Suprathermal Electron Population Research Articles

Modeling the electron cyclotron emission radiation signature from suprathermal electrons in a tokamak.

An Electron Cyclotron Emission (ECE) modeling code has been developed to model ECE radiation with an arbitrary electron momentum distribution, a small oblique angle, both ordinary (O-mode) and extraordinary polarizations (X-mode), and multiple cyclotron frequency harmonics. The emission and absorption coefficients are calculated using the Poynting theorem from the cold plasma dispersion and the electron-microwave interaction from the full anti-Hermitian tensor. The modeling shows several ECE radiation signatures that can be used to diagnose the population of suprathermal electrons in a tokamak. First, in an n = 2 X-mode (X2) optically thick plasma and oblique ECE view, the modeling shows that only suprathermal electrons, which reside in a finite region of the velocity and space domains, can effectively generate cyclotron emissions to the ECE receiver. The code also finds that the O1 mode is sensitive to suprathermal electrons of both a high v⊥ and v‖, while the X2 mode is dominantly sensitive to suprathermal electrons of a high v⊥. The modeling shows that an oblique ECE system with both X/O polarization and a broad frequency coverage can be used to effectively yield information of the suprathermal electron population in a tokamak.

Probing Turbulent Scattering Effects on Suprathermal Electrons in the Solar Wind: Modeling, Observations, and Implications

This study explores the impact of a turbulent scattering mechanism, akin to those influencing solar and galactic cosmic rays propagating in the interplanetary medium, on the population of suprathermal electrons in the solar wind. We employ a Fokker–Planck equation to model the radial evolution of electron pitch angle distributions under the action of magnetic focusing, which moves the electrons away from isotropy, and of a diffusion process that tends to bring them back to it. We compare the steady-state solutions of this Fokker–Planck equation with data obtained from the Solar Orbiter and Parker Solar Probe missions and find a remarkable agreement, varying the turbulent mean free path as the sole free parameter in our model. The obtained mean free paths are of the order of the astronomical unit, and display weak dependence on electron energy within the 100 eV–1 keV range. This value is notably lower than Coulomb collision estimates but aligns well with observed mean free paths of low-rigidity solar energetic particle events. The strong agreement between our model and observations leads us to conclude that the hypothesis of turbulent scattering at work on electrons at all heliospheric distances is justified. We discuss several implications, notably the existence of a low Knudsen number region at large distances from the Sun, which offers a natural explanation for the presence of an isotropic “halo” component at all distances from the Sun—electrons being isotropized in this distant region before traveling back into the inner part of the interplanetary medium.

Electron Heating by Magnetic Pumping and Whistler-mode Waves

The investigation of mechanisms responsible for the heating of cold solar wind electrons around the Earth’s bow shock is an important problem in heliospheric plasma physics because such heating is vitally required to run the shock drift acceleration at the bow shock. The prospective mechanism for electron heating is magnetic pumping, which considers electron adiabatic (compressional) heating by ultralow-frequency waves and simultaneous scattering by high-frequency fluctuations. Existing models of magnetic pumping have operated with external sources of such fluctuations. In this study, we generalize these models by introducing the self-consistent electron scattering by whistler-mode waves generated due to the anisotropic electron heating process. We consider an electron population captured within a magnetic trap created by ultralow-frequency waves. Periodical adiabatic heating and cooling of this population drives the generation of whistler-mode waves scattering electrons in the pitch-angle space. The combination of adiabatic heating and whistler-driven scattering provides electron acceleration and the formation of a suprathermal electron population that can further participate in the shock drift acceleration.

Toward the Unified Theory of SAID‐Linked Subauroral Arcs

AbstractWe present a unified approach to subauroral arcs within intense subauroral ion drifts (SAID), which explains the observed transition of a precursor Stable Auroral Red (SAR) arc into Strong Thermal Emission Velocity Enhancement (STEVE). This approach is based on the short‐circuiting concept of fasttime SAID as an integral part of a magnetospheric voltage generator between the innermost boundaries of the freshly injected plasma sheet electrons and ring current ions. Here, enhanced plasma turbulence rapidly heats the bulk plasma and accelerates suprathermal non‐Maxwellian “tails.” Heat and suprathermal electron transport rapidly elevate the ionospheric electron temperature—the source of a bright SAR arc. Through a substorm, the density altitude profile within the evolving ionospheric SAID channel transforms into a “fresh” F‐region trough with the E‐region valley. The ionospheric feedback instability within the depleted‐density SAID channel generates small‐scale, field‐aligned currents with parallel electric fields sufficient to produce the suprathermal electron population, exciting the STEVE and Picket Fence emissions. This approach also explains the inner electromagnetic structure of intense SAID, which is consistent with fine optical structures in STEVE and Picket Fence.

Electrostatic solitary waves in a bi-ion plasma with two suprathermal electron populations – application to Saturn’s magnetosphere

ABSTRACT Non-thermal particle distributions characterized by a high-energy tail are ubiquitous in space plasmas. They are usually described by a kappa distribution function, that has been shown to be an excellent fit in most real circumstances. Among other space missions, Cassini and Voyager have both recorded evidence of a coexistence of non-thermal electron populations (with different characteristics) in Saturn’s magnetosphere, and subsequent studies showed that these are well-described by using different tailor-fit realizations of the (parametrized) kappa distribution. Motivated by these observations we have formulated a multifluid plasma model incorporating two types of (positive) ions and two distinct kappa-distributed electron populations, in order to study electrostatic solitary waves (ESWs) in Saturn’s magnetosphere from first principles. Our analysis reveals that the spectral index (in fact, the κ parameter value related to the cold electron population mainly) is vital in explaining the difference among different types of non-linear structures. A comparison with spacecraft observations suggests that our theoretical model provides an efficient framework for the interpretation of ESW observations in Saturn’s magnetosphere. Our qualitative predictions may also apply to other planetary magnetospheres, where a similar multispecies plasma composition may be present.

Potential for electron acoustic mode in the coma of a weakly outgassing comet

This study aims to explore the electron acoustic (EA) mode in the vicinity of comet 67P/Churyumov–Gerasimenko, considering the existence of multiple electron species. In Region I, where all three suprathermal, warm and cold electron populations present and in Region II, where only warm and cold electrons present there is a chance to observe the EA mode for the plasma parameters measured at the comet. At smaller wave number (k), hotter electrons whereas at larger k, the cold electrons cause damping of the mode, and the mode only propagates at intermediate wave lengths. In these regions, it is determined that the mode observation enhances with the streaming speeds of hotter species, and it could be observed even for , which is not possible at lower or in the absence of drift speeds. In Region III, both warm and suprathermal electrons significantly damp the mode and hence, it is non-observable for the reported plasma parameters, and the mode could be only observable for the temperature ratio and surprisingly, the most favorable density for the observation is . Here, and stand for suprathermal and warm electron temperatures, respectively.

Experimental and numerical investigations of electron transport enhancement by electron-cyclotron plasma-wave interaction in tokamaks

Energy transfer from electron-cyclotron (EC) waves to the plasma is being routinely used in tokamaks to heat and drive current through the electron channel. Technical applications such as magnetohydrodynamic mode mitigation require power deposition with a high degree of localization. However, observations made in tokamaks show a broader distribution of suprathermal electrons than predicted by standard drift-kinetic codes. The present paper explores a possible wave-induced increase of electron turbulent transport that may explain the experimental data, using power-modulated EC waves in the Tokamak à Configuration Variable (TCV). In particular, an indirect measurement of the suprathermal electron population via hard x-rays exhibits an enhanced radial transport with increased wave power. This correlates well with the measured increase of the density fluctuation level during the power pulses, associated with the destabilization of ion temperature gradient modes and trapped electron modes and with stiff electron profiles. Forward bounce-averaged drift-kinetic simulations show that a radial diffusion model directly proportional to the wave power deposition is required to match the experimental data. The power dependency is confirmed by global flux-driven gyro-kinetic simulations using a realistic EC power source, computing turbulent transport from first principles and showing a radial increase of electron transport with increased wave power.

The Inner Structure of STEVE‐Linked SAID

AbstractWe found the inner electromagnetic structure of subauroral ion drifts (SAID) in the SAID‐STEVE events documented by the Swarm spacecraft and numerically simulated the ionospheric feedback instability (IFI) development for one of the four similar events. Good quantitative agreement of the modeling results with the observed features shows that the ionospheric feedback mechanism captures their basic underlying physics. Simulations require nonlinear saturation of the IFI‐generated dispersive Alfvén waves. That is, a strong driving field of STEVE‐linked SAID with a deep density trough leads to a nonlinear system of dispersive Alfvén waves coupled with the density perturbation and parallel electric fields. As shown earlier, these fields produce the suprathermal electron population and energy balance necessary for the STEVE and Picket Fence radiation. Therefore, our results predict their inner structure.

Strong Nonideal Electric Fields and Energy Dissipation Observed by MMS within Field-Aligned Current Layers in the Plasma Sheet of the Earth’s Magnetotail

We report the MMS observations of the intense spikes of field-aligned current (FAC) produced by magnetic reconnection at the plasma sheet (PS) field lines. The MMS was located tailward of a near-Earth X-line and the most intense spike of FAC with an electric current density of ∼70 nA/m2 was observed near the magnetic separatrix. The FAC structures located deeper in the PS were strongly filamented and consisted of several spikes with a thickness of ∼(1–2)ρe (ρe is the gyroradius of thermal electrons). We found that the FAC in these structures was carried by unmagnetized thermal and suprathermal electron populations (≥ 1 keV), which were ∼(20–80)% of the entire electron population. Strong nonideal electric fields up to ∼100 mV/m were detected in the FAC spike near the magnetic separatrix. The generation of these fields was mainly due to the anomalous resistivity, possibly caused by the electrostatic fluctuations. As a result, a significant energy dissipation of up to 1.3 nW/m3 occurred within the electron-scale FAC structure, which caused an increase in the electron temperature by a factor of two compared with that outside the FAC. Thus, MMS observations demonstrate that during the interval of the active X-line, the outer part of the PS consists of multiple electron-scale FAC layers/filaments in which a significant energy exchange between electrons and fields occurs. To investigate the stability of these filaments and estimate their lifetime, additional observations and theoretical studies are needed.

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.

Retracted: New Types of Electron Pitch Angle Distributions on Mars: Funnel and Skirt Distributions

AbstractUsing 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 , 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 , 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.

Solar Orbiter SWA Observations of Electron Strahl Properties Inside 1 AU

The Solar Wind Analyser (SWA) suite on Solar Orbiter includes an Electron Analyser System (SWA-EAS) which is capable of high temporal and angular resolution measurements of solar wind electrons in the energy range ∼1 eV to ∼5 keV. In this article we report early nominal phase observations of the suprathermal electron population at energies ≥70 eV (representative of the ’strahl’ population), and use a simple fitting routine and classification system to determine the characteristics of the distributions and determine the variations in their properties as a function of heliocentric distance and solar wind properties. We find that under our classification system a significant population of radially outward moving strahl beams is identifiable in the tested samples. These are seen in across solar wind speed regimes, but, consistent with earlier observations, are slightly more prevalent in high speed wind. These beams occur at all distances examined (∼0.43 to ∼1.0 AU), but do not show significant evolution with distance, suggesting a balance between focusing and scattering processes across the distance range covered. However, the data suggests that the beams broaden on average with increasing magnetic field strength and narrow on average with increasing solar wind speed. We also identify a small population, occurring in sporadic clusters, which have deficits in phase space density in the sunward moving part of the electron distribution. These clusters occur across the distance range sampled and show some variations in average properties with radial distance, suggesting they too are influenced by competing scattering and (de-)focusing processes. The implications for the origin and evolution of these electron populations derived from these new observations are explored.

Dust particle surface potential in fusion plasma with supra-thermal electrons

By solving the current balance equations, the effect of the supra-thermal electrons on the surface potential of a negatively charged dust particle in a fusion plasma is studied based on the orbital motion limited theory. A non-Maxwellian plasma is modeled by employing a q-non-extensive velocity distribution, where a decreased q-parameter (q < 1) yields increased population of supra-thermal electrons. It is found that with the decrease in the q-parameter, the surface potentials of tungsten and carbon dust particles become more negative. When the ion temperature or ion flow velocity is increased, the dust surface potential changes non-monotonically, which is first decreasing and then increasing. For a small q-parameter, the non-monotonic variation is more pronounced and the minimum value of surface potential moves toward higher values of the ion temperature or ion flow velocity. Due to the dependence of the dust particle surface potential on the supra-thermal electrons, the increased proportion of supra-thermal electrons causes the increase in the ion drag force and the decrease in the dust particle lifetime. Here, lifetime refers to the time taken for the dust surface temperature to increase from its initial value to the sublimation temperature (for carbon) or the melting temperature (for tungsten). The decrease in lifetime caused by supra-thermal electrons is significant as the dust particle radius is increased.

Switchbacks in the Young Solar Wind: Electron Evolution Observed inside Switchbacks between 0.125 au and 0.25 au

Switchbacks are localized deviations from the nominal Parker spiral field in the solar wind. In this study, we investigate the electron distributions inside switchbacks, focusing primarily on the suprathermal (halo and strahl) populations. We explore electron parameters in relation to the angle of rotation of the magnetic field from radial to determine whether electron distributions observed within switchbacks have any differences from those outside of switchbacks. Our observations reveal several trends in the suprathermal electron populations inside switchbacks. We find that the sunward deficit in the electron velocity distribution function typically observed near the Sun is filled in at larger rotation angles. This results in the suprathermal electron density and heat flux in the antistrahl direction changing from a negative to a positive value. On many days, we also observe a positive correlation between the halo density and rotation angle, and this may suggest that the growth of the halo may fill in the sunward deficit. We also find that strahl distributions have an increased average angular spread at large magnetic field rotation angles. The increase in suprathermal electron flux in the antistrahl direction, and the increase in strahl width, together could suggest that enhanced scattering occurs inside switchbacks. Electron core beta values tend to increase with the magnetic field rotation angle, mainly due to a decrease in magnetic pressure. An increase in electron beta may favor the growth of instabilities inside switchbacks. The Parker Solar Probe observations therefore support an enhanced role for wave–particle interactions in switchbacks.

On the Kinetic Theory of Subauroral Arcs

AbstractWe report on a novel scenario of subauroral arcs within strong subauroral ion drifts (SAID)‐STEVE and Picket Fence. Their explanation requires a local source of low‐energy, ε < 18.75 eV, suprathermal electrons, and N2 vibrational and electronic excitation below ∼270 km. We show that the ionospheric feedback instability in strong SAID flows with depleted density troughs generates intense, small‐scale field‐aligned currents and parallel electric fields below the F2 peak. With these fields, we employed a rigorous numerical solution of the Boltzmann kinetic equation for the distribution of ionospheric electrons and determined the power going to excitation and ionization of neutral gas (the energy balance). The obtained suprathermal electron population and energy balance at altitudes of ∼130–140 km are just what is necessary for Picket Fence. Concerning STEVE, the kinetic theory predictions are in a good qualitative agreement with its basic features, such as the enhanced continuum emissions. Besides, the theory predicts that subauroral arcs might have the transient phase with typical aurora‐like emissions that fade out afterward.

Is the Solar Wind Electron Strahl a Seed Population for the Earth’s Electron Radiation Belt?

1) Since the outer electron radiation belt is lost on occasion, the radiation belt needs seed electrons to rebuild. 2) The clear candidate for that seed population is energetic substorm-injected electrons in the dipolar magnetosphere. 3) The energetic substorm-injected electrons in the dipole come from the suprathermal electron population in the magnetotail plasma sheet, delivered by substorms. Scenario (1)–3) begs the question: Where do these magnetotail suprathermal electrons come from? We are hypothesizing that one source (perhaps the dominant source) is the energetic field-aligned electron strahl in the solar wind, which are electrons fresh from the solar corona.

Simulation of Proton Acceleration With Varying Laser Intensity in the Presence of Suprathermal Electrons

In the framework of high laser–plasma interaction, 1-D particle-in-cell (PIC) simulations are performed in order to optimize the role of electron suprathermality to generate high-quality proton beams useful for diverse applications, despite the presence of high nonlinear laser effects. For that, the electrons are supposed to obey the kappa distribution function discretized in the simulation code. Various characteristics of the proton beams, such as proton and electron densities, electric fields, front position, and energy spectra, have been analyzed as functions of electron suprathermality and laser amplitude. It is shown that, in the case of the presence of an important population of suprathermal electrons, the protons converge toward the same energy, leading to a quasi-monoenergetic beam and to good front position stability despite the presence of a large laser field. In addition, the two effects of electron suprathermality and laser intensity are added to enhance the proton acceleration. It turns out that in the analyzed range of laser intensities, the presence of the suprathermal electron population is beneficial for the proton acceleration process, which minimizes the nonlinear effects due to the laser field induced in high laser–plasma acceleration.

Approximate atomic models for fast computation of the Fokker–Planck equation in fusion plasmas with high-Z impurities and suprathermal electrons

With the choice of tungsten as a material for the ITER plasma facing components, the suprathermal electron interaction with non-fully ionized impurities emerged as an important issue in plasma modeling. Microwave heating and current drive systems, especially lower hybrid current drive, can generate a significant population of suprathermal electrons in the plasma. Also, in the case of the runaway electron generation and mitigation by massive gas injection, the collisions with impurities can have a significant impact on the electron drag force. A correct description of the fast electrons collisions with non-fully ionized impurities requires calculation of the atomic form factor. This can be done with ab initio models that are accurate, though time consuming in practical applications. In this paper, we compare existing approximations of the form factors, based on the Thomas–Fermi or Pratt–Tseng models. Ab initio density functional theory (DFT) calculations are used as a reference method to determine the accuracy of the compared models. Based on this analysis, we propose some modifications of the existing models, tuned with numerical parameter optimization, which provide a higher accuracy while maintaining a short computation time. These modifications include multiple exponents in the Pratt–Tseng model and fitting the parameters of the form factor equation to the DFT-based results. Some applications of the presented models to the calculation of the elastic and inelastic collision frequencies for Fokker–Planck equation are presented, showing a good agreement between the results obtained with the DFT method and the proposed models.

Nonlinear dynamics and phase space transport by chorus emission

Chorus emission in planetary magnetospheres is taken as working paradigm to motivate a short tutorial trip through theoretical plasma physics methods and their applications. Starting from basic linear theory, readers are first made comfortable with whistler wave packets and their propagation in slowly varying weakly non-uniform media, such as the Earth’s magnetosphere, where they can be amplified by a population of supra-thermal electrons. The nonlinear dynamic description of energetic electrons in the phase space in the presence of self-consistently evolving whistler fluctuation spectrum is progressively introduced by addressing renormalization of the electron response and spectrum evolution equations. Analytical and numerical results on chorus frequency chirping are obtained and compared with existing observations and particle-in-cell simulations. Finally, the general theoretical framework constructed during this short trip through chorus physics is used to draw analogies with condensed matter and laser physics as well as magnetic confinement fusion research. Discussing these analogies ultimately presents plasma physics as an exciting cross-disciplinary field to study.

Origin of the Weak Plasma Emission Line Detected by Voyager 1 in the Interstellar Medium: Evidence for Suprathermal Electrons

Abstract Recently, a very weak, nearly continuous plasma wave emission line has been discovered in the nearby interstellar medium at the electron plasma frequency. The new observations were made by the plasma wave instrument on the Voyager 1 spacecraft, which crossed into the interstellar medium in 2012 August. Several questions remained unanswered after the initial discovery. Why was the emission line not observed until several years after Voyager 1 entered the interstellar medium, what is the wavelength of the plasma oscillations responsible for the emission line, and what is the origin of the oscillations? Here, we provide answers to these questions. On the most important question, namely the origin of the oscillations, the evidence strongly suggest that the emission is driven by suprathermal electrons that excite plasma oscillations comparable to the quasi-thermal noise (QTN) that is commonly observed by space plasma wave instruments with long, thin electric dipole antennas. These results imply the existence of a relatively dense population of suprathermal electrons that could contribute significantly to the overall pressure in the interstellar medium. Although the similarities to the previous QTN observations are impressive, there is no certainty that the emissions are driven by thermal excitation, and other sources should be explored, such as the possibility that they are driven by pressure fluctuations associated with the short-wavelength cascade of interstellar turbulence.