Population Of Energetic Electrons Research Articles

Impact of Electron Precipitation on Brown Dwarf Atmospheres and the Missing Auroral H3+ Emission

Recent observations have demonstrated that very low-mass stars and brown dwarfs are capable of sustaining strong magnetic fields despite their cool and neutral atmospheres. These kilogauss field strengths are inferred based on strong, highly circularly polarized gigahertz radio emission, a consequence of the electron cyclotron maser instability. Crucially, these observations imply the existence of energetic nonthermal electron populations, associated with strong current systems, as are found in the auroral regions of the magnetized planets of the solar system. Intense auroral electron precipitation will lead to electron collisions with the H2 gas that should generate the ion H3+ . With this motivation, we targeted a sample of ultracool dwarfs, known to exhibit signatures associated with aurorae, in search of the K-band emission features of H3+ using the Keck telescopes on Maunakea. From our sample of nine objects, we found no clear indication of H3+ emission features in our low-to-medium-resolution spectra (R ∼ 3600). We also modeled the impact of an auroral electron beam on a brown dwarf atmosphere, determining the depth at which energetic beams deposit their energy and drive particle impact ionization. We find that the H3+ nondetections can be explained by electron beams of typical energies ≳2–10 keV, which penetrate deeply enough that any H3+ produced is chemically destroyed before radiating energy through its infrared transitions. Strong electron beams could further explain the lack of UV auroral detections and suggest that most or nearly all of the precipitating auroral energy must ultimately emerge as thermal emissions deep in brown dwarf atmospheres.

β-induced Alfvén eigenmodes and fishbone driven by energetic electrons in EAST electron heating dominant plasmas

β-induced Alfvén eigenmodes (e-BAE) and fishbone (e-fishbone) driven by the energetic electrons have been identified on the experimental advanced superconducting tokamak in electron cyclotron resonance heating (ECRH), low hybrid wave (LHW) together with neutral beam injection plasma and in pure radio frequency heating plasma, respectively. The e-BAE is predominantly located near the pedestal region, and its frequency typically displays a chirping characteristic. The frequency is notably associated with the count of energetic electrons with energy between 30 keV and 80 keV driven by LHW and undergoes three distinct phases. A direct correlation is established between the frequency of e-BAE and the global parameters, including electron temperature, Alfvén velocity and βp . The e-fishbone emerges in the core and becomes noticeable shortly after ECRH initiation, in an electron heated dominant and core confinement enhanced discharge. Experimental analyses suggest that the e-fishbone ties closely with both the population and energy distribution of energetic electrons. These observations can provide valuable insights into the behavior of energetic electrons, which play a crucial role in the formation and sustainability of transport barriers within hybrid plasma, both in the internal and pedestal regions.

Acceleration and transport of relativistic electrons in the jets of the microquasar SS 433.

SS 433 is a microquasar, a stellar binary system that launches collimated relativistic jets. We observed SS 433 in gamma rays using the High Energy Stereoscopic System (H.E.S.S.) and found an energy-dependent shift in the apparent position of the gamma-ray emission from the parsec-scale jets. These observations trace the energetic electron population and indicate that inverse Compton scattering is the emission mechanism of the gamma rays. Our modeling of the energy-dependent gamma-ray morphology constrains the location of particle acceleration and requires an abrupt deceleration of the jet flow. We infer the presence of shocks on either side of the binary system, at distances of 25 to 30 parsecs, and that self-collimation of the precessing jets forms the shocks, which then efficiently accelerate electrons.

Enhancement of Energetic-Electron Population During Magnetic Reconnection Associated with Strong Tearing Mode in Non-Disruptive HL-2A Plasmas

Enhancement of Energetic-Electron Population During Magnetic Reconnection Associated with Strong Tearing Mode in Non-Disruptive HL-2A Plasmas

Parametrization of Energetic Ion and Electron Fluxes in the Near‐Earth Magnetotail

AbstractThe near‐Earth plasma sheet region is the main source of energetic (tens to hundreds keV) ion and electron populations transported by convection and injections into the inner magnetosphere. Energetic ions from the plasma sheet contribute to the ring current, whereas energetic electrons contribute to the radiation belt seed population for further acceleration to relativistic energies. Near‐Earth plasma sheet energetic fluxes have been traditionally used to set boundary conditions for radiation belt and ring current models. This study provides an empirical parametrization for ∼75 keV flux intensity as a function of the geomagnetic activity index auroral electrojet and the equatorial magnetic field Bz. Such parametrization includes the dynamic magnetic field configuration in the near‐Earth plasma sheet and may be merged with empirical magnetic field models. We also provide models extending this parametrization to the [20, 300] keV of electron energy range and [75, 300] keV of ion energy range. The parametrization is developed based on THEMIS and Geostationary Operational Environmental Satellite measurements, and verified by comparison with MMS measurements in the near‐Earth plasma sheet. This parametrization incorporates meso‐scale transient flux variations associated with Bz perturbations into ring current and radiation belt simulations.

Enhanced laser-driven backward proton acceleration using micro-wire array targets

Micro-structured targets can be employed to enhance the coupling of laser energy to the high energy density plasma. Here we report on experimental measurement of enhanced proton beam energy from laser-driven micro-wire array (MWA) targets along the backward direction. An ultra-intense (∼2×1020W/cm2) laser pulse of ∼ 40 fs pulse duration interacts with the MWA structure and induces large population of energetic electrons. The enhanced sheath field efficiently accelerates protons both transversely and longitudinally. The spectrometers record proton cut-off energy of around 16 MeV and temperature 813keV along the backward direction, which is 20%−60% higher than that of a flat target under commensurate laser conditions. Comparison with particle-in-cell simulations suggests that the enhancement originates from the increased temperature and population of the hot electrons within the micro-wires. These measurements provide a direct probe of the high energy density plasma condition in laser-driven solid targets and a useful benchmark for further studies on laser-driven micro-structured targets.

Modeling of very high frequency large-electrode capacitively coupled plasmas with a fully electromagnetic particle-in-cell code

Phenomena taking place in capacitively coupled plasmas with large electrodes and driven at very high frequencies are studied numerically utilizing a novel energy- and charge-conserving implicit fully electromagnetic particle-in-cell (PIC)/Monte Carlo code ECCOPIC2M. The code is verified with three model problems and is validated with results obtained in an earlier experimental work (Sawada et al 2014 Japan. J. Appl. Phys. 53 03DB01). The code shows a good agreement with the experimental data in four cases with various collisionality and absorbed power. It is demonstrated that under the considered parameters, the discharge produces radially uniform ion energy distribution functions for the ions hitting both electrodes. In contrast, ion fluxes exhibit a strong radial nonuniformity, which, however, can be different at the powered and grounded electrodes at increased pressure. It is found that this nonuniformity stems from the nonuniformity of the ionization source, which is in turn shaped by mechanisms leading to the generation of energetic electrons. The mechanisms are caused by the interaction of electrons with the surface waves of two axial electric field symmetry types with respect to the reactor midplane. The asymmetric modes dominate electron heating in the radial direction and produce energetic electrons via the relatively inefficient Ohmic heating mechanism. In the axial direction, the electron energization occurs mainly through an efficient collisionless mechanism caused by the interaction of electrons in the vicinity of an expanding sheath with the sheath motion, which is affected by the excitation of the surface modes of both types. The generation of energetic electron populations as a result of such mechanisms is shown directly. Although some aspects of the underlying physics were demonstrated in the previous literature with other models, the PIC method is advantageous for the predictive modeling due to a complex interplay between the surface mode excitations and the nonlocal physics of the corresponding type of plasma discharges operated at low pressures, which is hard to reproduce in other models realistically.

Dust ion-acoustic dromions in Saturn’s magnetosphere

ABSTRACTMotivated by observations of localized electrostatic wavepackets by the Cassini – and (earlier) by Voyager 1 and 2 – mission(s) in Saturn’s magnetosphere, we have investigated the existence conditions and the dynamical evolution of localized multidimensional structures in the Saturnian dusty plasma environment. To this effect, we have adopted a plasma-fluid model for dust-ion acoustic excitations, taking into account the presence of a highly energetic (suprathermal, kappa-distributed) electron population in combination with massive dust dust particulates in the background. A multiple scales perturbation method was shown to lead to a Davey--Stewartson (DS) system of evolution equations, if one assumes perpendicular carrier wave propagation across the magnetic field (direction). The system is then shown to possess two regimes mainly, known in the literature as DS-I and DS-II. In the former case, if certain conditions are fulfilled, exponentially localized solutions are obtained, known as dromions. The combined effects of various physical parameters, such as the electron spectral index, the ambient magnetic field (strength), and the dust concentration, have been examined. A numerical investigation reveals that the dromion amplitude increases with higher dust concentration, whereas it decreases for lower κe (i.e. with an increase in the suprathermal electron population component). A stronger magnetic field results in higher amplitude but narrower dromions. Our results provide a comprehensive framework for modeling modulated electrostatic wavepackets, in direct comparison with experimental data in planetary environments, in Saturn’s magnetosphere and elsewhere.

The attributes of the dust-acoustic solitary and periodic structures in the Saturn's inner magnetosphere

Nonlinear characteristics of dust-acoustic (DA) structures including the localized and periodic waves in a plasma having Maxwellian ions and superthermal two-temperature electrons are investigated. The wave equations, including both Kadomtsev–Petviashvili (KP) and modified KP (mKP) equations, are derived using the reductive perturbation technique (RPT). The quantitative and qualitative characteristics of both compressive and rarefactive structures are studied. The Jacobi elliptic function expansion method (JEFEM) is employed for the purpose of quantitative analysis, while the qualitative behavior is studied by the dint of the dynamical system approach. The solutions to the mKP equation hold under a critical condition where the quadratic nonlinearity ceases to exist. It is noticed that the KP equation admits only rarefactive solitary waves (SWs), whereas the mKP equation admits both compressive and rarefactive SWs. It is found that the profile (amplitude and width) of both DA solitary and periodic structures are different at different radii of Saturn's inner magnetosphere. The effect of the kappa spectral index is studied, and it is found that when the population of energetic cold electrons is decreased, the solitary structure gets energized. Our study is applied to Saturn's inner magnetosphere where kappa distributed two-temperature electrons and dust grains with negative charge are observed by various satellite missions.

MAGNETOSPHERIC SUBSTORMS AND RELATIVISTIC ELECTRONS

The most recent findings on the dynamics of the outer radiation belt (ORB) and the physics of magnetospheric substorms are examined. Specifically, we investigate the relationship between storm time substorms and the energetic electron population that forms the ORB. Traditionally, storm time substorms have been considered as the primary source of energetic electrons, which are further accelerated during storms to contribute to the formation of the ORB. However, several observations have demonstrated that large magnetospheric substorms can generate high-energy electrons even in the absence of magnetic storms. Substorms introduce dispersionless injections of energetic electrons deep into the magnetosphere from the geosynchronous orbit during storm times. The injected electrons undergo additional acceleration via the betatron mechanism during the storm recovery phase, thus increasing the ORB population. To gain a better understanding of this process, it is crucial to study plasma sheet turbulence, substorm onset processes, and the brightening of auroral arcs. By analyzing the aforementioned findings, this study aims to highlight the need for reanalyzing of the role of auroral processes in the formation of the ORB.

Energetic electron lensing caused by Ganymede's magnetic field

The micro Advanced Stellar Compass (μASC) continuously monitors high energy particle fluxes in Jupiter's magnetosphere while providing attitude reference for the Juno Magnetic Field investigation. The μASC camera head unit (CHU) serves as a energetic particle sensor monitoring the radiation environment at Jupiter by sensing electrons greater than 15 ​MeV that pass through the CHU shielding. A comparison of the energetic electron population sampled during many orbits reveals events observed when Juno is traversing Ganymede's M-shell. We present observations of energetic electron depletions related to the interaction with Ganymede's magnetic field. The magnitude and extent of these depletions is associated with Juno's proximity to the magnetic field line passing through Ganymede. Analysis of the observations reveal that the charged particle depletion likely arises from electron scattering in Ganymede's magnetic field, observed within ∼13° phase angle from the moon, beyond which the flux tubes refill due to diffusion.

Fluid simulation of dust-acoustic solitary waves in the presence of suprathermal particles: Application to the magnetosphere of Saturn

The observation of dust in the rings of Saturn by instruments on board the Voyager 1, Voyager 2, and Cassini missions triggered our interest in exploring the evolution of electrostatic dust acoustic waves (DAWs) in the Saturnian magnetospheric dusty plasma. The salient features of dust-acoustic electrostatic solitary waves have been examined by means of numerical simulations that adopted a fluid algorithm. We considered highly energetic non-Maxwellian ion and electron populations, in combination with inertial dust. The ions and electrons were modeled by kappa distributions to account for the long-tailed particle distribution featuring a strong suprathermal component. At equilibrium, the initial density perturbation in the dust density was used to trigger the evolution of DASWs propagating in non-Maxwellian dusty plasma. Our main focus is to determine the comprehensive role of the dust concentration and the suprathermal index (kappa) of the ion and electron populations in the generation and evolution of DASWs. These simulation results are thought to be relevant for (and applicable in) existing experimental data in space, especially in the magnetosphere of Saturn, but also in other planetary plasma environments that are presumably characterized by the presence of charged dust.

Electron dynamics in small magnetospheres

Context. The planet Mercury possesses a small but highly dynamic magnetosphere in which the role and dynamics of electrons are still largely unknown. Aims. We aim to model the global dynamics of solar-wind electrons impinging on Mercury’s magnetosphere. Particular relevance is given to local acceleration processes and the global circulation patterns. Methods. The goals of this work are pursued by means of three-dimensional, fully kinetic particle-in-cell simulations modeling the interaction of the solar wind with the Hermean magnetosphere. This method allows a self-consistent representation of the plasma dynamics from the large planetary scale down to the electron kinetic scale. We carried out numerical simulations using two different solar-wind conditions: purely northward or purely southward interplanetary magnetic field direction. Results. We find a high plasma current (of the order of few μA m−2) flowing at the magnetospheric boundaries (bow shock and magnetopause) dominated by electrons. This current is driven by the small-scale electron physics resolved in our model. Furthermore, we observe strong electron acceleration up to tens of keV as a consequence of magnetic reconnection when the interplanetary magnetic field is directed southward. Such energetic electrons are partially trapped in the dipolar magnetic field of the planet mainly at nightside. Finally, by studying the distribution of electrons in our simulations along Mariner10 and BepiColombo first-Mercury-flyby trajectories, we propose that both spacecraft observed this energetic quasi-trapped electron population around closest approach.

Energetic electron scattering by kinetic Alfvén waves at strong magnetic field gradients of dipolarization front

Energetic electron scattering and precipitation from the Earth's plasma sheet to the ionosphere is an important contributor to magnetosphere–ionosphere coupling. In this study, we investigate the role of one of the most intense wave emissions, kinetic Alfvén waves (KAWs), in energetic electron scattering. We have evaluated the effect of KAWs on energetic electrons within a curved magnetic field configuration exhibiting sharp cross field gradients. The magnetic field in Earth's magnetotail plasma sheet with an embedded dipolarization front is used as a working example. Taking into account electron bounce motion and perpendicular guiding-center drifts, we have shown that electrons with energies of tens to hundreds of keV can be scattered by KAWs in pitch angle and momentum through Doppler-shifted Landau resonance near the magnetic equator. The bounce-averaged pitch-angle diffusion coefficients for near-loss-cone (∼2°) electrons are on the order of 10–7–10–6 rad2/s for a characteristic KAW amplitude of 1 mV/m and approach the strong diffusion limit of ∼10–4 rad2/s for amplitudes of greater than 10 mV/m. These results suggest that under such ambient conditions, KAWs can pitch-angle scatter energetic electron population into the loss cone. In Earth's plasma sheet, this scattering is, thus, very likely to cause significant precipitation during active times. The diffusion coefficients of energetic electrons at large pitch angles (∼45°–∼80°) are more than two orders of magnitude larger than those of electrons near the loss cone, suggesting that KAWs contribute to isotropization of anisotropic electrons due to adiabatic heating should they drift into the vicinity of the magnetic field gradient.

On the stability of 2D modulated electrostatic wavepackets in non-Maxwellian dusty plasma – application in Saturn’s magnetosphere

ABSTRACT Motivated by observations of localized electrostatic wavepackets by the Voyager 1 and 2 and Cassini missions in Saturn’s magnetosphere, we have investigated the evolution of modulated electrostatic wavepackets in a dusty plasma environment. The well-known dust-ion acoustic (DIA) mode was selected to explore the dynamics of multidimensional structures, by means of a Davey–Stewartson (DS) model, by taking into account the presence of a highly energetic (suprathermal, kappa-distributed) electron population in combination with heavy (immobile) dust in the background. The modulational (in)stability profile of DIA wavepackets for both negative as well as positive dust charge is investigated. A set of explicit criteria for modulational instability (MI) to occur is obtained. Wavepacket modulation properties in 3D dusty plasmas are shown to differ from e.g. Maxwellian plasmas in 1D. Stronger negative dust concentration results in a narrower instability window in the K (perturbation wavenumber) domain and to a suppressed growth rate. In the opposite manner, the instability growth rate increases for higher positive dust concentration and the instability window gets larger. In a nutshell, negative dust seems to suppress instability while positive dust appears to favour the amplitude modulation instability mechanism. Finally, stronger deviation from the Maxwell–Boltzmann equilibrium, i.e. smaller κe values, lead(s) to stronger instability growth in a wider wavenumber window – hence suprathermal electrons favour MI regardless of the dust charge sign (i.e. for either positive or negative dust). The wavepacket modulation properties in 2D dusty plasmas thus differ from e.g. Maxwellian plasmas in 1D, both quantitatively and qualitatively, as indicated by a generalized dispersion relation explicitly derived in this paper (for the amplitude perturbation). Our results can be compared against existing experimental data in space, especially in Saturn’s magnetosphere.

Type III radio bursts and excitation of Langmuir waves by energetic electrons

Context.Solar activity occurs not only in terms of the well-known 11-year Sun spot cycle but also in terms of short-lived phenomena as radio bursts. For instance, type III radio bursts are the most common phenomenon of this activity in the Sun’s radio radiation. In dynamic radio spectra, they appear as short-lived stripes of enhanced radio emission rapidly drifting from high to low frequencies. They are regarded as the radio signature of beams of energetic electrons travelling along magnetic field lines in the corona. The radio emission is thought to be plasma emission, that is to say the radio emission happens near the electron plasma frequency and/or its harmonics. Plasma emission means, that energetic electrons excite Langmuir waves, which convert into radio waves.Aims.Initially, energetic electrons are injected in a small region in the corona. Due to their spatio-temporal evolution, they develop a beam-like velocity distribution function (VDF), which is able to excite Langmuir waves. The aim of the paper is to study the spatio-temporal behaviour of the generation of Langmuir waves under coronal cirumstances and its effect on type III radio bursts.Methods.The generation of Langmuir waves is treated by means of the Maxwell-Vlasov equations. The results are discussed by employing plasma parameters usually found in the corona, for instance at the 150 MHz level.Results.The Langmuir waves associated with the type III bursts are not generated by a monoenergetic electron beam, but by a population of energetic electrons with a broad velocity distribution. Hence, the Langmuir waves are produced by different parts of the energetic electron population at different times and positions.Conclusions.In the case of type III bursts, the velocities derived from their drift rates in dynamic radio spectra are not the velocities of electrons, which generate the onset of the type III burst at a given frequency. That can lead to an apparent accelerated motion of the type III radio burst source.

Dynamical Modulation of Solar Flare Electron Acceleration due to Plasmoid-shock Interactions in the Looptop Region

Abstract A fast-mode shock can form in the front of reconnection outflows and has been suggested as a promising site for particle acceleration in solar flares. Recent developments in the study of magnetic reconnection have shown that numerous plasmoids can be produced in a large-scale current layer. Here we investigate the dynamical modulation of electron acceleration in the looptop region when plasmoids intermittently arrive at the shock by combining magnetohydrodynamics simulations with a particle kinetic model. As plasmoids interact with the shock, the looptop region exhibits various compressible structures that modulate the production of energetic electrons. The energetic electron population varies rapidly in both time and space. The number of 5–10 keV electrons correlates well with the compression area, while that of >50 keV electrons shows good correlation with the strong compression area but only moderate correlation with shock parameters. We further examine the impacts of the first plasmoid, which marks the transition from a quasi-steady shock front to a distorted and dynamical shock. The number of energetic electrons is reduced by ∼20% at 15–25 keV and nearly 40% for 25–50 keV, while the number of 5–10 keV electrons increases. In addition, the electron energy spectrum above 10 keV evolves softer with time. We also find that double or even multiple distinct sources can develop in the looptop region when the plasmoids move across the shock. Our simulations have strong implications to the interpretation of nonthermal looptop sources, as well as the commonly observed fast temporal variations in flare emissions, including the quasi-periodic pulsations.

Enhanced laser-driven proton acceleration with gas–foil targets

We study numerically the mechanisms of proton acceleration in gas–foil targets driven by an ultraintense femtosecond laser pulse. The target consists of a near-critical-density hydrogen gas layer of a few tens of microns attached to a $2\ \mathrm {\mu }$ m-thick solid carbon foil with a contaminant thin proton layer at its back side. Two-dimensional particle-in-cell simulations show that, at optimal gas density, the maximum energy of the contaminant protons is increased by a factor of $\sim$ 4 compared with a single foil target. This improvement originates from the near-complete laser absorption into relativistic electrons in the gas. Several energetic electron populations are identified, and their respective effect on the proton acceleration is quantified by computing the electrostatic fields that they generate at the protons’ positions. While each of those electron groups is found to contribute substantially to the overall accelerating field, the dominant one is the relativistic thermal bulk that results from the nonlinear wakefield excited in the gas, as analysed recently by Debayle et al. (New J. Phys., vol. 19, 2017, 123013). Our analysis also reveals the important role of the neighbouring ions in the acceleration of the fastest protons, and the onset of multidimensional effects caused by the time-increasing curvature of the proton layer.

Leptonic or Hadronic Emission: The X-Ray Radiation Mechanism of Large-scale Jet Knots in 3C 273

Abstract A comprehensively theoretical analysis of the broadband spectral energy distributions (SEDs) of large-scale jet knots in 3C 273 is presented to reveal their X-ray radiation mechanism. We show that these SEDs cannot be explained with a single-electron population model when the Doppler boosting effect is either considered or not. By adding a more energetic electron (the leptonic model) or proton (the hadronic model) population, the SEDs of all knots are well represented. In the leptonic model, the electron population that contributes the X-ray emission is more energetic than the one responsible for the radio-optical emission by almost two orders of magnitude; the derived equipartition magnetic field strengths (B eq) are ∼0.1 mG. In the hadronic model, protons with energy ∼20 PeV are required to interpret the observed X-rays; the B eq values are several mG, larger than those in the leptonic model. Based on the fact that no resolved substructures are observed in these knots and the fast cooling time of the high-energy electrons does not easily explain the observed X-ray morphologies, we argue that the two distinct electron populations accelerated in these knots are unreasonable and their X-ray emission is attributed to the proton synchrotron radiation accelerated in these knots. In cases where these knots have relativistic motion toward the observer, the super-Eddington issue of the hadronic model can be avoided. Multiwavelength polarimetry and γ-ray observations with high resolution may be helpful to discriminate these models.

On Whistler Mode Wave Relation to Electron Field‐Aligned Plateau Populations

AbstractWhistler mode waves are among the most intense electromagnetic emissions and play an important role in the energy redistribution between electron populations in the Earth inner magnetosphere through wave‐particle resonant interactions. Usually generated by transversely anisotropic plasma sheet electron populations ( 10–30 keV) through cyclotron resonance, whistler mode waves can effectively accelerate a small fraction of the seed population of energetic electrons ( 100 keV) up to relativistic energies. However, these waves can be efficiently damped through simultaneous interactions with much more numerous suprathermal electrons ( 0.1–1 keV) via Landau resonance. Recent observations indeed show that electron distributions accompanied by intense whistler mode emissions often contain a plateau‐like electron population at energies close to the energy of Landau resonance with the waves. However, simultaneous observations of these waves and of the related plateau population does not prove a causal relationship. Here, we test the hypothesis that such a plateau population may have been formed by whistler mode waves generated earlier, or by other types of waves. Combining analytical estimates and spacecraft observations, we show that this plateau population is often unlikely to be formed by whistler mode waves alone. We suggest three alternative scenarios that can lead to the formation of plateau populations and test these scenarios based on spacecraft observations. We show that a plateau population can be formed by ultralow frequency electric fields (carried by kinetic Alfven waves or time domain structures) often accompanying injections of plasma sheet electrons—the energy source for whistler mode waves. We also discuss the possible role of ionospheric secondary electrons.