection velocities, and energy fluxes of precipitating electrons) are more enhanced in the southern hemisphere. We suggest that these interhemispheric asymmetries might cause a stronger outflow from the southern hemisphere than from the northern hemisphere, to produce the observed asymmetric field‐aligned distributions for the low‐energy H+. Key Points: Low‐energy (E < 100 eV) H+ field‐aligned distributions are asymmetric in the outer magnetosphereConditions in the southern hemisphere ionosphere are favorable and result in asymmetric outflowThe observed low‐energy H+ pitch‐angle distributions are consistent with asymmetric outflow as the cause [ABSTRACT FROM AUTHOR]

Numerical calculations are performed for the non-stationary non-equilibrium distribution function of electrons in gas methane CH4 under high-energy electrons with their initial energy 1 keV. The basic elementary processes of collisions of electrons with methane molecules have been considered. Expenditures of electron power into the various processes of molecule excitation, dissociation and ionization are computed so that to obtain the rates of interaction processes of electrons with methane molecules CH4. [ABSTRACT FROM AUTHOR]

Using high‐resolution Solar Wind Electron Analyzer data from the Mars Atmosphere and Volatile EvolutioN spacecraft, we report two new types of electron pitch angle distributions (PADs) in the Martian plasma environment. The first type of electron PADs, showing pitch angle primally around 45° $45{}^{\circ}$, is termed funnel distribution, which was observed near the terminator and at about 1,200 km altitude. The second type of electron PADs, showing pitch angle primally around 135° $135{}^{\circ}$, is termed skirt distribution, which was detected on the nightside and at about 1,250 km altitude. The electrons of funnel and skirt distributions do not exhibit any photoelectron signatures and are shown to originate from the solar wind. Through the fitting analysis, we find that the electrons showing both funnel and skirt PADs are thermal electron populations and suprathermal electron populations. In addition, the possibilities that such two types of electron pitch angle distributions corresponding to magnetic field configurations are also discussed. Plain Language Summary: Electron pitch angle distribution (PAD) plays an important role in studying electron dynamics and magnetic field topologies. Various types of electron PADs in the Martian space environment have been widely reported and studied, including pancake, one‐sided loss cone, isotropy, double field‐aligned beam, butterfly, and so on. In this study, we report two new types of electron PADs, funnel and skirt distributions. The funnel distribution shows pitch angle primally around 45°, and the skirt distributions shows pitch angle primally around 135°. Additionally, we also analyze the sources and populations of these electrons showing funnel and skirt distributions, and discuss the magnetic field configurations that correlate to these two distributions. Such electron PADs may be important for understanding the Martian plasma environment, especially electron precipitation and ion escape. Key Points: In Martian space environment, we report two new types of electron pitch angle distributions, funnel and skirt distributionsThe funnel distribution shows pitch angles primally around 45° $45{}^{\circ}$, and the skirt distribution shows pitch angles primally around 135° $135{}^{\circ}$The electrons of funnel and skirt distributions are identified as thermal and suprathermal electron populations originating from solar wind [ABSTRACT FROM AUTHOR]

Cyclotron resonance with magnetosonic waves was usually considered unimportant for the Van Allen radiation belt electron dynamics because it occurs at energies that are too high. Here, we show that for magnetosonic waves with frequencies above the lower hybrid frequency and normal angles below 87° in the high‐density plasmasphere, the electron cyclotron resonances of ±1, ±2, and ±3 orders can extend down to energies of 10–100s keV. These high‐frequency, non‐quasi‐perpendicular magnetosonic waves are found to occur predominantly in the duskside outer plasmasphere, with an amplitude on the order of 10 pT following strong substorms. They probably originate from midlatitudes of the outer plasmasphere, different from the commonly known magnetosonic waves generated near the equator. Plain Language Summary: Earth's outer radiation belt is composed of electrons with energies from hundreds of keV to several MeV. How electrons are accelerated to high energy and scattered in pitch‐angle are two fundamental questions in the radiation belt dynamics. For commonly existing magnetosonic waves, Landau resonance, bounce resonance, and transit‐time scattering have been well established as candidate mechanisms. However, electron cyclotron resonance with magnetosonic waves was usually considered unimportant because it occurs at energies that are too high. Here, we show that the electron cyclotron resonant energy can extend down to 10–100s keV for magnetosonic waves with frequencies high enough and normal angles off‐90° enough. These high‐frequency, non‐quasi‐perpendicular magnetosonic waves are statistically found to gather in the duskside outer plasmasphere, with an amplitude on the order of 10 pT following strong substorms. Their source region is probably located at midlatitudes of the outer plasmasphere, different from the normal magnetosonic waves generated near the equator. Future works may need to quantify the contribution of these magnetosonic waves to the radiation belt electron dynamics via the fundamental and harmonic cyclotron resonances. Key Points: Electron cyclotron resonance can extend to 10–100s keV for the high‐frequency, non‐quasi‐perpendicular, plasmaspheric magnetosonic wavesThe f > flh magnetosonic waves occur mainly in the duskside outer plasmasphere, with an average amplitude of 10 pT following strong substormsThe f > flh magnetosonic waves are probably generated by the proton Bernstein‐mode instability at midlatitudes in the outer plasmasphere [ABSTRACT FROM AUTHOR]

Photoelectrons are crucial to atmospheric physics. They heat the atmosphere, strengthen planetary ambipolar electric fields, and enhance the outflow of ions to space. However, there exist only a handful of measurements of their energy spectrum near the peak of photoproduction. We present calibrated energy spectra of pristine photoelectrons at their source by a prototype Dual Electrostatic Analyzer (DESA) instrument flown on 11 July 2021 aboard the Dynamo‐2 sounding rocket (NASA № 36.357). Photopeaks arising from 30.4 nm He‐II spectral line were observed throughout the flight above 120 km. DESA also successfully resolved the rarely observed N2 absorption feature. Below 10 eV observations were in good agreement with the GLOW suprathermal electron. Above 10 eV fluxes substantially deviated from the model by as much as an order of magnitude. Plain Language Summary: We designed, built, and flew a new scientific instrument for the measurement of photoelectrons which are created when sunlight shines on the upper atmosphere. The instrument was launched on a suborbital rocket from NASA Wallops Flight Facility just before 2 p.m. on 11 July 2021. The rocket flew to an altitude of 131 km before splashing down in the Atlantic Ocean 8 min later. The instrument gathered scientific data during the flight, measuring the energy spectrum of electrons in Earth's ionosphere. Historical observations of electron spectra at these altitudes are extremely rare, and are often uncalibrated and/or not archived. We present calibrated observations of the pristine spectra of Earth's electrons near their source as a reference for future computer modeling and exploration of Earth's ionosphere. Key Points: We present in situ observations from a plasma spectrometer flown on a rocket to 131 km in the daytime mid‐latitude ionosphereThe instrument returned calibrated measurements of the energy spectra of pristine photoelectrons near the peak of productionThe N2 absorption feature and He‐II photopeaks were partially resolved. Observations are compared with the GLOW electron model [ABSTRACT FROM AUTHOR]

We report a concurrent event of unusual high‐frequency Electromagnetic ion cyclotron (EMIC) and Magnetosonic (MS) waves in correspondence with butterfly distributions for sub‐MeV electrons and flux decay of MeV electrons, observed by Van Allen Probe A on 16 October 2017. The scattering effects are quantitatively investigated by test particle simulations. The results indicate that the unusual EMIC waves mainly scatter electrons from hundreds of keV to several MeV at pitch angles <60° while the MS waves mainly influence sub‐MeV and MeV electrons at pitch angles >60°. Their combined scattering effects are suitably analyzed to be justified as the linear superposition of the effect induced by each wave mode. The Fokker‐Planck simulations exhibit the formation of butterfly pitch angle distributions for sub‐MeV electrons and the obvious flux decay for electrons ∼1.8 MeV at all pitch angles, showing similar variation trends to the observations. Our study suggests that the occurrence of the two waves can significantly influence the electrons over the whole pitch angle range, which can help us better understand the radiation belt dynamics. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves and Magnetosonic (MS) waves are commonly observed in the Earth's magnetosphere and play important roles in energetic electron dynamics. Usually, EMIC waves and MS waves scatter electrons at different energies and pitch angles. The peak frequency of the H+ band EMIC wave is usually much lower than the equatorial proton gyrofrequency. Recently, a different type of H+ band EMIC waves, named unusual high‐frequency EMIC waves since their peak frequency is close to the equatorial proton gyrofrequency, has been reported. Studies on the unusual high‐frequency EMIC waves found that these waves are capable of scattering sub‐MeV and MeV electrons that can also be influenced by MS waves. Moreover, observations confirm that unusual high‐frequency EMIC waves can be well connected with MS waves. In the present study, we quantitatively investigate the combined scattering effect of both wave modes on radiation belt electrons and simulate the evolution of the electron phase space densities under the impact of both waves. The simulation results show similar evolution trends as the observations, indicating the importance of incorporating these two waves and evaluating their combined effects on radiation belt particle dynamics. Key Points: Concurrent unusual high‐frequency Electromagnetic ion cyclotron (EMIC) and Magnetosonic (MS) waves are observed with butterfly pitch angle distributions (PADs) of sub‐MeV electrons and flux decay of MeV electronsThe two wave modes can simultaneously affect sub‐MeV and MeV electrons, leading to significant pitch angle scatteringThe combined effect of both wave modes can form distinct butterfly PADs of sub‐MeV electrons and produce flux decay of MeV electrons [ABSTRACT FROM AUTHOR]

Electron cyclotron harmonic (ECH) waves, potential drivers for diffuse aurora precipitation, have been extensively investigated for decades. The generation mechanism of ECH waves, however, remains an open question. Theoretical work in 1970s has demonstrated that ECH waves can be excited by loss cone distributions of hot plasma sheet electrons. Recent THEMIS spacecraft observations, however, indicate that the waves can also be excited by low energy electron beams. Utilizing interferometry techniques to analyze the phase difference between electric potentials measured by individual probes on Electric Field Instrument antenna pairs on THEMIS spacecraft, we compute the wavenumber of both beam‐driven ECH waves and loss‐cone‐driven ECH waves. These wavenumber measurements as well as other wave properties obtained from spacecraft measurements prove to be consistent with expectation from linear instability analysis. This provides us with independent verification of the generation mechanism and linear dispersion relation of beam‐driven and loss‐cone‐driven ECH waves. Our statistical results demonstrate that the median value of the wave vectors of beam‐driven ECH waves, characterized by wave normal angles (θB⇀,k⇀ $\theta \left(\stackrel{\rightharpoonup }{B},\stackrel{\rightharpoonup }{k}\right)$) less than 80°, is 0.011 m−1; and that of loss‐cone‐driven ECH waves, characterized by wave normal angles larger than 85°, is 0.00765 m−1. Direct wavenumber measurements of ECH waves allow us to better understand the interaction between ECH waves and electrons in Earth's magnetosphere. Key Points: We compute the wavenumber of both beam‐driven Electron cyclotron harmonic (ECH) waves and loss‐cone‐driven ECH waves using the interferometry techniqueThe direct wavelength measurements as well as other wave properties are consistent with expectations from linear instability analysisOur statistical results demonstrate that the median value of the wave vector of ECH waves is 0.0105 m−1 [ABSTRACT FROM AUTHOR]

A one-dimensional numerical simulation of the parameters of the front of a negative-polarity ionization wave in Xe under the action of an electrostatic field was carried out in the context of the development and propagation of a streamer in gas. The estimates were made within the continuum model in the local electron energy approximation. To solve the problem, analytical approximations of the ionization-transport coefficients of electrons in Xe were constructed in the range of the reduced field strength of 10–3–15 000 Td based on known experimental and theoretical data. Numerical solutions were obtained for the velocity and thickness of the wave front, and the electron density behind it. On the basis of the solutions obtained, the dependence of two dimensionless parameters on the field strength was estimated, which show the ratio of the average electron energy relaxation rate to the rate of perturbations in the gas induced by the propagating front. For a physically justified use of the continuum model in estimates, it is necessary that both parameters significantly exceed unity. In particular, their values are close to 10 at field strength of 3450 Td and average electron energy of about 170 eV, making the application of the model problematic at higher field strengths. [ABSTRACT FROM AUTHOR]

The properties of ion acoustic solitary and periodic structures are studied in magnetized two-ion component (O+ − H+ − e) plasmas with (r, q) distributed electrons. It is found that two modes of ion acoustic waves, namely, fast and slow modes, can propagate in such a plasma. The nonlinear Zakharov–Kuznetsov equation is derived using the well-known reductive perturbation method. Employing the theory of planar dynamical systems, the system under consideration is found to admit compressive (hump) and rarefactive (dip) solitary structures and periodic wave solutions. It is found that behavior of propagation of nonlinear ion acoustic solitary structures is different for fast and slow modes owing to the difference in physics of the two modes. The effect of the double spectral indices r and q is thoroughly explored. It is shown that altering the shape of the distribution function through these indices radically alter the propagation characteristics of nonlinear ion acoustic waves. The ratio of concentration of heavy (O+) to light ions (H+) is found to change the fast mode, whereas the temperature ratio is observed to alter the slow mode. We have applied our study to the upper ionosphere where bi-ion plasmas and the presence of non-Maxwellian electrons have been observed by various satellite missions. [ABSTRACT FROM AUTHOR]

It is known that in dense gases in sufficiently strong electric fields, electrons can be continuously accelerated, when they receive more energy from the field than is lost in collisions with atoms and molecules of the medium (runaway electrons). In this work, we study the effect of electron angular scattering in elementary ionization and excitation acts of atoms on the acceleration of electrons in strong fields. To this end, a computer code is developed on the basis of the Monte Carlo technique, and the kinetics of electrons in helium is numerically simulated. In the setting corresponding to the configurations of laboratory experiments with electron "swarms," the code is tested by comparing the calculated kinetic characteristics of the electron swarm (ionization coefficient, drift velocity) with the measurement data in different types of experiments. Numerical simulation is performed for a gas of motionless helium atoms with a concentration N equal to the Loschmidt number 1019 cm–3, in the fields with the strength E from 50 to 300 kV cm–1. The generation rate νhe of electrons with energies in the range from 0.25 to 10 keV is calculated, which is recommended for the use in a source of high-energy electrons in problems of the numerical simulation of gas discharges developing in strong electric fields with the participation of runaway electrons. It is shown that different models of electron anisotropic scattering in inelastic interactions with atoms can lead to multiple differences in the rate νhe values. [ABSTRACT FROM AUTHOR]

We evaluate the location, extent, and energy range of electron precipitation driven by ElectroMagnetic Ion Cyclotron (EMIC) waves using coordinated multisatellite observations from near‐equatorial and Low‐Earth‐Orbit (LEO) missions. Electron precipitation was analyzed using the Focused Investigations of Relativistic Electron Burst Intensity, Range and Dynamics (FIREBIRD‐II) CubeSats, in conjunction either with typical EMIC‐driven precipitation signatures observed by Polar Orbiting Environmental Satellites (POES) or with in situ EMIC wave observations from Van Allen Probes. The multievent analysis shows that electron precipitation occurred in a broad region near dusk (16–23 MLT), mostly confined to 3.5–7.5 L‐shells. Each precipitation event occurred on localized radial scales, on average ∼0.3 L. Most importantly, FIREBIRD‐II recorded electron precipitation from ∼200 to 300 keV to the expected ∼MeV energies for most cases, suggesting that EMIC waves can efficiently scatter a wide energy range of electrons. Plain Language Summary: ElectroMagnetic Ion Cyclotron (EMIC) waves occur in the Earth's magnetosphere and are excited during injections of hot ions from the magnetotail toward Earth. EMIC waves are known to cause precipitation of ∼MeV electrons into the upper atmosphere, which can lead to enhancements of ionization and chemical reactions, but some studies suggest that this energy can extend down to a few hundred keV as well. In this study, we focus on identifying the location, extent and energy range of electron precipitation driven by EMIC waves by analyzing multiple events using coordinated multisatellite observations. We found that energetic electron precipitation primarily occurred on the duskside and within ∼7.5 Earth Radii (RE). The precipitation was localized radially (on average ∼0.3 RE), and the energy range of precipitation extended from at least ∼200–300 keV up to ∼MeV. Our findings are important to understand the effects of EMIC waves on energetic electron precipitation, including their dependence on location, extent, and energy. Key Points: Conjunctions between FIREBIRD‐II and RBSP or POES with clear EMIC‐driven precipitation signatures are analyzedFIREBIRD‐II systematically observed EMIC‐driven electron precipitation with an energy range extending from ∼200 to 300 keV up to ∼1 MeVElectron precipitation occurred in a broad region (16–23 MLT, L < 7.5), but the extent per event was highly localized (on average ∼0.3 L) [ABSTRACT FROM AUTHOR]

Arrival-time spectra (ATS) of an electron swarm in N2 from 50 Td to 700 Td (1 Td = 10−17 Vcm2) were measured by double-shutter drift tube, and then the first Townsend ionization coefficient, mean-arrival-time drift velocity, longitudinal diffusion coefficient, and longitudinal third-order transport coefficient were obtained from the measured ATS. This is the first successful measurement of the third-order transport coefficient in a gas. Furthermore, we went back to an expression of the longitudinal diffusion coefficient in terms of the α parameter, which is the electron transport coefficient derived from ATS, and then demonstrated that the third-order and higher-order electron transport coefficients, which are traditionally ignored in the electron swarm experiment, should be considered to obtain the longitudinal diffusion coefficient properly at moderate and high reduced electric fields. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is a universal process where energy is transferred from magnetic fields to particles. However, it remains unclear whether this transfer of energy includes the direct energization of particles to high energies (i.e., >10s of keV). We compare the spectral indices of energetic (50–200 keV) electrons from six magnetotail electron diffusion region candidates encountered by MMS with those from "quiet‐time" plasma sheet crossings to test whether energetic electron enhancements result from direct energization at/near the reconnection site or simply focusing high‐energy populations that do not originate from the nearby X‐line and already exist in the plasma sheet along the newly reconnected field lines. The study suggests that active reconnection can be a source of localized electron energization. However, the EDR spectra are not outside the range of those from the quiet‐time plasma sheet, which surprisingly still often have very energetic (up to and beyond 200 keV) electrons present. Plain Language Summary: Magnetic reconnection is a fundamental process in plasma physics and is hypothesized to be an efficient process to accelerate particles up to very high energies by transferring energy held in the magnetic field to the particles. Similar to how a taught rubber band can snap, releasing its latent energy—so too do magnetic fields "snap" in reconnection; as the field lines relax to their normal state, they transfer their energy to particle. This paper addresses an outstanding question of whether magnetic reconnection in fact produces high energy electrons. While many computer simulations have predicted this, most in situ observations have focused on energization at much lower energies than we look at here. However, this paper compares a handful of events very near the region where reconnection is believed to occur to a "control group" of nonactive regions not near a reconnection site and finds that the events nearest the reconnection do tend to have more energetic electrons. This provides evidence that the energetic electrons are in fact coming from the reconnection site. Key Points: Six candidate EDRs identified by the MMS team are compared to a statistical database of quiet‐time plasmasheet crossingsThe spectral indices of energetic electrons near the EDRs are indeed harder than the plasmasheet crossings, suggesting local energizationEven the quiet‐time plasmasheet crossings include cases with surprisingly hard spectra, perhaps sourced by distant tail reconnection [ABSTRACT FROM AUTHOR]

We report the measurement of the absolute double differential cross sections (DDCS) of secondary electrons emitted due to the ionization of N2 molecule in collisions with fast electrons having energies between 3 and 5 keV. The emitted electrons with energies from 1–500 eV have been measured for different forward and backward emission angles. The measured DDCS have been compared with the state-of-the-art first Born approximation with correct boundary condition (CB1) model calculations as well as with the classical trajectory Monte Carlo (CTMC) method. From the measured DDCS, the single differential cross sections (SDCS) as a function of the emission energies have been computed and eventually the total ionization cross sections (TCS) have been derived. The TCS values are also compared with a semi-empirical calculation, namely, the CSP-ic (complex scattering potential-ionization contribution) model. [ABSTRACT FROM AUTHOR]

Atmospheric escape is a fundamental process in the long‐term habitability evolution of terrestrial planets. Recent observations on Mars have found the concurrence of the severe ionospheric erosion and the large‐amplitude, quasi‐perpendicular magnetosonic waves. However, whether and then how these magnetosonic waves had contributed to the ionospheric erosion remains unclear. Here we propose a new candidate mechanism, electron Landau heating by magnetosonic waves, for the Martian ionospheric erosion. In contrast to the cyclotron resonance with oxygen atomic and molecular ions above the escape energies, the magnetosonic waves Landau‐resonate with the thermal electrons at energy channels of the order of 0.01–0.1 eV. Through the Landau resonance over tens of minutes, the large‐amplitude (12 nT) magnetosonic waves can heat the topside ionospheric electrons to ∼2 times the normal temperature. The topside ionospheric electron heating could result in the enhancement of the ambipolar electric potential and eventually facilitate the escape of ionospheric plasma. Plain Language Summary: An atmosphere of sufficient density is a fundamental condition for a habitable terrestrial planet. Mars as a terrestrial planet in the habitable zone of our solar system lost much of its atmosphere over billions of years and has thus become a unique testing ground to understand the planetary atmospheric loss processes. Recent observations on Mars have shown that the severe erosion of the ionosphere, an ionized part of the upper atmosphere, was accompanied by the large‐amplitude, quasi‐perpendicular magnetosonic waves. However, whether and then how these magnetosonic waves had contributed to the ionospheric erosion remains unclear. On the basis of space observations and numerical calculations, we propose a new physical mechanism, electron Landau heating by magnetosonic waves, for the Martian ionospheric erosion. The large‐amplitude magnetosonic waves can heat the topside ionospheric electrons to ∼2 times the normal temperature over tens of minutes, enhance the ambipolar electric potential, and eventually facilitate the escape of ionospheric plasma. The electron Landau heating process may also contribute to the ionospheric erosion of other terrestrial planets within and beyond our solar system. Key Points: Magnetosonic waves are difficult to cyclotron‐resonate with Martian ionospheric oxygen atomic and molecular ions below the escape energiesStrong magnetosonic waves can Landau‐heat Martian topside ionospheric electrons to 2 times the normal temperature over tens of minutesElectron Landau heating by magnetosonic waves may facilitate the Martian ionospheric erosion by enhancing the ambipolar electric field [ABSTRACT FROM AUTHOR]

Electron cyclotron harmonic (ECH) waves play an important role in the magnetosphere‐ionosphere coupling. They are usually considered to be generated by the Bernstein‐mode instability with electron loss cone distributions. By analyzing the Van Allen Probes wave data, we present the direct evidence of the nonlinear interactions between ECH waves in the magnetosphere. Substorm‐injected electrons excite primary ECH waves in a series of structureless bands between multiples of the electron gyrofrequency. Nonlinear interactions between the primary ECH waves produce secondary waves at sum‐ and difference‐frequencies of the primary waves. Our results suggest that the nonlinear wave‐wave interactions can redistribute the primary ECH wave energy over a broader frequency range and hence potentially affect the magnetospheric electrons over a broader range of pitch angles and energies. Plain Language Summary: Electron cyclotron harmonic (ECH) waves significantly contribute to the magnetosphere‐ionosphere coupling via resonating with hot electrons. The resonance effect strongly depends on the wave frequency distributions. Both theoretical studies and laboratory experiments made the point that nonlinear wave‐wave interactions can contribute to the frequency spectrum broadening of ECH waves. However, whether such nonlinear interactions occur in the magnetospheric plasma environment remains to be investigated. In this letter, we present, for the first time, the direct evidence of the nonlinear interactions between magnetospheric ECH waves. The nonlinear wave‐wave interactions redistribute the primary ECH wave energy over a broader frequency range. Hence, these waves are expected to have the ability to resonate with the electrons over a broader range of pitch angles and energies. Key Points: We present the direct evidence of the nonlinear interactions between electron cyclotron harmonic (ECH) waves in the magnetospherePrimary ECH waves are generated by the Bernstein‐mode instability of substorm‐injected electronsSum‐ and difference‐frequency interactions between the primary ECH waves can redistribute wave energy over a broader frequency range [ABSTRACT FROM AUTHOR]

The Juno spacecraft has been orbiting Jupiter for the past 3 years. During this interval, Juno has collected large datasets of plasma, magnetic field, and wave measurements in Jupiter's magnetosphere. In this study, we conduct statistics on the plasma and wave characteristics in Jupiter's magnetodisk (nightside magnetosphere beyond 20 Jupiter radii, RJ). Distributions of electron fluxes and waves in the magnetodisk exhibit different characteristics in two regions: the plasma sheet with dense plasma and weak wave intensity and the plasma sheet boundary layer (PSBL) occupied by strong waves and rarefied plasma. We discuss that these waves can be generated by field‐aligned high‐energy electrons in the PSBL. We further compare plasma and wave properties in the PSBL of Jupiter's magnetodisk with the PSBL in Earth's magnetotail. This comparison suggests that the PSBL in Jupiter's magnetodisk may be formed by transient magnetic reconnection in the Jovian magnetosphere. The dawn‐dusk asymmetry of the PSBL properties further supports this scenario: the PSBL is more pronounced in the dawn flank of Jupiter's magnetodisk, where magnetic reconnection is predicted to occur by models of the rotation‐dominated magnetosphere. We further discuss properties of the Jovian PSBL and implications on magnetic reconnection in Jupiter's magnetosphere. Key Points: Our results reveal the statistical properties of plasma sheet boundary layer in Jupiter's magnetodiskStrong electromagnetic waves with E/cB∼1 are observed at the plasma sheet boundary layerObserved dawn‐dusk asymmetry of the plasma sheet boundary layer is consistent with expected asymmetry of transient reconnection [ABSTRACT FROM AUTHOR]

Ionospheric outflow is driven by an ambipolar electric field induced due to the separation of electrons and ions in a gravitational field when equilibrium along a magnetic field line is lost. A model of ionospheric outflow at Saturn was developed using transport equations to estimate the number of charged particles that flow from the auroral regions into the magnetosphere. The model evaluates the outflow from 1,400 km in altitude above the 1 bar level, to 3 RS along the field line. The main ion constituents evaluated are R+ and R3+. We consider the centrifugal force exerted on the particles due to a fast rotation rate, along with the effects of field‐aligned currents present in the auroral regions. The total number flux from both auroral regions is found to be 5.5–13.0×1027 s−1, which relates to a total mass source of 5.5–17.7 kg s−1. These values are on average an order of magnitude higher than expected without the additional effects of centrifugal force and field‐aligned currents. We find the ionospheric outflow rate to be comparable to the lower estimates of the mass loading rate from Enceladus and are in agreement with recent Cassini observations. This additional mass flux into the magnetosphere can substantially affect the dynamics and composition of the inner and middle magnetosphere of Saturn. Key Points: An ionospheric outflow model is developed for use at Saturn's auroral regionsThe presence of field‐aligned currents and centrifugal forces enhances outflow by an order of magnitudePredicted total outflow flux rate of 5.5–13.0×1027 s−1 is comparable to flux calculated from Cassini data [ABSTRACT FROM AUTHOR]

In this study, the ion composition of flux transfer events (FTEs) observed within the magnetosheath proper is examined. These FTEs were observed just upstream of the Earth's postnoon magnetopause by the National Aeronautics and Space Administration (NASA) Magnetospheric Multiscale (MMS) spacecraft constellation. The minor ion characteristics are described using energy spectrograms, flux distributions, and ion moments as the constellation encountered each FTE. In conjunction with electron data and magnetic field observations, such observations provide important contextual information on the formation, topologies, and evolution of FTEs. In particular, minor ions, when combined with the field‐aligned streaming of electrons, are reliable indicators of FTE topology. The observations are also placed (i) in context of the solar wind magnetic field configuration, (ii) the connection of the sampled flux tube to the ionosphere, and (iii) the location relative to the modeled reconnection line at the magnetopause. While protons and alpha particles were often depleted within the FTEs relative to the surrounding magnetosheath plasma, the He+ and O+ populations showed clear enhancements either near the center or near the edges of the FTE, and the bulk plasma flow directions are consistent with magnetic reconnection northward of the spacecraft and convection from the dayside toward the flank magnetopause. Key Points: Long‐duration magnetosheath FTEs observed under at postnoon local times and under similar solar wind conditions are compared and contrastedMinor ions from the magnetosphere are observed at higher energies at the edges than at the center of the FTEsThe cores of the magnetosheath FTEs are on closed field lines [ABSTRACT FROM AUTHOR]

Coherent electron flux oscillations of hundreds of keV are often observed by the Van Allen Probes in the magnetosphere during quiet times in association with ultralow frequency (ULF) waves. They are observed in the form of periodic flux fluctuations, with a drift frequency that is energy dependent, but are not associated with drift echoes following storm‐ or substorm‐related energetic particle injections. Instead, they are associated with the resonant interaction of electrons with ULF waves and are an indication of ongoing electron radial diffusion. To investigate details of such flux oscillations, particle‐tracing simulations are conducted under the effect of realistic, broadband ULF electric and consistent magnetic fluctuations. Virtual detectors are simulated along spacecraft orbits and the results are compared to measurements. Through a parametric study, it is found that the width of electron energy channels is a critical parameter affecting the observed amplitude of flux oscillations, with narrower energy channel widths enabling the observation of higher‐amplitude flux oscillations; this potentially explains why such features were not observed regularly before the Van Allen Probes era, as previous spacecraft generally had lower energy resolution, which only enabled the observation of large‐amplitude drift echoes following a storm or substorm. Results are confirmed using the Magnetic Electron Ion Spectrometer (MagEIS) ultrahigh energy resolution data. Energy width effects are quantified through a parametric simulation study that matches flux oscillation observations during a period that is characterized by extremely quiet conditions, where the Van Allen Probes observed flux oscillations over multiple days. Key Points: Electron flux oscillations at 100 s keV energy are often observed in the radiation belts, associated with broadband ULF wavesObservations of electron flux oscillations are dependent on detector energy channel widthElectron flux oscillations are indicators of ongoing radial transport processes [ABSTRACT FROM AUTHOR]

Radiation belt electrons can be accelerated and scattered by magnetosonic waves in the Earth's magnetosphere, and the scattering rate of electrons is sensitive to the wave normal angle. However, observationally it is difficult to identify the wave normal angle within a few degrees. In this study, using 2‐D particle‐in‐cell (PIC) simulations, we investigate the wave normal angle distribution of magnetosonic waves excited by ring distribution protons. Both the linear theory and simulations have shown that the wave normal angles are distributed over a narrow range (82°–89°) with a major peak at about 85° during the linear growth stage when the proton ring velocity is close to the Alfven speed. In addition, 2‐D PIC simulations further demonstrated that the waves tend to have larger wave normal angles (84°–89°) during the saturation stage since the waves with smaller wave normal angles are dissipated faster. It is also found that wave normal angles decrease with the increase of wave frequency. With the increase of the ring velocity of the proton ring distribution, the perpendicular wavenumber of excited magnetosonic waves decreases, which leads to the decrease of the wave normal angle. The simulation results provide a valuable insight to understand the property of magnetosonic waves, and the findings are useful for the global simulations of radiation belt dynamics. Key Points: Using 2‐D PIC simulations, the wave normal angle distribution of magnetosonic waves has been investigatedThe wave normal angles are distributed over a narrow range (82°–89°) during the linear growth stageThe wave normal angles decrease with the increase of the ring velocity of the proton ring distribution [ABSTRACT FROM AUTHOR]

The multifluid (MF) magnetohydrodynamic model of Mars is improved by solving an additional electron pressure equation. Through the electron pressure equation, the electron temperature is calculated based on the effects from various electron‐related heating and cooling processes (e.g., photoelectron heating, electron‐neutral collision, and electron‐ion collision), and thus, the improved model can calculate the electron temperature and the electron pressure force terms self‐consistently. Model results of a typical case using the MF with electron pressure equation included model are compared in detail to identical cases using the MF and multispecies models to identify the effect of the improved physics. We find that when the electron pressure equation is included, the general interaction patterns are similar to those with no electron pressure equation. However, the MF with electron pressure equation included model predicts that the electron temperature is much larger than the ion temperature in the ionosphere, consistent with both Viking and Mars Atmosphere and Volatile EvolutioN (MAVEN) observations. Using our numerical model, we also examined in detail the relative importance of different forces in the plasma interaction region. All three models are also applied to a MAVEN event study using identical input conditions; overall, the improved model matches best with MAVEN observations. All of the simulation cases are examined in terms of the total ion loss, and the results show that the inclusion of the electron pressure equation increases the escape rates by 50–110% in total mass, depending on solar condition and strong crustal field orientation, clearly demonstrating the importance of the ambipolar electric field in facilitating ion escape. Key Points: For the first time, the effect of the ambipolar electric field is self‐consistently included in the global multifluid MHD modelThe ambipolar electric field plays a significant role in driving ion loss from Mars. The ion mass loss can be enhanced by more than 50%The improved model matches best with MAVEN observations in comparison with previous models [ABSTRACT FROM AUTHOR]

We develop a full relativistic test particle code to model the combined electron scattering effect of Landau and bounce resonances with magnetosonic waves. Test particle simulations of magnetosonic wave‐electron interactions indicate that the two resonances coexist to affect radiation belt electrons at different energies and pitch angles, and the resultant combined pitch angle scattering and energy diffusion can reach the rates of ~10−4 and ~10‐5 s, respectively, for electrons ~40–500 keV at pitch angles ~   70 ° –   80° for the given wave model (~200 pT) inside the plasmapause at L = 4.5. Comparisons with the quasi‐linear theory results show that the test particle combined scattering rates are generally an order of magnitude weaker, possibly because the electrons are moved out of the Landau resonance by the advective effect of the bounce resonance. Our investigation demonstrates that the Landau and bounce resonances with magnetosonic waves cannot be treated independently or additively in terms of quasi‐linear theory to simulate the associated radiation belt electron dynamics. Plain Language Summary: The radiation belt electrons are believed to resonate with magnetosonic (MS) waves in ways of Landau resonance and bounce resonance. In previous studies, the MS wave‐particle interactions were separately evaluated through numerical calculations, that is, the formulism of quasi‐linear theory for Landau resonance and bounce resonance. However, each time the quasi‐linear theory calculations can involve only one resonance mechanism and lack the exploration upon the net scattering effect caused by MS waves. Here we use the test particle simulations to examine the wave‐particle interactions including both the Landau and bounce resonances with MS waves, and quantify the resultant radiation belt electron scattering coefficients for quantitative comparisons with the quasi‐linear theory results. Key Points: Landau and bounce resonances induced by MS waves can coexist to scatter radiation belt electronsTest particle computations of combined pitch angle scattering and energy diffusion coefficients are generally weaker than those obtained from the quasi‐linear theoryMS wave‐driven electron scattering can be dominated by the bounce resonance [ABSTRACT FROM AUTHOR]

Near equatorial (fast) magnetosonic waves, characterized by high magnetic compressibility, are whistler‐mode emissions destabilized by proton shell/ring distributions. In the past, substorm proton injections are widely known to intensify magnetosonic waves in the inner magnetosphere. Here we report the unexpected observations by the Van Allen Probes of the magnetosonic wave quenching associated with the substorm proton injections under both high‐ and low‐density conditions. The enhanced proton thermal pressure distorted the background magnetic field configuration and the cold plasma density distribution. The reduced phase velocities locally allowed the weak growth or even damping of magnetosonic waves. Meanwhile, the spatially irregularly varying refractive indices might suppress the cumulative growth of magnetosonic waves. For intense injections, this wave quenching region could extend over 2 hr in magnetic local time and 0.5 Earth radii in radial distance. These results provide a new understanding of the generation and distribution of magnetosonic waves. Plain Language Summary: Magnetosonic waves are the near‐equatorially confined electromagnetic emissions between the proton gyrofrequency and the lower hybrid frequency in the magnetosphere. Theoretical and observational studies have demonstrated the potential of the magnetosonic waves to accelerate the radiation belt electrons. It is therefore important to understand the generation process and the spatiotemporal distribution of the magnetosonic waves. The substorm activities injecting hot protons into the inner magnetosphere are conventionally considered to intensify the magnetosonic waves. By analyzing the wave and particle data of the Van Allen Probes, we find a magnetosonic wave quenching region related to the distortion of the background magnetic field configuration and the cold plasma density distribution closely following the substorm injection front. We propose two possible causes for this phenomenon: (1) local weak growth or even damping of the magnetosonic waves with the reduced phase velocities and (2) suppression of the magnetosonic wave cumulative growth in the disturbed medium. This new finding may help refine the modeling of the magnetosonic waves and then the radiation belt electrons. Key Points: The enhanced thermal pressure of hot protons can distort the geomagnetic field configuration and the cold plasma density distributionThe magnetosonic wave quenching region can emerge closely following the substorm injection front under both high‐ and low‐density conditionsThe magnetosonic wave quenching region can extend over 2 hr in magnetic local time and 0.5 Earth radii in radial distance [ABSTRACT FROM AUTHOR]

We consider the formation of a potential drop over the Earth's polar cap during geomagnetically quiet daytime. The observed potential drop is primarily defined by the hydrogen, photoelectron, and polar rain fluxes ratios and depends strongly on the energy distribution of the photoelectron flux. Polar rain is an essential component of the model required for plasma quasi‐neutrality. The potential distribution along the magnetic field line has two regions, with a small, gradual, potential drop of 3–4 V and a potential jump. The value of the potential jump depends on the hydrogen ion to photoelectron flux ratio and is also controlled by polar rain electrons. With quasi‐neutrality required at its upper boundary, the jump only occurs in the presence of polar rain and its location depends on the polar rain flux. Model predictions compare well with FAST observations presented by Kitamura et al. (2012, https://doi.org/10.1029/2011JA017459). Key Points: Formation of field‐aligned electrostatic potential drops in the sunlit polar cap during geomagnetically quiet conditions is consideredThe role of the polar rain electrons in the formation of electrostatic potential jumps in the polar cap region is studiedThe influence of photoelectron energy distribution function on the polar cap potential drop is analyzed [ABSTRACT FROM AUTHOR]

To understand the variations of the O+ ions in the quiet‐time plasma sheet between the regions of cold‐dense plasma sheet (CDPS) and hot plasma sheet (HPS), we conduct three event studies. These studies investigate the O+ densities in the two regions and how they are correlated with the strength of two magnetospheric sources important to ion outflows: the soft electron flux and Poynting flux toward the ionosphere. The CDPS is characterized by two‐component ions (one hot component mixed with one cold component), while the HPS ions consist of only one single hot component. Comparing the O+ density between the CDPS and HPS of the same event, the average CDPS O+ density was higher by a factor of ~2–5. Compared to the HPS, the soft electron flux source within the CDPS was higher, consistent with the fact that the soft electron precipitation and O+ upward number fluxes observed in the ionosphere were also higher within the CDPS. In the plasma sheet, broadband ultralow‐frequency electric and magnetic field waves with the characteristics of kinetic Alfvén waves were often more intense within the CDPS, providing a stronger Poynting flux source. In addition, electron resonant interaction with kinetic Alfvén waves results in acceleration along the magnetic fields and, thus, may drive the observed soft electron precipitation. These correlations suggest that the higher soft electron precipitation and Poynting flux coming from the magnetospheric CDPS likely produce larger ionospheric O+ outflows back to the magnetosphere, thus resulting in the higher O+ density within the CDPS. Key Points: O+ densities in cold‐dense plasma sheet in the three quiet‐time events were higher than those in hot plasma sheet by a factor of ~2‐5Higher soft electron fluxes in the magnetosphere and soft electron precipitation in the ionosphere in cold‐dense than hot plasma sheetMore intense kinetic Alfven waves within the CDPS, providing stronger Poynting flux downward to the ionosphere [ABSTRACT FROM AUTHOR]

With the advantage of fast plasma measurements of the Magnetospheric Multiscale (MMS) mission in the magnetotail, we first investigate the particle carriers of field‐aligned currents (FACs) in the plasma sheet boundary layer for three cases. In all cases, electrons are the main carriers of FACs, while the contribution of ions can be neglected. The analysis of these cases indicate that thermal electrons (energy range from 0.5Te to 5Te, where Te is the electron parallel temperature) are the main carrier of the FACs. However, cold electrons (energy less than 0.5Te) can also significantly contribute to the FACs. In one of three cases, suprathermal electrons (energy greater than 5Te) contribute but a small portion to the total current. The difference between the cases may depend on the local dynamics in the magnetotail. Then a statistical analysis was performed, which also shows that the thermal electrons are dominant in most of the FACs, while the cold electrons can be dominant in some cases. However, the suprathermal electrons cannot support the FACs solely, and they and the cold electrons are more often supporting the FACs together with the thermal electrons. Key Points: The FACs observed in the PSBL are mainly carried by electrons, while ionic contributions are less importantFurther investigation using particle energy flux demonstrates that the main contribution to the FACs comes from thermal electronsWeak suprathermal and cold electron contributions to the FACs are observed in some cases [ABSTRACT FROM AUTHOR]

In this paper we report a rare and fortunate event of fast magnetosonic (MS, also called equatorial noise) waves modulated by compressional ultralow frequency (ULF) waves measured by Van Allen Probes. The characteristics of MS waves, ULF waves, proton distribution, and their potential correlations are analyzed. The results show that ULF waves can modulate the energetic ring proton distribution and in turn modulate the MS generation. Furthermore, the variation of MS intensities is attributed to not only ULF wave activities but also the variation of background parameters, for example, number density. The results confirm the opinion that MS waves are generated by proton ring distribution and propose a new modulation phenomenon. Key Points: The PSD of ring proton is antiphase modulated by ULF waveA rare and fortunate MS wave modulated by ULF wave is reported for the first timeLinear theory and perpendicular propagation account for the modulation of MS [ABSTRACT FROM AUTHOR]

Energetic particle injections are characterized by dispersive or dispersionless increases in observed particle flux. Observations from National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) mission have revealed transient events displaying injection‐like dispersed reductions in energetic (~30–600 keV) electron flux in the dawnside magnetosphere. Although initially believed to be the result of magnetopause losses, drift tracing of the electrons suggests a source for these drift‐dispersed flux dropouts in the near‐to‐postmidnight magnetotail suggesting that they are likely related to similar signatures previously observed at geosynchronous orbit. We suggest that the dozen examples presented are signatures of "flux‐reduced injections" resulting from earthward injection in the presence of a negative phase space density radial gradient as supported by observed phase space density versus L‐shell profiles. These events also display varying pitch angle responses inconsistent with a singular loss mechanism, leading to the suggestion that they result from preconditioning of the magnetotail source region prior to the injection. Plain Language Summary: Energetic particles are suddenly and abruptly pushed earthward through near‐Earth space via processes called "injections." These are commonly seen by spacecraft as sudden time‐ and energy‐dependent increases in the number of observed particles and are known to play an important role in sourcing the planet's radiation belts. Investigating the processes governing energetic particle transport is critical to better understand fundamental plasma processes, the dynamics of Earth's radiation belts, and the effects of space weather. This paper investigates a set of similar, but unique, signatures where the number of particles suddenly decreases instead of increasing. The analysis presented here surprisingly reveals that the particles observed in these signatures come from the magnetotail, which raises new questions about what generates these signatures. We investigate two hypotheses to explain the observed decrease in particles: (1) that a process removes the particles from the system and (2) that a process similar to "normal" injections moves a population of decreased particles into an area where there are more particles (hence generating a local decrease). The analysis finds that the secondary hypothesis is far more likely to be the primary driver while the first cannot be ruled out entirely as a secondary contributor. Key Points: MMS/FEEPS has repeatedly observed energy/time‐dispersed transient reductions in energetic electron flux in the dawnside magnetosphereDrift tracing consistently indicates that these signatures are from a postmidnight magnetotail sourceResults suggest that these features result from earthward injection in the presence of a negative radial gradient in phase space density [ABSTRACT FROM AUTHOR]

We test the hypothesis that their dominant driver of a planetary ambipolar electric field is the ionospheric electron pressure gradient (∇Pe). The ionospheres of Venus and Mars are mapped using Langmuir probe measurements from NASA's Pioneer Venus Orbiter (PVO) and Mars Atmosphere and Volatile EvolutioN (MAVEN) missions. We then determine the component of the ionospheric potential drop that can be explained by the electron pressure gradient drop along a simple draped field line. At Mars, this calculation is consistent with the mean potential drops measured statistically by MAVEN. However, at Venus, contrary to our current understanding, the thermal electron pressure gradient alone cannot explain Venus' strong ambipolar field. These results strongly motivate a return to Venus with a comprehensive plasmas and fields package, similar to that on MAVEN, to investigate the physics of atmospheric escape at Earth's closest analog. Plain Language Summary: Every planet with an atmosphere generates a weak electric field, called an "ambipolar field," which plays a critical role in the escape of the ionosphere. Until recently, these fields had never been measured due to their low strength. However, by measuring the subtle shifts in the energies of electrons generated in the ionosphere, the total potential drop associated with this field has recently been measured at both Venus and Mars. These measurements permit us to make the first investigation of the fundamental physics that underpins this field. Specifically, we test the long‐held hypothesis that ambipolar fields are primarily generated by the gradient of electron pressure along magnetic field lines that connect the ionosphere to space. We find the potential drop at Mars is consistent with theory, but Venus' field is 10 times stronger than expected. Key Points: We map the ionospheres of Venus and Mars to investigate whether ambipolar fields are generated by the thermal electron pressure gradientMars' ambipolar potential drop is consistent with what would be expected from existing theory (∼0.7 V peaking at 220 km)Venus' potential drop (10 V) far exceeds what can be explained by the mean electron pressure gradient (1 V), motivating further research [ABSTRACT FROM AUTHOR]

We report for the first time, simultaneous ion, electron, magnetic field vector and electric field wave measurements made possible by Mars Atmosphere and Volatile EvolutioN, during ion energy flux spikes in low‐altitude radial crustal magnetic fields on the Mars dayside. Observations show energetic electrons and ions (E > 25 eV) precipitating on magnetic field lines assumed as closed. Ions (E < 1.4 keV) display broad velocity distributions toward Mars, showing ions flowing from higher altitude possibly after magnetic reconnection or loss cone filling from pitch angle scattering effects. Precipitating ions (E < 1.4 keV) show nonadiabatic features depending on ion mass and energy and returning ions (E < 1.4 keV) show evidence of conserving the first adiabatic invariant in a mirror field. We observe magnetic field perturbations up to 60 nT, electric field wave amplitudes up to 38 mV/m, and brief periods of peaked electron spectra. At ∼175 km and at times Mars Atmosphere and Volatile EvolutioN is below the mirroring altitude of electrons, we observe mirroring and transverse heating of H+ ions alongside increased electric field wave amplitude fluctuations. It suggests field aligned potential drops result from different mirror altitudes of ions and electrons. Ions E > 1.4 keV (O+) occur as injected accelerated ion beams and ions heated after energization or deceleration. Energy dispersed kilo‐electron‐volt ions suggest a selection effect in radial magnetic fields for lower‐energy Marsward ions, compared to reflection of higher‐energy anti‐Sunward ions. Precipitating kilo‐electron‐volt ions show energy deposition rates of 3.6 ×10−6 W/m2 and sputtering escape rates from precipitating O+ ions of 1.5 ×105/(cm2.s) and 2.1 ×106/(cm2.s) are calculated. Plain Language Summary: Mars Atmosphere and Volatile EvolutioN arrived at Mars on 21 September 2014 to study the effects of the solar wind interaction on the upper atmosphere. This study reports the raining of both energetic electrons and multiple populations of ions at low altitudes and over regions of ancient magnetic field from the Martian crust. These observations are associated with large magnetic field deflections and large electric field fluctuations. The results are important for understanding plasma transport and associated electrodynamics where the crustal magnetic fields point vertically toward or away from the planet. Such analysis was not possible with previous missions, which did not include the complete set of space plasma instruments carried by Mars Atmosphere and Volatile EvolutioN. Some ions display behaviors not expected for the small curvature of the crustal magnetic fields; ions that are locally tied that would require much larger magnetic field structures or ions injected along the direction of the magnetic field. Oxygen ions that originate from Mars that have been picked up by the solar wind and moving at very high speeds are selected over regions of vertical magnetic field to travel directly toward the planet. These ions will have head on impacts with Mars to heat and blast atmosphere out into space. Key Points: Investigated plasma transport and associated electrodynamics during ion energy flux spikes in radial crustal fields on the Mars daysidePrecipitating ion and electron energy flux spikes on closed field consisting of ion beams E > 1.4 keV and broad ion distributions E < 1.4 keVConsidered impact of low‐altitude precipitating O+ ions E > 25 eV on localized heating and atmospheric escape [ABSTRACT FROM AUTHOR]

Cold electron runaway means that free electrons in gases are accelerated by electric fields from eV energies to energies above tens of keV where they can be accelerated further. To run away, the electrons need to overcome a barrier at intermediate energies where they can lose much energy in collisions. When they have reached the runaway regime, they can produce high‐energy radiation by bremsstrahlung that can be detected as (terrestrial) gamma ray flashes. When can thermal electrons from active discharges like streamers and leaders reach the runaway regime? The deterministic approach to this question is based on an energy‐dependent electron friction that has to be overcome by electric acceleration. Taking the stochastic nature of the electron molecule collisions into account, we find (1) that the classical friction curve in the energy regime up to 1 keV does not characterize the mean electron energy, but rather, it seems to approximate the upper limit of the electron energy distribution and (2) that electrons can "tunnel" through the barrier when the field is close to 3 MV/m, below the so‐called cold runaway threshold (or critical field) of approximately 26 MV/m in air at standard temperature and pressure. (3) This is only true in the bulk perspective where the electron liberation and attachment in a given electric field is taken into account in the continuously refreshing electron ensemble. In a flux simulation that follows individual electrons as long as they are free, electron attachment reduces electron runaway very strongly in air, differently to what we observe in nitrogen. Key Points: We evaluate the stationary sub‐keV electron energy distribution in air and nitrogen, in different electric fieldsFriction is not a systematic approximation for the energy loss of nonrelativistic electronsCold electrons can accelerate to above 1‐keV energy in electric fields close to 3 MV/m, well below the so‐called runaway threshold [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]

The fortunate location of Cluster and the THEMIS P3 probe in the near-Earth plasma sheet (PS) (at X ~-7- -9 RE/ allowed for the multipoint analysis of properties and spectra of electron and proton injections. The injections were observed during dipolarization and substorm current wedge formation associated with braking of multiple bursty bulk flows (BBFs). In the course of dipolarization, a gradual growth of the BZ magnetic field lasted ~13 min and it was comprised of several BZ pulses or dipolarization fronts (DFs) with duration ≥1 min. Multipoint observations have shown that the beginning of the increase in suprathermal (>50 keV) electron fluxes - the injection boundary - was observed in the PS simultaneously with the dipolarization onset and it propagated dawnward along with the onset-related DF. The subsequent dynamics of the energetic electron flux was similar to the dynamics of the magnetic field during the dipolarization. Namely, a gradual linear growth of the electron flux occurred simultaneously with the gradual growth of the BZ field, and it was comprised of multiple short (~ few minutes) electron injections associated with the BZ pulses. This behavior can be explained by the combined action of local betatron acceleration at the BZ pulses and subsequent gradient drifts of electrons in the flux pile up region through the numerous braking and diverting DFs. The nonadiabatic features occasionally observed in the electron spectra during the injections can be due to the electron interactions with high-frequency electromagnetic or electrostatic fluctuations transiently observed in the course of dipolarization. On the contrary, proton injections were detected only in the vicinity of the strongest BZ pulses. The front thickness of these pulses was less than a gyroradius of thermal protons that ensured the nonadiabatic acceleration of protons. Indeed, during the injections in the energy spectra of protons the pronounced bulge was clearly observed in a finite energy range ~70-90 keV. This feature can be explained by the nonadiabatic resonant acceleration of protons by the bursts of the dawn-dusk electric field associated with the BZ pulses. [ABSTRACT FROM AUTHOR]

The cusp has important effects on the transportation of particles and their energy from the solar wind to the magnetosphere, and ionosphere, and high-altitude atmosphere. The cusp can be considered to be a part of the magnetospheric boundary layer with weaker magnetic fields. It has been studied since 1971 by different satellite observations. Despite many years of investigation, some problems, such as the boundaries, shapes, and method of construction, remain to be solved. The double cusp was first reported by Wing using the observation of the DMSP satellite. He also compared the results of observations with the results of a 2D MHD simulation. In this study, by performing simulations and analyzing the results, we report the observation of a V-shaped double-cusp structure under the northward Interplanetary Magnetic Field (IMF). In our simulation, the double cusp was seen only for electrons, although a weak double cusp was observed for ions as well. We showed that this double cusp occurred because of electron precipitation from different sources of solar wind and magnetosphere with different magnetic field strengths. In previous studies of the double cusp, there were debates on its spatial structure or on its temporal behavior due to the cusp movement caused by the sharp solar wind effects on the magnetosphere shape. Here we report the spatial detection of the double cusp similar to the one observed by the DMSP satellite, but for the northward IMF case. Also, we investigate the asymmetry along the dawn-dusk side of the magnetosphere using our 3D PIC simulation code. [ABSTRACT FROM AUTHOR]

Radiation belt electron flux dropouts are a kind of drastic variation in the Earth's magnetosphere, understanding of which is of both scientific and societal importance. Using electron flux data from a group of 14 satellites, we report multi-satellite simultaneous observations of magnetopause and atmospheric losses of radiation belt electrons during an event of intense solar wind dynamic pressure pulse. When the pulse occurred, magnetopause and atmospheric loss could take effect concurrently contributing to the electron flux dropout. Losses through the magnetopause were observed to be efficient and significant at L ࣡ 5, owing to the magnetopause intrusion into L ~ 6 and outward radial diffusion associated with sharp negative gradient in electron phase space density. Losses to the atmosphere were directly identified from the precipitating electron flux observations, for which pitch angle scattering by plasma waves could be mainly responsible. While the convection and substorm injections strongly enhanced the energetic electron fluxes up to hundreds of keV, they could delay other than avoid the occurrence of electron flux dropout at these energies. It is demonstrated that the pulse-time radiation belt electron flux dropout depends strongly on the specific interplanetary and magnetospheric conditions and that losses through the magnetopause and to the atmosphere and enhancements of substorm injection play an essential role in combination, which should be incorporated as a whole into future simulations for comprehending the nature of radiation belt electron flux dropouts. [ABSTRACT FROM AUTHOR]

The propagation of electron acoustic solitary waves (EASWs) in a magneto-rotating electron-positron-ion (epi) plasma containing cold dynamical electrons, nonthermal electrons and positrons obeying Cairns' distribution have been explored in the stationary background of massive positive ions. Through the linear dispersion relation (LDR) the effects of nonthermal components, magnetic field and rotation have been analyzed, wherein, various limiting cases have been deduced from the LDR. For nonlinear analysis, Korteweg-de Vries (KdV) equation is obtained using the reductive perturbation technique. It is found that in the presence of nonthermal positrons both hump and dip type solitons appear to excite, the structural properties of these solitary waves change drastically with magneto-rotating effects. The present work may be employed to explore and to understand the formation of electron acoustic solitary structures in the space and laboratory plasmas with nonthermal electrons and positrons under magneto-rotating effects. [ABSTRACT FROM AUTHOR]

The absorption of telemetry radiosignals at frequencies of 250 and 75 MHz, transmitted from rockets, was observed in the ARAKS and Zarnitza 2 rocket experiments, respectively, with electron pulses in the ionosphere. The signals were registered with ground receivers. Four cases of complete signal absorption on the propagation path were observed in the ARAKS experiment. The radio absorption at frequencies substantially higher than the plasma and upper hybrid frequencies can be related to wave scattering by plasma inhomogeneities. It has been indicated that plasma inhomogeneities were generated when electrostatic oscillations damped in the region with decreased plasma density at a decrease in the natural oscillation phase volume in the frequency-wave vector space with decreasing plasma density. The observed radio absorption could be related to reflectionless wave scattering in an inhomogeneous plasma structure. [ABSTRACT FROM AUTHOR]

The problem of small amplitude electron-acoustic solitary waves (EASWs) is discussed using the reductive perturbation theory in magnetized plasmas consisting of cold electrons, hot electrons obeying nonextensive distribution and stationary ions. The presented investigation shows that the presence of nonextensive distributed hot electrons (due to the effects of long-range interactions) causes a reduction in the soliton amplitude while its width increases. The effects of the population ratio of hot to cold electrons and also the effects of the presence of magnetic field in this situation are also discussed. [ABSTRACT FROM AUTHOR]

Electrons from the Sun, often accelerated by magnetospheric processes, produce low-density plasmas in the upper atmospheres of planets and their satellites. The secondary electrons can produce further ionization, dissociation and excitation, leading to enhancement of chemical reactions and light emission. Similar processes are driven by photoelectrons produced by sunlight in upper atmospheres during daytime. Sunlight and solar electrons drive the same processes in the atmospheres of comets. Thus for both understanding of planetary atmospheres and in predicting emissions for comparison with remote observations it is necessary to simulate the processes that produce upper atmosphere plasmas. In this review, we describe relevant models and their applications and address the importance of electron-impact excitation cross sections, towards gaining a quantitative understanding of the phenomena in question. [ABSTRACT FROM AUTHOR]

In this paper, a homogeneous system of unmagnetized collisionless plasma consisting of a cold electron fluid, low-temperature ion obeying Boltzmann-type distribution and high-temperature ion obeying non-thermal distribution is considered. The perturbation method with two different forms of stretching will be considered to drive the KdV and modified KdV (mKdV) equations. The Agrawal's method is applied to formulate the time-fractional KdV and mKdV equations. A variational iteration method is used to solve these equations. The results show that the fractional order of the differential equations can be used to modify the shape of the solitary pulse instead of adding higher order dissipation terms to the equations. This study may be useful to construct the compressive and rarefactive electrostatic potential pulses associated with the broadband electrostatic noise type emissions. [ABSTRACT FROM PUBLISHER]

The location of magnetic reconnection in the midtail during a substorm was studied in many researches. Here we present multi-point THEMIS observations of a reconnection event in the near-Earth magnetotail during substorm. In this event, THEMIS probes stayed in the near-Earth and midtail region aligning along the magnetotail. This allows reconnection evolution to be probed simultaneously from about -10RE to -23RE down tail. The Hall current related electron streams were observed at the same time by two probes far away from the reconnection site. Before near-Earth reconnection involved the tail lobe magnetic field, the reconnection site was restricted in earthward -23RE. When reconnection involved into the tail lobe region, the reconnection site started to retreat gradually. [ABSTRACT FROM AUTHOR]

A case study is presented to test the role several different sources may play in populating the exterior cusp with energetic (E >40 keV) electrons. The properties of the electrons measured in the exterior cusp are compared with the expected signatures of three sources in order to weigh the role each source may contribute in populating this region. The potential sources probed are: (1) solar energetic electrons entering the cusp from the solar wind; (2) an equatorial magnetospheric source with particles drifting to high latitudes on the dayside; and (3) a local source accelerating the electrons in situ. From the observations it is likely that local acceleration is the primary source of the energetic electrons in the exterior cusp during this event. [ABSTRACT FROM AUTHOR]

Large amplitude ion-acoustic and electronacoustic waves in an unmagnetized multi-component plasma system consisting of cold background electrons and ions, a hot electron beam and a hot ion beam are studied using Sagdeev pseudo-potential technique. Three types of solitary waves, namely, slow ion-acoustic, ion-acoustic and electron-acoustic solitons are found provided theMa ch numbers exceed the critical values. The slow ion-acoustic solitons have the smallest critical Mach numbers, whereas the electron-acoustic solitons have the largest critical Mach numbers. For the plasma parameters considered here, both type of ion-acoustic solitons have positive potential whereas the electron-acoustic solitons can have either positive or negative potential depending on the fractional number density of the cold electrons relative to that of the ions (or total electrons) number density. For a fixed Mach number, increases in the beam speeds of either hot electrons or hot ions can lead to reduction in the amplitudes of the ion-and electron-acoustic solitons. However, the presence of hot electron and hot ion beams have no effect on the amplitudes of slow ion-acoustic modes. Possible application of this model to the electrostatic solitary waves (ESWs) observed in the plasma sheet boundary layer is discussed. [ABSTRACT FROM AUTHOR]

Proceeding with the analysis of Amata et al. (2005), we suggest that the general feature for the local transport at a thin magnetopause (MP) consists of the penetration of ions from the magnetosheath with gyroradius larger than the MP width, and that, in crossing it, the transverse potential difference at the thin current sheet (TCS) is acquired by these ions, providing a field-particle energy exchange without parallel electric fields. It is suggested that part of the surface charge is self-consistently produced by deflection of ions in the course of inertial drift in the non-uniform electric field at MP. Consideration of the partial moments of ions with different energies demonstrates that the protons having gyroradii of roughly the same size or larger than the MP width carry fluxes normal to MP that are about 20% of the total flow in the plasma jet under MP. This is close to the excess of the ion transverse velocity over the cross-field drift speed in the plasma flow just inside MP (Amata et al., 2005), which conforms to the contribution of the finite-gyroradius inflow across MP. A linkage through the TCS between different plasmas results from the momentum conservation of the higher-energy ions. If the finite-gyroradius penetration occurs along the MP over ∼1.5RE from the observation site, then it can completely account for the formation of the jet under the MP. To provide the downstream acceleration of the flow near the MP via the cross-field drift, the weak magnetic field is suggested to rotate from its nearly parallel direction to the unperturbed flow toward being almost perpendicular to the accelerated flow near the MP. We discuss a deceleration of the higher-energy ions in the MP normal direction due to the interaction with finite-scale electric field bursts in the magnetosheath flow frame, equivalent to collisions, providing a charge separation. These effective collisions, with a nonlinear frequency proxy of the order of the proton cyclotron one, in extended turbulent zones are a promising alternative in place of the usual parallel electric fields invoked in the macro-reconnection scenarios. Further cascading towards electron scales is supposed to be due to unstable parallel electron currents, which neutralize the potential differences, either resulted from the ion- burst interactions or from the inertial drift. The complicated MP shape suggests its systematic velocity departure from the local normal towards the average one, inferring domination for the MP movement of the non-local processes over the small-scale local ones. The measured Poynting vector indicates energy transmission from the MP into the upstream region with the waves triggering impulsive downstream flows, providing an input into the local flow balance and the outward movement of the MP. Equating the transverse electric field inside the MP TCS by the Hall term in the Ohm's law implies a separation of the different plasmas primarily by the Hall current, driven by the respective part of the TCS surface charge. The Hall dynamics of TCS can operate either without or as a part of a macro-reconnection with the magnetic field annihilation. [ABSTRACT FROM AUTHOR]

We present a review of recent analytical and numerical studies of the dynamics of electron and ion holes in a collisionless plasma. The new results are based on the class of analytic solutions which were found by Schamel more than three decades ago, and which here work as initial conditions to numerical simulations of the dynamics of ion and electron holes and their interaction with radiation and the background plasma. Our analytic and numerical studies reveal that ion holes in an electron-ion plasma can trap Langmuir waves, due the local electron density depletion associated with the negative ion hole potential. Since the scale-length of the ion holes are on a relatively small Debye scale, the trapped Langmuir waves are Landau damped. We also find that colliding ion holes accelerate electron streams by the negative ion hole potentials, and that these streams of electrons excite Langmuir waves due to a streaming instability. In our Vlasov simulation of two colliding ion holes, the holes survive the collision and after the collision, the electron distribution becomes flat-topped between the two ion holes due to the ion hole potentials which work as potential barriers for low-energy electrons. Our study of the dynamics between electron holes and the ion background reveals that standing electron holes can be accelerated by the selfcreated ion cavity owing to the positive electron hole potential. Vlasov simulations show that electron holes are repelled by ion density minima and attracted by ion density maxima. We also present an extension of Schamel's theory to relativistically hot plasmas, where the relativistic mass increase of the accelerated electrons have a dramatic effect on the electron hole, with an increase in the electron hole potential and in the width of the electron hole. A study of the interaction between electromagnetic waves with relativistic electron holes shows that electromagnetic waves can be both linearly and nonlinearly trapped in the electron hole, which widens further due to the relativistic mass increase and ponderomotive force in the oscillating electromagnetic field. The results of our simulations could be helpful to understand the nonlinear dynamics of electron and ion holes in space and laboratory plasmas. [ABSTRACT FROM AUTHOR]

We present a review of recent analytical and numerical studies of the dynamics of electron and ion holes in a collisionless plasma. The new results are based on the class of analytic solutions which were found by Schamel more than three decades ago, and which here work as initial conditions to numerical simulations of the dynamics of ion and electron holes and their interaction with radiation and the background plasma. Our analytic and numerical studies reveal that ion holes in an electron-ion plasma can trap Langmuir waves, due the local electron density depletion associated with the negative ion hole potential. Since the scale-length of the ion holes are on a relatively small Debye scale, the trapped Langmuir waves are Landau damped. We also find that colliding ion holes accelerate electron streams by the negative ion hole potentials, and that these streams of electrons excite Langmuir waves due to a streaming instability. In our Vlasov simulation of two colliding ion holes, the holes survive the collision and after the collision, the electron distribution becomes flat-topped between the two ion holes due to the ion hole potentials which work as potential barriers for low-energy electrons. Our study of the dynamics between electron holes and the ion background reveals that standing electron holes can be accelerated by the self-created ion cavity owing to the positive electron hole potential. Vlasov simulations show that electron holes are repelled by ion density minima and attracted by ion density maxima. We also present an extension of Schamel's theory to relativistically hot plasmas, where the relativistic mass increase of the accelerated electrons have a dramatic effect on the electron hole, with an increase in the electron hole potential and in the width of the electron hole. A study of the interaction between electromagnetic waves with relativistic electron holes shows that electromagnetic waves can be both linearly and nonlinearly trapped in the electron hole, which widens further due to the relativistic mass increase and ponderomotive force in the oscillating electromagnetic field. The results of our simulations could be helpful to understand the nonlinear dynamics of electron and ion holes in space and laboratory plasmas. [ABSTRACT FROM AUTHOR]