Nonthermal Electrons Research Articles

Nonlinear dust ion acoustic shock wave structures in solar F corona region

Shock wave formation in dusty plasma consisting of mobile positive and negative ions, nonthermal electrons, and negatively charged static dust particles is theoretically studied in the presence of a magnetic field. Using the reductive perturbation technique, the basic set of fluid equations is reduced to the three-dimensional Zakarov–Kuznetsov Burgers nonlinear wave equation. The dissipation generated by ion kinematic viscosity in the system is taken into consideration. Distinct features of the dispersive (oscillatory) shock-like structures are discussed for different plasma parameters such as ion kinematic viscosities, ion density ratio, dust density ratio, nonthermal electrons, and magnetic field effect. It is observed that at large value of ion viscosities, nonthermal electrons, and ion density ratio, the formation of compressive shock strength increases. However, at certain critical values of nonthermal electrons, ion density ratio, the transition in the polarity of shock wave occurs within this range of plasma parameters. The considered parameters are reliable in the Solar F Corona (Dust corona) region. The findings of this work may be helpful in understanding shock formation in the Solar F Corona (Dust corona) region and astrophysical related plasma environments.

Conditions for Whistler Generation by Nonthermal Electrons in Flare Loops

Constraints are imposed on whistler generation in solar-flare loops. It is shown that the effective generation of whistlers is possible only under specific conditions: when the anisotropy of nonthermal electrons is sufficiently strong, the magnetic field is relatively weak (less than 200 G), or the plasma density is relatively high (more than 1010 cm–3). Otherwise, whistlers in flare loops will be strongly absorbed by the nonthermal electrons accelerated in the flare.

Hadronic High-energy Emission from Magnetically Arrested Disks in Radio Galaxies

Abstract We propose a novel interpretation that gamma rays from nearby radio galaxies are hadronic emission from magnetically arrested disks (MADs) around central black holes (BHs). The magnetic energy in MADs is higher than the thermal energy of the accreting plasma, where the magnetic reconnection or turbulence may efficiently accelerate nonthermal protons. They emit gamma rays via hadronic processes, which can account for the observed gamma rays for M87 and NGC 315. Nonthermal electrons are also accelerated with protons and produce MeV gamma rays, which is useful to test our model by proposed MeV satellites. The hadronic emission from the MADs may significantly contribute to the GeV gamma-ray background and produce the multi-PeV neutrino background detectable by IceCube-Gen2. In addition, gamma rays from MADs provide electron–positron pairs through two-photon pair production at the BH magnetosphere. These pairs can screen the vacuum gap, which affects high-energy emission and jet-launching mechanisms in radio galaxies.

Electron Energy Spectra and e–e Bremsstrahlung from Anisotropic Electron Distributions in Extreme Solar Flares

We study the electron energy spectra of two powerful solar flares SOL2003-10-28T 11:06:16 (GOES class X17.2) and SOL2002-07-23T00:18:16 (X4.8) on the basis of HXR and gamma-ray spectra obtained from RHESSI data. Electron-electron (e-e) bremsstrahlung makes a significant contribution to the X-ray flux at energies above 500 keV. In X-ray flux calculations different electron pitch-angle distributions were considered: quasi-transverse and quasi-longitudinal. It was shown that breaks in hard X-ray spectra occur on energies $\epsilon\approx100$ keV with photon spectral-index difference $\Delta\gamma\ge 1$ in the case of quasi-longitudinal anisotropy and hard power-law electron energy spectra. We deduce the parameters of non-thermal electrons energy spectra from RHESSI data for the extreme solar flares of 2003 October 28 and 2002 July 23. After including e-e bremsstrahlung, the non-thermal electrons energy spectra obtained in the thick-target model can be fitted using power-law with spectral index $\delta\approx 5$. For the 2002 July 23 flare, there is a pronounced asymmetry towards the southern footpoint, starting from energies above 100 keV. The separation of HXR sources at energies above 100 keV may be related to a feature of relativistic electron transport in flaring loops.

Probing Particle Acceleration through Broadband Early Afterglow Emission of MAGIC Gamma-Ray Burst GRB 190114C

Abstract Major Atmospheric Gamma Imaging Cerenkov Telescopes (MAGIC) detected the gamma-ray afterglow of GRB 190114C, which can constrain microscopic parameters of the shock-heated plasma emitting non-thermal emission. Focusing on the early afterglow of this event, we numerically simulate the spectrum and multi-wavelength light curves with constant and wind-like circumstellar medium using a time-dependent code. Our results show that the electron acceleration timescale at the highest energies is likely shorter than 20 times the gyroperiod to reproduce the GeV gamma-ray flux and its spectral index reported by Fermi. This gives an interesting constraint on the acceleration efficiency for Weibel-mediated shocks. We also constrain the number fraction of non-thermal electrons f e, and the temperature of the thermal electrons. The early optical emission can be explained by the thermal synchrotron emission with f e ≲ 0.01. On the other hand, the X-ray light curves restrict efficient energy transfer from protons to the thermal electrons, and f e ∼ 1 is required if the energy fraction of the thermal electrons is larger than ∼10%. The parameter constraints obtained in this work give important clues to probing plasma physics with relativistic shocks.

Observation of non-thermal electron formation during the thermal quench of shattered pellet injection shutdowns in DIII-D

Formation of non-thermal (hot) electrons is studied during the thermal quench of shattered pellet injection (SPI) discharge shutdowns in the DIII-D tokamak. Both pure neon and mixed neon/deuterium shutdowns are studied. High-energy radiation (SXR and EUV) brightness levels show indications of non-thermal electron formation during the TQ. The non-thermal densities are of order 1018 m−3 and non-thermal temperatures are estimated to be of order 10 keV. The non-thermal electrons appear to be mostly lost during the subsequent current quench, as no signal of relativistic runaway electrons is typically seen at the end of the current quench. The non-thermals appear to have an effect on the deposition profile of the SPI pellets, especially at the end of the TQ, due to increased pellet ablation.

Establishing nonthermal regimes in pump-probe electron relaxation dynamics

Time- and angle-resolved photoemission spectroscopy (TR-ARPES) accesses the electronic structure of solids under optical excitation, and is a powerful technique for studying the coupling between electrons and collective modes. One approach to infer electron-boson coupling is through the relaxation dynamics of optically-excited electrons, and the characteristic timescales of energy redistribution. A common description of electron relaxation dynamics is through the effective electronic temperature. Such a description requires that thermodynamic quantities are well-defined, an assumption that is generally violated at early delays. Additionally, precise estimation of the non-thermal window -- within which effective temperature models may not be applied -- is challenging. We perform TR-ARPES on graphite and show that Boltzmann rate equations can be used to calculate the time-dependent electronic occupation function, and reproduce experimental features given by non-thermal electron occupation. Using this model, we define a quantitative measure of non-thermal electron occupation and use it to define distinct phases of electron relaxation in the fluence-delay phase space. More generally, this approach can be used to inform the non-thermal-to-thermal crossover in pump-probe experiments.

Hot X-ray onsets of solar flares

ABSTRACT The study of the localized plasma conditions before the impulsive phase of a solar flare can help us understand the physical processes that occur leading up to the main flare energy release. Here, we present evidence of a hot X-ray ‘onset’ interval of enhanced isothermal plasma temperatures in the range of 10–15 MK over a period of time prior to the flare’s impulsive phase. This ‘hot onset’ interval occurs during the initial soft X-ray increase and definitely before any detectable hard X-ray emission. The isothermal temperatures, estimated by the Geostationary Operational Environmental Satellite X-ray sensor, and confirmed with data from the Reuven Ramaty High Energy Solar Spectroscopic Imager, show no signs of gradual increase, and the ‘hot onset’ phenomenon occurs regardless of flare classification or configuration. In a small sample of four representative flare events, we tentatively identify this early hot onset soft X-ray emission to occur within footpoint and low-lying loop regions, rather than in coronal structures, based on images from the Atmospheric Imaging Assembly. We confirm this via limb occultation of a flaring region. These hot X-ray onsets appear before there is evidence of collisional heating by non-thermal electrons, and hence challenge the standard modelling techniques.

Radio and X-Ray Observations of Short-lived Episodes of Electron Acceleration in a Solar Microflare

Abstract Solar flares are sudden energy release events in the solar corona, resulting from magnetic reconnection, that accelerate particles and heat the ambient plasma. During a flare, there are often multiple temporally and spatially separated individual energy release episodes that can be difficult to resolve depending on the observing instrument. We present multiwavelength imaging and spectroscopy observations of multiple electron acceleration episodes during a GOES B1.7-class two-ribbon flare on 2012 February 25, observed simultaneously with the Karl G. Jansky Very Large Array at 1–2 GHz, the Reuven Ramatay High Energy Solar Spectroscopic Imager in X-rays, and the Solar Dynamics Observatory in extreme ultraviolet. During the initial phase of the flare, five radio bursts were observed. A nonthermal X-ray source was seen to be cotemporal but not cospatial with the first three radio bursts. Their radio spectra are interpreted as optically thick gyrosynchrotron emission. By fitting the radio spectra with a gyrosynchrotron model, we derive the magnetic field strength and nonthermal electron spectral parameters in each acceleration episode. Notably, the nonthermal parameters derived from X-rays differ considerably from the nonthermal parameters inferred from the radio. The observations are indicative of multiple cotemporal acceleration episodes during the impulsive phase of a solar microflare. The X-ray and radio burst sources likely originate from separate electron distributions in different magnetic loops.

A Multiwavelength Study of Distant Blazar PKS 0537-286

We report the results of broadband observations of distant blazar PKS 0537-286 (z = 3.1) using data spanning more than ten years from the Fermi Large Area Telescope together with Swift UVOT/XRT archival data taken between 2005 and 2017. In the γ -ray band, the peak flux above 100 MeV, F$_{γ}$ = (6 23 ± 0 56) ∙ 10$^{-7}$ photon cm$^{-2}$ s$^{-1}$ observed on MJD 57874 within one week, corresponds to L$_{γ}$ = 2 46 ∙ 10$^{49}$ erg s$^{-1}$ isotropic γ -ray luminosity. The Swift XRT data analyses show that the X-ray emission is characterized by a significantly hard photon index, Γ$_{X-ray}$ ≤ 1.3 , and an X-ray flux of 4 ∙ 10$^{-12}$ erg cm$^{-2}$ s$^{-1}$, which is almost constant over twelve years. The spectral energy distribution is modeled within one-zone leptonic models assuming the emission region is within the broad-line region. The observed X-ray and γ -ray data are modeled as inverse Compton scattering of (i) only synchrotron photons and (ii) synchrotron and external photons on the electron population that produces the radioto-optical emission. The modeling shows that the nonthermal electrons in the jet of PKS 0537-286 have a hard power-law index (<1.9) and that the jet should be particle dominated with a luminosity within 10$^{45}$-10$^{46}$ erg s$^{-1}$.

A low-frequency radio halo survey of the South Pole Telescope SZ-selected clusters with the GMRT

ABSTRACT The presence of non-thermal electrons and large-scale magnetic fields in the intracluster medium is known through the detection of megaparsec (Mpc) scale diffuse radio synchrotron emission. Although a significant amount of progress in finding new diffuse radio sources has happened in the last decade, most of the investigation has been constrained towards massive low-redshift clusters. In this work, we explore clusters with redshift z &amp;gt; 0.3 in search of diffuse radio emission, at 325 MHz with the Giant Metrewave Radio Telescope. This campaign has resulted in the discovery of two new radio haloes (SPT-CL J0013−4906 and SPT-CL J0304−4401) along with two other detections (SPT-CL J2031−4037 and SPT-CL J2248−4431), previously reported (at 325 MHz) in the literature. In addition, we detect a halo candidate in one cluster in our sample, and upper limits for haloes are placed in eight clusters where no diffuse emission is detected. In the P1.4–LX plane, the detected haloes follow the observed correlation, whereas the upper limits lie above the correlation line, indicating the possibility of future detection with sensitive observations.

Electron Acceleration during Macroscale Non-Relativistic Magnetic Reconnection

The first self-consistent simulations of electron acceleration during magnetic reconnection in a macroscale system are presented. Consistent with solar flare observations the spectra of energetic electrons take the form of power-laws that extend more than two decades in energy. The drive mechanism for these nonthermal electrons is Fermi reflection in growing and merging magnetic flux ropes. A strong guide field is found to suppress the production of nonthermal electrons by weakening the Fermi drive mechanism. For a weak guide field the total energy content of nonthermal electrons dominates that of the hot thermal electrons even though their number density remains small. Our results are benchmarked with the hard x-ray, radio and extreme ultra-violet (EUV) observations of the X8.2-class solar flare on September 10, 2017.

Nonthermal electron and ion acceleration by magnetic reconnection in large laser-driven plasmas

Magnetic reconnection is a fundamental plasma process that is thought to play a key role in the production of nonthermal particles associated with explosive phenomena in space physics and astrophysics. Experiments at high-energy-density facilities are starting to probe the microphysics of reconnection at high Lundquist numbers and large system sizes. We have performed particle-in-cell (PIC) simulations to explore particle acceleration for parameters relevant to laser-driven reconnection experiments. We study particle acceleration in large system sizes that may be produced soon with the most energetic laser drivers available, such as at the National Ignition Facility. In these conditions, we show the possibility of reaching the multi-plasmoid regime, where plasmoid acceleration becomes dominant. Our results show the transition from X point to plasmoid-dominated acceleration associated with the merging and contraction of plasmoids that further extend the maximum energy of the power-law tail of the particle distribution for electrons. We also find for the first time a system-size-dependent emergence of nonthermal ion acceleration in driven reconnection, where the magnetization of ions at sufficiently large sizes allows them to be contained by the magnetic field and energized by direct X point acceleration. For feasible experimental conditions, electrons and ions can attain energies of ϵ max , e / k B T e > 100 and ϵ max , i / k B T i > 1000. Using PIC simulations with binary Monte Carlo Coulomb collisions, we study the impact of collisionality on plasmoid formation and particle acceleration. The implications of these results for understanding the role reconnection plays in accelerating particles in space physics and astrophysics are discussed.

The Lyα Emission in Solar Flares. I. A Statistical Study on Its Relationship with the 1–8 Å Soft X-Ray Emission

Abstract We statistically study the relationship between the Lyα and 1–8 Å soft X-ray (SXR) emissions from 658 M- and X-class solar flares observed by the Geostationary Operational Environmental Satellite during 2006–2016. Based on the peak times of the two wave band emissions, we divide the flares into three types. Type I (III) has an earlier (a later) peak time in the Lyα emission than that in the SXR emission, while type II has nearly the same peak time (within the time resolution of 10 s) between the Lyα and SXR emissions. In these 658 flares, we find that there are 505 (76.8%) type I flares, 10 (1.5%) type II flares, and 143 (21.7%) type III flares, and that the three types appear to have no dependence on the flare duration, flare location, or solar cycle. Besides the main peak, the Lyα emission of the three type flares also shows sub-peaks which can appear in the impulsive or gradual phase of the flare. It is found that the main-peak (for type I) and subpeak (for type III) emissions of Lyα that appear in the impulsive phase follow the Neupert effect in general. This indicates that such Lyα emissions are related to the nonthermal electron beam heating. While the main-peak (for type III) and subpeak (for type I) emissions of Lyα that appear in the gradual phase are supposed to be primarily contributed by the thermal plasma that cools down.

Effects of nonthermal electrons and ion beams on ion-acoustic double layers in warm ion plasma

The effects of nonthermal electrons and ion beams on the ion acoustic double layers in warm ion plasma have been studied using the pseudopotential technique. Sagdeev potential of ion-acoustic wave in the plasma has been derived and the solution of ion acoustic double layers has been obtained. The critical value of nonthermal parameter of electrons for existence of double layers is obtained and discussed. The profiles of double layers in the plasma are drawn taking different values of density and temperature of ion beams, and the nonthermal parameter of electrons obeying the critical condition. It has been shown that both compressive and rarefactive double layers are excited in the plasma depending on the values of nonthermal electrons and ion beam parameters. The width of double layers is also graphically discussed. The results are new and may be useful in understanding the acceleration processes of charged particles in space plasma.

The Depth and the Vertical Extent of the Energy Deposition Layer in a Medium-class Solar Flare

Abstract We analyze here variations of the position and the vertical extent of the energy deposition layer (EDL) in the C1.6 GOES-class solar flare observed at 10:20 UT on 2012 September 10. The variations of the EDL are contrasted with the variations of the spectra and emission intensities recorded in the Hα line with very high time resolution using the MSDP spectrograph at Białków Observatory. The flare radiated hard X-rays (HXR) detectable up to a energy of 70 keV. A numerical model of the flare used in the analysis assumes that the nonthermal electrons (NTEs) carried the external energy to the flare. The energy flux of the NTEs was derived from a nonthermal component seen in RHESSI spectra. The main geometrical parameters of the flare were derived using restored RHESSI imaging data. We found that the variations of the X-ray fluxes recorded in various energy bands and the variations of the Hα intensities were well-correlated in time during the pre-impulsive and impulsive phases of the flare, and they agreed with the variations of the calculated position and vertical extent of the EDL. The variations of the emission noticed in various parts of the Hα line profile were caused by individual episodes of energy deposition by the beams of NTEs of various energy spectra on various depths in the chromospheric plasma. These results supplement our previous findings for the solar flare on 2013 June 21, having nearly the same GOES class of C1.1 but HXR emission below 34 keV only.

Polarization-resolved femtosecond pump-probe spectroscopy for Au nanodisks at the LSP resonance

The formation and relaxation of plasmon-induced hot electrons in Au nanodisks (AuNDs) with relatively large particle size was studied by femtosecond pump-probe spectroscopy. We temporally resolved the plasmon-induced formation of nonthermal electrons through intraband transitions and subsequent redistribution of nonthermal electrons to thermal electrons. The transient extinction spectra were found to exhibit clear dichroism for the pump polarization on a time scale of a few hundred femtoseconds. The electric field distribution was calculated using the Finite-difference time-domain (FDTD) method to be inhomogeneous inside the AuNDs under irradiation by linearly polarized light. The formation and relaxation of hot electrons occur locally inside the AuNDs, which results in the observed ultrafast dichroic response.

RESIK and RHESSI observations of the 20 September 2002 flare

Context. Soft X-ray spectra (3.33 Å–6.15 Å) from the RESIK instrument on CORONAS-F constitute a unique database for the study of the physical conditions of solar flare plasmas, enabling the calculation of differential emission measures. The two RESIK channels for the shortest wavelengths overlap with the lower end of the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spectral energy range, which is located around 3 keV, making it possible to compare both data sets. Aims. We aim to compare observations from RESIK and RHESSI spectrometers and cross-correlate these instruments. Observations are compared with synthetic spectra calculated based on the results of one-dimensional hydrodynamical (1D-HD) modelling. The analysis was performed for the flare on 20 September 2002 (SOL2002-09-20T09:28). Methods. We estimated the geometry of the flaring loop, necessary for 1D-HD modelling, based on images from RHESSI and the Extreme-Ultraviolet Imaging Telescope aboard the Solar and Heliospheric Observatory. The distribution of non-thermal electrons (NTEs) was determined from RHESSI spectra. The 1D-HD model assumes that non-thermal electrons with a power-law spectrum were injected at the apex of the flaring loop. The NTEs then heat and evaporate the chromosphere, filling the loop with hot and dense plasma radiating in soft X-rays. The total energy of electrons was constrained by comparing observed and calculated fluxes from Geostationary Operational Environmental Satellite 1–8 Å data. We determined the temperature and density at every point of the flaring loop throughout the evolution of the flare, calculating the resulting X-ray spectra. Results. The synthetic spectra calculated based on the results of hydrodynamic modelling for the 20 September 2002 flare are consistent within a factor of two with the observed RESIK spectra during most of the duration of the flare. This discrepancy factor is probably related to the uncertainty on the cross-calibration between RESIK and RHESSI instruments.

Effect of non-thermal electron and positron on the dust ion acoustic solitary wave in the presence of relativistic thermal magnetized ions

The propagation characteristics of small-amplitude electrostatics solitary wave in magnetized dusty plasma in the presence of relativistic thermal ion with non-thermal electron and positron are investigated. Here, Zakharov–Kuznetsov (Z–K) equation is studied to observe the evolution of dust ion acoustic solitary waves with the help of perturbative procedures. In the present plasma system, the Mach number (phase velocity) is found to be in the supersonic range and it decreases (increases) with normalized positron density (thermal ion parameter). On the other hand, the plasma system behaves in the least nonlinear way when the non-thermal parameter of electrons and positrons has the lowest value allowing the solitary wave potential to propagate with maximum value. It is found that the non-thermal parameters of electrons and positrons, relativistic factor and dust density, can effectively control the variation of Mach number. The solitary wave potential has the least value when the thermal ion parameter increases, whereas it is the highest when the positron density has the highest.

Stopbands in fast ion-acoustic soliton propagation revisited

It was recently discovered that, in a multispecies plasma with two positive ion species (one cold and one warm adiabatic) and Boltzmann-distributed electrons, a “stopband” could occur, i.e., there was a range of velocities where no fast ion-acoustic solitons could propagate between two ranges where propagation was possible. Several extensions were subsequently investigated, including the effects of the cool ions having finite temperature and of nonthermal electron distributions. Efforts were made to estimate existence domains in plasma parameter space, often by ad hoc arguments, but the illustrations were invariably restricted to the same specific set of parameter values or their neighborhoods. In contrast, here, a systematic and structured study is given: physical arguments determine a range of compositional parameters so that a Sagdeev pseudopotential analysis establishes in a consistent way the various curves limiting the existence domains in parameter space. This is done for four models, namely, Boltzmann, nonthermal Cairns, superthermal kappa, and nonextensive Tsallis electron distributions, and for each, existence domains are plotted in the space of ion charge-to-mass ratio and inverse electron temperature, and detailed examples are presented. Contrary to reports in the literature, stopbands are shown to exist for large deviations from isothermality. However, their range in parameter space is shifted from that obtained for Boltzmann electrons. This establishes that the stopband phenomenon is robust and governed primarily by the cold and warm ion properties and the electron temperature, the form of the electron distribution having only a quantitative effect.