Energy Of Non-thermal Electrons Research Articles

Excitation and relaxation of nonthermal electron energy distributions in metals with application to gold

Excitation and relaxation of nonthermal electron energy distributions in metals with application to gold

Solar Flare Ribbon Fronts. I. Constraining Flare Energy Deposition with IRIS Spectroscopy

Spectral lines formed at lower atmospheric layers show peculiar profiles at the “leading edge” of ribbons during solar flares. In particular, increased absorption of the BBSO/GST He i λ10830 line, as well as broad and centrally reversed profiles in the spectra of the Mg ii and C ii lines observed by the IRIS satellite, has been reported. In this work, we aim to understand the physical origin of such peculiar IRIS profiles, which seem to be common of many, if not all, flares. To achieve this, we quantify the spectral properties of the IRIS Mg ii profiles at the ribbon leading edge during four large flares and perform a detailed comparison with a grid of radiative hydrodynamic models using the RADYN+FP code. We also studied their transition region (TR) counterparts, finding that these ribbon front locations are regions where TR emission and chromospheric evaporation are considerably weaker compared to other parts of the ribbons. Based on our comparison between the IRIS observations and modeling, our interpretation is that there are different heating regimes at play in the leading edge and the main bright part of the ribbons. More specifically, we suggest that bombardment of the chromosphere by more gradual and modest nonthermal electron energy fluxes can qualitatively explain the IRIS observations at the ribbon leading front, while stronger and more impulsive energy fluxes are required to drive chromospheric evaporation and more intense TR emission in the bright ribbon. Our results provide a possible physical origin for the peculiar behavior of the IRIS chromospheric lines in the ribbon leading edge and new constraints for the flare models.

Investigation of the afterpeaks in pulsed microwave argon plasma at atmospheric pressure

We studied the energy transport process in pulsed microwave argon plasmas at atmospheric pressure, focusing on the optical emission burst during the pulse-off time called the afterpeak. Guided by experimental observations using nanosecond time resolution imaging and spectroscopic diagnostics, we developed a global simulation model considering time-varying reaction rate coefficients and non-thermal electron energy distribution. Experimental and simulation results show that the afterpeak can be maximized by choosing an appropriate pulse period. Our analysis of the generation and consumption of excited argon species reveals that the rapid drop in electron temperature during the inter-pulse time reduces the diffusive loss of ions and enhances the recombination reactions, which produce the afterpeak. We also reveal that the radiation trapping and high energy level argon must be considered to simulate the afterpeak in atmospheric conditions. The improved understanding of the afterpeak dynamics can be utilized to optimize the power coupling and/or generation of reactive species.

Multiwavelength observations of MAXI J1820+070 during its outburst decay and subsequent mini-outburst

ABSTRACT We present results from quasi-simultaneous multiwavelength observations of the Galactic black hole X-ray transient MAXI J1820+070 during the decay of the 2018 outburst and its entire subsequent mini-outburst in March 2019. We fit the X-ray spectra with phenomenological and Comptonizaton models and discuss the X-ray spectral evolution in comparison with the multiwavelength behaviour of the system. The system showed a rebrightening in UV/Optical/NIR bands 7 d after the soft-to-hard transition during the main outburst decay while it was fading in X-rays and radio. In contrast, the mini-outburst occurred 165 d after the hard state transition of the initial outburst decay and was detected in all wavelengths. For both events, the measured time-scales are consistent with those observed in other black hole systems. Contemporaneous hard X-ray/soft γ-ray observations indicate a non-thermal electron energy distribution at the beginning of the UV/Optical/NIR rebrightening, whereas a thermal distribution can fit the data during the hard mini-outburst activity. The broad-band spectral energy distributions until the rebrightening are consistent with the irradiated outer accretion disc model. However, both the SEDs produced for the peak of rebrightening and close to the peak of mini-outburst provided good fits only with an additional power law component in the UV/Optical/NIR frequency ranges, which is often interpreted with a jet origin.

On the calculation of superstatistics ionization rates of neutral Hélium

Electron-impact ionization (EII) can be important in dynamic systems where atoms are suddenly exposed to higher electron temperatures. EII can also have a significant effect on the charge state distribution for plasmas with a non-thermal electron energy distribution. In this paper, we present the effects of hot electrons on the calculation of ionization rates for neutral Helium using superstatistics. We also exchange the distribution function with the effective Boltzmann factor of superstatistics. The ionization rates of neutral Helium are obtained from cross sections obtained by FAC code, are weighted by a non-maxwellian distribution function. The results are compared to those reported by Kato. et. al. We note that non-maxwellian energy distribution for different fractions of hot electrons showed the effects of these rates on the fractions of hot electrons and the forms of ionization rates.

Calculation of ionization rates of Beryllium ions (Be2+) using the superstatistics distributions

Electron-impact ionization (EII) can be important in dynamic systems where ions are suddenly exposed to higher electron temperatures. EII can also have a significant effect on the charge state distribution for plasmas with a non-thermal electron energy distribution. In this paper, we present the effects of hot electrons on the calculation of ionization rates for Be2+ using the superstatistics, and we also exchange the distribution function with the effective Boltzmann factor of superstatistics. The ionization rates of Be2+ generate from cross sections obtained by FAC code, weighted by a non–Maxwellian distribution function. We note that non-Maxwellian energy distribution for different fractions of hot electrons showed the effects of these rates on the fractions of hot electrons and the forms of ionization rates.

Rate coefficients for electron-impact dissociation of O3+ to singly charged fragments

Rate coefficients for electron-impact dissociation of O3 + to the O+ and O2 + fragments are calculated for the new, recommended cross section data set and for various collisional conditions. Two sets of the cross section data, measured recently by different experimental groups, are used. These cross sections differ significantly with each other, but are renormalized and optimized to the coherent data base. Rate coefficients for the ozone cation fragmentation are determined using the Maxwellian and the non-thermal electron energy distribution functions (EEDF). In the case of Maxwellian distribution, mean electron energies cover the range from zero up to 2 keV. Non-thermal electron energy distribution functions are adopted from the recent electron observations by the 3-D plasma and energetic particles experiment on the WIND spacecraft. The non-thermal rates are evaluated for the mean electron energies from 4 to 80 eV. The role of the possible contribution of electron-impact dissociation of O3 + to the ozone layer depletion has been emphasized.

General relativistic MHD simulations of non-thermal flaring in Sagittarius A*

ABSTRACT Sgr A* exhibits regular variability in its multiwavelength emission, including daily X-ray flares and roughly continuous near-infrared (NIR) flickering. The origin of this variability is still ambiguous since both inverse Compton and synchrotron emission are possible radiative mechanisms. The underlying particle distributions are also not well constrained, particularly the non-thermal contribution. In this work, we employ the GPU-accelerated general relativistic magnetohydrodynamics code H-AMR to perform a study of flare flux distributions, including the effect of particle acceleration for the first time in high-resolution 3D simulations of Sgr A*. For the particle acceleration, we use the general relativistic ray-tracing code bhoss to perform the radiative transfer, assuming a hybrid thermal+non-thermal electron energy distribution. We extract ∼60 h light curves in the sub-millimetre, NIR and X-ray wavebands, and compare the power spectra and the cumulative flux distributions of the light curves to statistical descriptions for Sgr A* flares. Our results indicate that non-thermal populations of electrons arising from turbulence-driven reconnection in weakly magnetized accretion flows lead to moderate NIR and X-ray flares and reasonably describe the X-ray flux distribution while fulfilling multiwavelength flux constraints. These models exhibit high rms per cent amplitudes, $\gtrsim 150{{\ \rm per\ cent}}$ both in the NIR and the X-rays, with changes in the accretion rate driving the 230 GHz flux variability, in agreement with Sgr A* observations.

FOXSI-2 Solar Microflares. II. Hard X-ray Imaging Spectroscopy and Flare Energetics.

We study the nature of energy release and transfer for two sub-A class solar microflares observed during the second Focusing Optics X-ray Solar Imager (FOXSI-2) sounding rocket flight on 2014 December 11. FOXSI is the first solar-dedicated instrument to utilize focusing optics to image the Sun in the hard X-ray (HXR) regime, sensitive to energies of 4-20 keV. Through spectral analysis of the microflares using an optically thin isothermal plasma model, we find evidence for plasma heated to ~10 MK and emission measures down to ~1044 cm-3. Though nonthermal emission was not detected for the FOXSI-2 microflares, a study of the parameter space for possible hidden nonthermal components shows that there could be enough energy in nonthermal electrons to account for the thermal energy in microflare 1, indicating that this flare is plausibly consistent with the standard thick-target model. With a solar-optimized design and improvements in HXR focusing optics, FOXSI-2 offers approximately five times greater sensitivity at 10 keV than the Nuclear Spectroscopic Telescope Array for typical microflare observations and allows for the first direct imaging spectroscopy of solar HXRs with an angular resolution at scales relevant for microflares. Harnessing these improved capabilities to study small-scale events, we find evidence for spatial and temporal complexity during a sub-A class flare. This analysis, combined with contemporaneous observations by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, indicates that these microflares are more similar to large flares in their evolution than to the single burst of energy expected for a nanoflare.

Rate coefficients for electron-impact dissociation of O3+ to singly charged fragments

Rate coefficients for electron-impact dissociation of O3+ to the O+ and O2+ fragments are calculated for the new, recommended cross section data set and for various collisional conditions. Two sets of the cross section data, measured recently by different experimental groups, are used. These cross sections differ significantly with each other, but are renormalized and optimized to the coherent data base. Rate coefficients for the ozone cation fragmentation are determined using the Maxwellian and the non-thermal electron energy distribution functions (EEDF). In the case of Maxwellian distribution, mean electron energies cover the range from zero up to 2 keV. Non-thermal electron energy distribution functions are adopted from the recent electron observations by the 3-D plasma and energetic particles experiment on the WIND spacecraft. The non-thermal rates are evaluated for the mean electron energies from 4 to 80 eV. The role of the possible contribution of electron-impact dissociation of O3+ to the Ozone layer depletion has been emphasized.

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.

Role of non-thermal electrons in ultrafast spin dynamics of ferromagnetic multilayer

Understanding of ultrafast spin dynamics is crucial for future spintronic applications. In particular, the role of non-thermal electrons needs further investigation in order to gain a fundamental understanding of photoinduced demagnetization and remagnetization on a femtosecond time scale. We experimentally demonstrate that non-thermal electrons existing in the very early phase of the photoinduced demagnetization process play a key role in governing the overall ultrafast spin dynamics behavior. We simultaneously measured the time-resolved reflectivity (TR-R) and the magneto-optical Kerr effect (TR-MOKE) for a Co/Pt multilayer film. By using an extended three-temperature model (E3TM), the quantitative analysis, including non-thermal electron energy transfer into the subsystem (thermal electron, lattice, and spin), reveals that energy flow from non-thermal electrons plays a decisive role in determining the type I and II photoinduced spin dynamics behavior. Our finding proposes a new mechanism for understanding ultrafast remagnetization dynamics.

Global Energetics of Solar Flares. VIII. The Low-energy Cutoff

Abstract One of the key problems in solar flare physics is the determination of the low-energy cut-off: the value that determines the energy of nonthermal electrons and hence flare energetics. We discuss different approaches to determine the low-energy cut-off in the spectrum of accelerated electrons: (i) the total electron number model, (ii) the time-of-flight model (based on the equivalence of the time-of-flight and the collisional deflection time), (iii) the warm target model of Kontar et al., and (iv) the model of the spectral cross-over between thermal and nonthermal components. We find that the first three models are consistent with a low-energy cutoff with a mean value of ≈10 keV, while the cross-over model provides an upper limit for the low-energy cutoff with a mean value of ≈21 keV. Combining the first three models we find that the ratio of the nonthermal energy to the dissipated magnetic energy in solar flares has a mean value of q E = 0.57 ± 0.08, which is consistent with an earlier study based on the simplified approximation of the warm target model alone (q E = 0.51 ± 0.17). This study corroborates the self-consistency between three different low-energy cutoff models in the calculation of nonthermal flare energies.

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.

On the maximum energy of protons in the hotspots of AGN jets

We study particle acceleration and magnetic field amplification in the termination shocks (hotspots) of radiogalaxy jets. The cut-off of the synchrotron spectrum in the hotspots of powerful radiogalaxies is typically observed between infrared and optical frequencies, indicating that the maximum energy of non-thermal electrons accelerated at the jet termination shock is about 1 TeV for a canonical magnetic field of 100 μG. Based on theoretical considerations and observational data we show that the maximum energy of electrons cannot be constrained by synchrotron losses as usually assumed, unless the jet density is unreasonable large and most of the jet kinetic energy goes to non-thermal electrons. The maximum energy is ultimately determined by the ability to scatter particles back and forth the shock, and this limit applies to both electrons and protons. Therefore, the maximum energy of protons is also about 1 TeV when radiative cooling is not efficient. We show that non-resonant hybrid (Bell) instabilities generated by the streaming of cosmic rays can grow fast enough to amplify the jet magnetic field up to 100 μG and accelerate particles up to the maximum energies observed in the hotspots of radiogalaxies.

Binary neutron star merger remnants as sources of cosmic rays below the “Ankle”

We investigate non-thermal electron and nuclei energy losses within the binary neutron star merger remnant produced by the event GW170817. The lack of a cooling feature within the detected synchrotron emission from the source is used to constrain the magnetic field at the mG level, assuming that this emission is electron synchrotron in origin, and that the accelerated spectrum in the electrons follows the form dN/dEe∝Ee−2. The level of subsequent gamma-ray emission from the source is demonstrated to provide a further constraint on the source magnetic field strength. We also put forward alternative strong ( ∼ G) magnetic field scenarios able to support this emission. For such stronger fields, the photo-disintegration of non-thermal nuclei within the source is considered, and a bottleneck period of ∼ 5–30 days is found when this process peaks. We find that this class of source is in principle able to support the population of cosmic rays detected at Earth below the “ankle”.

Measurement of vector magnetic field in a flare kernel with a spectropolarimetric observation in He i 10830 Å

Abstract A flare kernel associated with a C4 class flare was observed in a spectral window including the He i triplet 10830 Å and Si i 10827 Å with a spectropolarimeter on the Domeless Solar Telescope at Hida Observatory on 2015 August 9. The observed Stokes profiles of the He i triplet in the flare kernel in its post-maximum phase are well reproduced through inversions considering the Zeeman and the Paschen–Back effects with a three-slab model of the flare kernel, in which two slabs which have upward and downward velocities produce emissions and one slab produces an absorption. The magnetic field strength inferred from the emission components of the He i line is 1400 G, which is significantly stronger than 690 G that is observed at the same location in the same line 6.5 hr before the flare. In addition, the photospheric magnetic field vector derived from the Si i10827 Å is similar to that of the flare kernel. To explain this result, we suggest that the emission in the He i triplet during the flare is produced in the deep layer, around which bombardment of non-thermal electrons leads to the formation of a coronal temperature plasma. Assuming a hydrogen column density at the location where the He i emissions are formed, and a power-law index of non-thermal electron energy distribution, we derived the low-energy cutoff of the non-thermal electron as 20–30 keV, which is consistent with that inferred from hard X-ray data obtained by RHESSI.

New estimation of non-thermal electron energetics in the giant solar flare on 28 October 2003 based on Mars Odyssey observations

A new estimation of the total number and energy of the non-thermal electrons produced in the giant (>X17) solar flare on 2003 October 28 is presented based on the analysis of the observations of the hard X-ray (HXR) emission by the High Energy Neutron Detector (HEND) onboard the Mars Odyssey spacecraft orbiting Mars. Previous estimations of the non-thermal electron energy based on the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) data were incomplete since RHESSI missed the peak of the flare impulsive phase. In contrast, HEND observed the whole flare. We used two models to estimate the energy of the non-thermal electrons: the cold thick target model and the warm thick target model. More specifically, in the second case we employed an approximation which relates the pitch-angle averaged injection spectrum with the electron spectrum integrated over the emitting source. We found that, depending on the model used and the low-energy cutoff (Ec) of the non-thermal electrons, the estimate of their total energy in the entire flare can vary from 2.3×1032 to 6.2×1033 ergs. The lowest estimate, 2.3×1032 ergs, obtained within the cold thick target model and fixed Ec=43 keV, is consistent with the previous estimate. In this case, non-thermal electrons accelerated in the peak of the flare impulsive phase missed by RHESSI contained approximately 40% of the total energy of non-thermal electrons of the entire flare. The highest value, 6.2×1033 ergs, obtained with the cold thick target model and fixed Ec=10 keV, looks abnormally high, since it exceeds the total non-potential magnetic energy of the parent active region and the total bolometric energy radiated in the flare. Our estimates also show that the total number and energetics of the HXR-producing electrons in the flare region is a few orders of magnitude higher than of the population of energetic electrons injected into interplanetary space.

Synchrotron Polarization Radiative Transfer: Relativistic Thermal Electron Contribution

Abstract Relativistic thermal electrons moving in a large-scale magnetic field can produce synchrotron radiation. Linear synchrotron polarization can also be produced by the relativistic thermal electrons. In this paper, we utilize a hybrid thermal–nonthermal electron energy distribution to calculate circular synchrotron polarization. We further compute the radiative transfer of the synchrotron polarization in the optical and radio bands when we consider the contribution of the thermal electrons. We attempt to apply the polarization results to some astrophysical objects, such as kilonova like AT 2017gfo/GW170817, the fast radio burst (FRB), the gamma-ray burst afterglow, and the supernova remnant. The large optical depth of radiative transfer affects the small polarization degrees of these populations when the media surrounding the synchrotron sources take heavy absorption to the polarized photons. We need a strong magnetic field in our model to reproduce the linear and circular polarization properties that were observed in FRB 140514. This indicates that FRBs have a neutron star origin.

Evidence that particle acceleration in hotspots of FR II galaxies is not constrained by synchrotron cooling

We study the hotspots of powerful radiogalaxies, where electrons accelerated at the jet termination shock emit synchrotron radiation. The turnover of the synchrotron spectrum is typically observed between infrared and optical frequencies, indicating that the maximum energy of non-thermal electrons accelerated at the shock is ≲ TeV for a canonical magnetic field of ∼100 μG. We show that this maximum energy cannot be constrained by synchrotron losses as usually assumed, unless the jet density is unreasonably large and most of the jet upstream energy goes to non-thermal particles. We test this result by considering a sample of hotspots observed at radio, infrared and optical wavelengths.