Energy Of Energetic Electrons Research Articles

Energetic electrons and their contribution to the breakdown of a point-plane air gap with a positive nanosecond pulse

It is found that the polarity reversal phenomenon occurs in the nanosecond pulse breakdown experiment. Two-dimensional axisymmetric particle-in-cell/Monte Carlo collisions’ calculation is used to observe energetic electrons at positive nanosecond pulse voltage in atmospheric air and compared with previous calculation results under negative polarity in an attempt to explain the polarity reversal phenomenon. During the evolution of the positive polarity discharge, the difference in spatial net charge distribution leads to distortion of the electric field, which makes the electric field in the area of the ionization channel head very high, exceeding the threshold at which electrons can enter the high-energy state. The simulation results show that although electrons travel in the opposite direction to the ionization channel, energetic electrons can also be generated during the positive polarity discharge’s evolution, which may differ from what some researchers expected. However, it is also found that the maximum energy of energetic electrons under positive nanosecond pulse voltage is lower than that under negative nanosecond pulse voltage (only about 1/4). This may be mainly because in the case of positive polarity, the energetic electrons in the head of the ionization channel will move to the low-field intensity region inside the ionization channel and cannot be accelerated continuously. However, it must be pointed out that in the case of positive polarity, energetic electrons still contribute significantly to the rapid breakdown of the air gap. This study provides a deeper understanding of the physics of nanosecond pulse discharge.

Investigations into growth of whistlers with energy of energetic electrons

Runaway electrons have great significance in fusion plasmas as they are considered to be responsible for the excitation of instabilities and for damage to the first wall components. The threshold for the generation of the runaway electron population in tokamaks strongly depends on the toroidal magnetic field and bulk electron temperature. The presence of these runaway electrons with a critical density and magnetic field value are correlated with the magnetosonic whistler activity. The Rosenbluth condition applies to the destabilization of upper hybrid modes, that remains mostly stable in large-scale tokamaks like Joint European Tokamakand International Thermonuclear Experimental reactor. On the other hand, whistlers with a frequency in the lower hybrid range are more likely to be destabilized in a lower magnetic field and temperature, along with the whistlers’ excitation threshold. The process of destabilization of whistlers by energetic electrons thus has a finite overlap between fusion and basic plasma devices, and the outcome from the latter can be scalable to fusion machines. We report that in the linear large volume plasma device, the destabilization of quasi-longitudinal whistlers, driven by the reflected particles from the mirror, takes place in the bulk plasma region i.e. away from the mirror in the energetic belt region, and follows the frequency ordering of . Our explanation involves an alternate limit of quasi-linear analysis, commonly employed for recovering the instability threshold of runaways scattering off whistlers in fusion plasmas, and the growth rate due to the reflected particle is proportional to the inverse square root of electron temperature, where the reflected particle fraction is (Sharma and Vlahos 1984 Astrophys. J. 280 405). The measurements required to obtain the energy scaling of runaway electrons with whistler instability are critical for large devices but remain scarce. We have emphasized the analytic correlation between the two regimes in these investigations.

PIC Simulation of Double Plasma Resonance and Zebra Pattern of Solar Radio Bursts

Abstract The latest study has reported that plasma emission can be generated by energetic electrons of Dory–Guest–Harris distribution via the electron cyclotron maser instability (ECMI) in plasmas characterized by a large ratio of plasma oscillation frequency to electron gyro-frequency (ω pe /Ω ce ). In our study, on the basis of the ECMI-plasma emission mechanism, we examine the double plasma resonance (DPR) effect and the corresponding plasma emission at both harmonic (H) and fundamental (F) bands using particle-in-cell simulations with various ω pe /Ω ce . This allows us to directly simulate the feature of the zebra pattern (ZP) observed in solar radio bursts for the first time. We find that (1) the simulations reproduce the DPR effect nicely for the upper hybrid and Z modes, as seen from their variation of intensity and linear growth rate with ω pe /Ω ce , (2) the intensity of the H emission is stronger than that of the F emission by ∼2 orders of magnitude and varies periodically with increasing ω pe /Ω ce , while the F emission is too weak to be significant (therefore, we suggest that it is the H emission accounting for solar ZPs), (3) the peak-valley contrast of the total intensity of H is ∼4, and the peak lies around integer values of ω pe /Ω ce (=10 and 11) for the present parameter setup. We also evaluate the effect of energy of energetic electrons on the characteristics of ECMI-excited waves and plasma radiation. The study provides novel insight on the physical origin of ZPs of solar radio bursts.

Harmonic Electron Cyclotron Maser Emission Excited by Energetic Electrons Traveling inside a Coronal Loop

Abstract A complete understanding of solar radio bursts requires developing numerical techniques that can connect large-scale activities with kinetic plasma processes. As a starting point, this study presents a numerical scheme combining three different techniques: (1) extrapolation of the magnetic field overlying a specific active region in order to derive the background field, (2) guiding-center simulation of the dynamics of millions of particles within a selected loop to reveal the integral velocity distribution function (VDF) around certain sections of the loop, and (3) particle-in-cell simulation of kinetic instabilities driven by energetic electrons initiated by the obtained distributions. Scattering effects at various levels (weak, moderate, and strong) due to wave turbulence-particle interaction are considered using prescribed timescales of scattering. It was found that the obtained VDFs contain strip-like and loss-cone features with positive gradient, and both features are capable of driving electron cyclotron maser emission, which is a viable radiation mechanism for some solar radio bursts, in particular, solar radio spikes. The strip-like feature is important in driving the harmonic X mode, while the loss-cone feature can be important in driving the fundamental X mode. In the weak-scattering case, the rate of energy conversion from energetic electrons to X2 can reach up to , where is the initial kinetic energy of energetic electrons. The study demonstrates a novel way of exciting the X2 mode in the corona during solar flares and provides new sight into how escaping radiation can be generated within a coronal loop.

Energetic Electron Acceleration by Ion-scale Magnetic Islands in Turbulent Magnetic Reconnection: Particle-in-cell Simulations and ARTEMIS Observations

Abstract Particle-in-cell simulations show that numerous ion-scale magnetic islands can be formed in a turbulent magnetic reconnection region. These islands can confine and accelerate electrons to high energies effectively, causing the energetic electron energy flux to peak at the center of each island. ARTEMIS spacecraft observations of turbulent reconnection outflows in Earth’s magnetotail reveal that peaks in the energetic electron energy flux are well correlated with bipolar signatures of reconnected magnetic field, which are indicative of ion-scale magnetic islands, providing observational evidence for this acceleration process.

The Role of Energy Diffusion in the Deposition of Energetic Electron Energy in Solar and Stellar Flares

Abstract During solar flares, a large fraction of the released magnetic energy is carried by energetic electrons that transfer and deposit energy in the Sun’s atmosphere. Electron transport is often approximated by a cold thick-target model, assuming that electron energy is much larger than the temperature of the ambient plasma, and electron energy evolution is modeled as a systematic loss. Using kinetic modeling of electrons, we reevaluate the transport and deposition of flare energy. Using a full collisional warm-target model (WTM), we account for electron thermalization and for the properties of the ambient coronal plasma such as its number density, temperature and spatial extent. We show that the deposition of nonthermal electron energy in the lower atmosphere is highly dependent on the properties of the flaring coronal plasma. In general, thermalization and a reduced WTM energy loss rate leads to an increase of nonthermal energy transferred to the chromosphere, and the deposition of nonthermal energy at greater depths. The simulations show that energy is deposited in the lower atmosphere initially by high-energy nonthermal electrons, and later by lower energy nonthermal electrons that partially or fully thermalize in the corona, over timescales of seconds, unaccounted for in previous studies. This delayed heating may act as a diagnostic of both the injected nonthermal electron distribution and the coronal plasma, vital for constraining flare energetics.

Effect of the Temperature of Background Plasma and the Energy of Energetic Electrons on Z-mode Excitation

Abstract It has been suggested that the Z-mode instability driven by energetic electrons with a loss-cone type velocity distribution is one candidate process behind the continuum and zebra pattern of solar type-IV radio bursts. Both the temperature of background plasma (T 0) and the energy of energetic electrons (v e ) are considered to be important to the variation of the maximum growth rate (γ max). Here we present a detailed parameter study on the effect of T 0 and v e , within a regime of the frequency ratio ( ). In addition to γ max, we also analyze the effect on the corresponding wave frequency ( ) and propagation angle (θ max). We find that (1) γ max generally decreases with increasing v e , while its variation with T 0 is more complex depending on the exact value of v e . (2) With increasing T 0 and v e , presents stepwise profiles with jumps separated by gradual or very weak variations, and due to the warm plasma effect on the wave dispersion relation can vary within the hybrid band (the harmonic band containing the upper hybrid frequency) and the higher band. (3) The propagation is either perpendicular or quasi-perpendicular, and θ max presents variations in line with those of , as constrained by the resonance condition. We also examine the profiles of γ max with for different combinations of T 0 and v e to clarify some earlier calculations which show inconsistent results.

Dust charging and levitating in a sheath of plasma containing energetic particles**Project supported by the National Natural Science Foundation of China (Grant No. 11475223), the National Magnetic Confinement Fusion Science Program of China (Grant No. 2015GB101003), and the JSPS-NRF-NSFC A3 Foresight Program in the field of Plasma Physics (Grant Nos. 11261140328 and 2012K2A2A6000443).

The structure of the sheath in the presence of energetic particles is investigated in the multi-fluid framework. Based on the orbital motion limited (OML) theory, the dust grain charging inside the sheath of plasma containing energetic particles is examined for the carbon wall, and then the effect of the energetic particles on the stationary dust particle inside the sheath is discussed through the trapping potential energy. It is found that with the increase of energetic ion concentration or energy, the size of dust staying in levitation equilibrium decreases and the levitating position is much closer to the wall. In the case of deuterium ions as energetic ions, the bigger dust particle can be trapped by the sheath than in the case of hydrogen ions as energetic ions. When the energetic electron component is present, the levitating position of dust particle in the sheath depends strongly on the energetic electron. The levitating dust particle is closer to the wall as the energetic electron energy or concentration is increased. In addition, with the increase of temperature of thermal background ion, the size of dust particle trapped by the sheath decreases and the levitating positions of dust particles with the same size radius inside the sheath move toward the wall. Our results can be helpful in investigating the property of the sheath where the energetic particle component is present.

Diagnosing the Source Region of a Solar Burst on 26 September 2011 by Using Microwave Type-III Pairs

This work reports a peculiar and interesting train of microwave type III pair bursts in the impulsive rising phase of a solar flare on 2011 September 26. The observations include radio spectrometers at frequency of 0.80 - 2.00 GHz, hard X-ray (RHESSI and FERMI), EUV images of SWAP/PROBA-2 and magnetogram of HMI/SDO. By using a recently developed method (Tan et al. 2016a), we diagnosed the plasma density, temperature, plasma beta, magnetic field near the source region, the energy of energetic electrons and the distance between the acceleration region and the emission start sites of type III bursts. From the diagnostics, we find that: (1) The plasma density, temperature, magnetic field, and the distance between the acceleration region and the emission start sites almost have no obvious variations during the period of type III pair trains, while the energy of electrons has an obvious peak value which is consistent to the hard X-ray emission. (2) The plasma beta is much higher than an unity showing a highly dynamic process near the emission start site of type III bursts. (3) Although the reversed-slope type III branches drift slower at one order of magnitude than that of the normal type III branches, the related downgoing and upgoing electrons still could have same order of magnitude of energy. These facts indicate that both of the upgoing and downgoing electrons are possibly accelerated by similar mechanism and in a small source region. This diagnostics can help us to understand the microphysics in the source region of solar bursts.

Plasma sheath in the presences of non-Maxwellian energetic electrons and secondary emission electrons

The formation of a sheath in front of a carbon or tungsten material plane immersed in a plasma containing non-Maxwellian energetic electrons and secondary emission electrons is studied using a 1D model. In the model, energetic electrons are described by the electron energy distribution function (EEDF) and secondary electron emission (SEE) is produced by the electrons impinging on the wall. It is found that SEE coefficient depends on not only the sheath potential but also the EEDF profile of energetic electrons when a non-Maxwellian energetic electron component is present. The energetic electrons and associated secondary emission electrons can strongly modify ion velocity at sheath edge, floating potential and I–V probe characteristic. Due to the interdependence between SEE coefficient originating from the impact of non-Maxwellian energetic electrons on the wall and the sheath potential, with the increase in the energy of energetic electrons, a sudden jump phenomenon can be found in the profiles of SEE coefficient and other quantities such as floating potential and ion velocity at the sheath edge for tungsten wall, while for carbon wall they are the continuous variation. To begin with, the energetic electron component does not dominate the sheath, and I–V probe characteristic depends on both the EEDF profile of energetic electrons and material properties. Once the energetic electron component dominates the sheath, the analysis of I–V probe characteristic will yield the energy of energetic electrons.

Remote sensing and modeling of energetic electron precipitation into the lower ionosphere using VLF/LF radio waves and field aligned current data

Abstract. A model for the development of electron density height profiles based on space time distributed ionization sources and reaction rates in the lower ionosphere is described. Special attention is payed to the definition of an auroral oval distribution function for energetic electron energy input into the lower ionosphere based on a Maxwellian energy spectrum. The distribution function is controlled by an activity parameter which is defined proportional to radio signal amplitude disturbances of a VLF/LF transmitter. Adjusting the proportionality constant allows to model precipitation caused VLF/LF signal disturbances using radio wave propagation calculations and to scale the distribution function. Field aligned current (FAC) data from the new Swarm satellite mission are used to constrain the spatial extent of the distribution function. As an example electron precipitation bursts during a moderate substorm on the 12 April 2014 (midnight–dawn) are modeled along the subauroral propagation path from the NFR/TFK transmitter (37.5 kHz, Iceland) to a midlatitude site.

A statistical study of electron acceleration behind the dipolarization fronts in the magnetotail

AbstractWe investigate the electron acceleration behind dipolarization fronts (DFs) in the magnetotail from −25 RE to −10 RE through the examination of the energetic electron energy flux (>30 keV) with the observations from Time History of Events and Macroscale Interactions during Substorms (THEMIS). Statistical results of 133 DF events are presented based on the data set from January to April of the years 2008 and 2009. As the DFs propagate earthward, the acceleration of energetic electrons behind the DFs is found to take place over several RE along the tail. The increase in energetic electron energy flux can reach 2–4 orders of magnitude. The dominant acceleration mechanisms are different in the midtail (XGSM ≤ −15 RE) and the near‐Earth region (−15 < XGSM ≤ −10 RE). In the midtail, the majority of DF events show that the dominant electron acceleration mechanism is betatron acceleration. In the near‐Earth region, betatron acceleration is dominant in ~46% DF events while Fermi acceleration is dominant in ~39% DF events.

Significance of Wave-Particle Interaction Analyzer for direct measurements of nonlinear wave-particle interactions

Abstract. In the upcoming JAXA/ERG satellite mission, Wave Particle Interaction Analyzer (WPIA) will be installed as an onboard software function. We study the statistical significance of the WPIA for measurement of the energy transfer process between energetic electrons and whistler-mode chorus emissions in the Earth's inner magnetosphere. The WPIA measures a relative phase angle between the wave vector E and velocity vector v of each electron and computes their inner product W, where W is the time variation of the kinetic energy of energetic electrons interacting with plasma waves. We evaluate the feasibility by applying the WPIA analysis to the simulation results of whistler-mode chorus generation. We compute W using both a wave electric field vector observed at a fixed point in the simulation system and a velocity vector of each energetic electron passing through this point. By summing up Wi of an individual particle i to give Wint, we obtain significant values of Wint as expected from the evolution of chorus emissions in the simulation result. We can discuss the efficiency of the energy exchange through wave-particle interactions by selecting the range of the kinetic energy and pitch angle of the electrons used in the computation of Wint. The statistical significance of the obtained Wint is evaluated by calculating the standard deviation σW of Wint. In the results of the analysis, positive or negative Wint is obtained at the different regions of velocity phase space, while at the specific regions the obtained Wint values are significantly greater than σW, indicating efficient wave-particle interactions. The present study demonstrates the feasibility of using the WPIA, which will be on board the upcoming ERG satellite, for direct measurement of wave-particle interactions.

Evolution of the Solar Flare Energetic Electrons in the Inhomogeneous Inner Heliosphere

Solar flare accelerated electrons escaping into the interplanetary space and seen as type III solar radio bursts are often detected near the Earth. Using numerical simulations we consider the evolution of energetic electron spectrum in the inner heliosphere and near the Earth. The role of Langmuir wave generation, heliospheric plasma density fluctuations, and expansion of magnetic field lines on the electron peak flux and fluence spectra is studied to predict the electron properties as could be observed by Solar Orbiter and Solar Probe Plus. Considering various energy loss mechanisms we show that the substantial part of the initial energetic electron energy is lost via wave-plasma processes due to plasma inhomogeneity. For the parameters adopted, the results show that the electron spectra changes mostly at the distances before $\sim20 R_\odot$. Further into the heliosphere, the electron flux spectra of electrons forms a broken power-law relatively similar to what is observed at 1 AU.

Wave-particle interactions in non-uniform plasma and the interpretation of hard X-ray spectra in solar flares

Context. High energy electrons accelerated during solar flare are abundant in the solar corona and in the interplanetary space. Commonly, the number and the energy of non-thermal electrons at the Sun is estimated using hard X-ray (HXR) spectral observations (e.g. RHESSI) and a single-particle collisional approximation. Aims. To investigate the role of the spectrally evolving Langmuir turbulence on the population of energetic electrons in the solar corona. Methods. We numerically simulate the relaxation of a power-law non-thermal electron population in a collisional inhomogeneous plasma including wave-particle, and wave-wave interactions. Results. The numerical simulations show that the long-time evolution of electron population above 20 keV deviates substantially from the collisional approximation when wave-particle interactions in non-uniform plasma are taken into account. The evolution of Langmuir wave spectrum towards smaller wavenumbers, due to large-scale density fluctuations and wave-wave interactions, leads to an effective acceleration of electrons. Furthermore, the time-integrated spectrum of non-thermal electrons, which is normally observed with HXR above 20 keV, is noticeably increased due to acceleration of non-thermal electrons by Langmuir waves. Conclusions. The results show that the observed HXR spectrum, when interpreted in terms of collisional relaxation, can lead to an overestimated number and energy of energetic electrons accelerated in the corona.

Ionization of Type Ia Supernovae: Electron Impact, Photon Impact, or Charge Transfer?

The ionization structure of the nebular Type Ia supernova iron core is determined by treating important atomic processes with the best available evaluated atomic data and by assuming both Chandrasekhar-mass and sub-Chandrasekhar-mass white dwarf explosion models. The electron impact ionization rates are calculated by analyzing the energetic electron energy deposition processes. Charge transfer processes between neutral and ionized iron-group elements are included in the calculations using the recently available rate coefficients. The effect of photoionization due to the recombination photons is critically evaluated by carrying out a detailed cascade calculation using the recently improved state-specific rate coefficients of radiative recombination and cross sections of photoionization of iron. It is concluded that the ionization structure is mainly controlled by a combination of the energetic electron impact ionization and charge transfer, while the photoionization plays only a minor role and its effect on the synthetic spectra is small.

Optimization of energetic electron energy degradation calculations

The optimization of the energy degradation and reapportionment computer algorithms within theoretical computations of the effects and behavior of photoelectrons and other energetic electrons is established within a framework of discrete losses resulting from inelastic collisions with a neutral gas and interactions with the ambient electron gas. The basic method described guarantees energy conservation, while allowing large increases in the energy cell sizes with increasing energy with minimal distortion of the total excitation rates of neutrals and heating of the ambient electrons over what would otherwise be possible. When energy resolution of the spectra can be sacrified, extensions to very large energies are economically feasible using approximately seven cells per decade of energy without significant distortions in the energy reapportionment to other species. The emphasis is not on what particular set of cross sections should be used but rather on how to adapt the energy degradation procedure and the energy grid to the characteristics of the cross sections over the energy range of interest. For example, accurate determination of the ambient electron heating rate in terrestrial applications requires fairly small energy cells at low energies (usually less than 0.2 eV wide below 10 eV), while secondary ionization rates and most neutral excitation rates place no severe limitations on the energy cell sizes. Cell widths much larger than the excitation thresholds are possible and can be large fractions of the mean cell energy. At energies above 30 eV, cell widths that are up to 40% of the mean cell energy appear adequate for some applications not requiring high spectral resolution.

Acceleration of Runaway Electrons and Joule Heating in Solar Flares

D.C. electric fields provide the simplest and most direct means of accelerating electrons out of a thermal plasma. Most solar flare models result in the production of D.C. electric fields. On the other hand, microwave and hard X–ray observations of flares provide specific requirements for the number and energy of energetic electrons produced during a flare, and the timescales involved in accelerating them. The microwave emission from flares is understood to be gyrosynchrotron radiation from electrons with energies of 100 keV or greater. The hard X–ray emission (≳ 25 keV) can be interpreted as being either thick–target bremsstrahlung from non–thermal electrons (thin–target radiation may also contribute to the X–ray emission, but the process is less efficient), or thermal bremsstrahlung from hot, impulsively heated plasma. Hence, it is of interest to study the electric field acceleration of “runaway” electrons and the simultaneous Joule heating of the thermal plasma in light of these results from flare observations, without recourse to a specific flare model. Some of the results of such a study are summarized here.