Thermal Electrons in Mildly Relativistic Synchrotron Blast Waves

Numerical models of collisionless shocks robustly predict an electron distribution composed of both thermal and nonthermal electrons. Here, we explore in detail the effect of thermal electrons on the emergent synchrotron emission from subrelativistic shocks. We present a complete “thermal + nonthermal” synchrotron model and derive properties of the resulting spectrum and light curves. Using these results, we delineate the relative importance of thermal and nonthermal electrons for subrelativistic shock-powered synchrotron transients. We find that thermal electrons are naturally expected to contribute significantly to the peak emission if the shock velocity is ≳0.2c, but would be mostly undetectable in nonrelativistic shocks. This helps explain the dichotomy between typical radio supernovae and the emerging class of “AT2018cow-like” events. The signpost of thermal electron synchrotron emission is a steep optically-thin spectral index and a ν 2 optically-thick spectrum. These spectral features are also predicted to correlate with a steep postpeak light-curve decline rate, broadly consistent with observed AT2018cow-like events. We expect that thermal electrons may be observable in other contexts where mildly relativistic shocks are present and briefly estimate this effect for gamma-ray burst afterglows and binary–neutron-star mergers. Our model can be used to fit spectra and light curves of events and accounts for both thermal and nonthermal electron populations with no additional physical degrees of freedom.

Multi-component dense plasma with ion and super-thermal electrons with kappa distribution

The soliton waves are one of the nonlinear phenomena which can propagate in the different types of plasma such as multiple particles of plasma, nonthermal plasma, and space plasma. Using the Sagdeev potential technique, the stability conditions of the soliton waves in the nonthermal plasma have been theoretically studied. One of the significant factors that can affect the propagation of the soliton waves is the distribution function such as nonMaxwellian distribution function or Kappa distribution function. In this paper, we try to investigate the soliton wave in the unmagnetized multi-component plasma consisting of the nonthermal electron and the nonthermal ion, positron and dust with Kappa distribution function. Then by using the Sagdeev potential, the nonlinear equation for the potential is obtained and then the compression and rarefaction soliton waves are computed with the numerical method for this nonlinear wave. Finally, by imposing the Sagdeev potential condition, we discuss the stability of these soliton waves.

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

<p>Solar flares release enormous magnetic energy into the corona, producing the heating of ambient plasma and acceleration of particles. The flaring process is complex and often shows multiple spatially separated temporal individual episodes of energy releases, which can be hard to resolve based on the instrument capability. We analysed the multi-wavelength imaging and spectroscopy observations of multiple electron acceleration episodes during a GOES B1.7-class two-ribbon flare observed simultaneously with the Karl G. Jansky Very Large Array (VLA) at 1--2 GHz, the Reuven Ramatay High Energy Solar Spectroscopic Imager (RHESSI) in X-rays, and the Solar Dynamics Observatory in extreme ultraviolet (EUV).<br>We observed a total of six radio bursts. First three bursts were co-temporal, but not co-spatial nonthermal X-ray source and represent multiple electron acceleration episodes. We model the radio spectra by optically thick gyrosynchrotron emission and estimate the magnetic field strength and nonthermal electron spectral parameters in each acceleration episode. We note that the nonthermal parameters derived from X-rays differ considerably from the nonthermal parameters inferred from the radio and originates in the lower corona. Although co-temporal, our multi-wavelength analysis shows that different electron populations produce multiple acceleration episodes in radio and X-rays wavelengths. </p>

Effect of Coulomb Focusing on the Electron–Atom Bremsstrahlung Cross Section for Tungsten and Iron in Nonthermal Lorentzian Plasmas

The Coulomb focusing effect on the electron–atom bremsstrahlung spectrum is investigated in nonthermal Lorentzian plasmas. The universal expression of the cross section of nonrelativistic electron–atom bremsstrahlung process is obtained by the solution of the Thomas-Fermi equation with the effective atomic charge. The effective Coulomb focusing for the electron–atom bremsstrahlung cross section near the threshold domain is also investigated by adopting the modified Elwert-Sommerfeld factor with the mean effective charge for the bremsstrahlung process. In addition, the bremsstrahlung emission rates are obtained by considering encounters between nonthermal electrons and atoms such as Fe and W atoms. We found that the bremsstrahlung emission rates for nonthermal electron–atoms are lower than those for thermal plasmas. Various nonthermal effects on the bremsstrahlung emission rates in Lorentzian plasmas are also discussed.

Evolution of Flare-accelerated Electrons in the Solar Corona and Chromosphere Revealed by Spatially Resolved Microwave and Hard X-Ray Analysis

<p>Hard X-ray (HXR) and microwave (MW) observations are highly complementary for studying electron acceleration and transport processes in solar flares. In recent years, a new effort has been made in the MW domain using new high-resolution, multifrequency data from The Expanded Owens Valley Solar Array (EOVSA) and a breakthrough numerical modeling infrastructure that enables us to study properties of high-energy electrons in unprecedented cadence and quantitative detail. This study introduces the observation of an M1.2 flare that occurred on 2017 September 9 and analyzes the evolution of the nonthermal electrons in the corona based on EOVSA MW spectral imaging data. We find a significant spectral hardening of the MWemitting nonthermal electron population in the corona, using EOVSA lower-frequency (<7 GHz) observations over a selected 4-minute window of the flare's impulsive phase. We compare this spectral evolution with the evolution of the spectral index of nonthermal electrons emitting in the chromosphere, derived from HXR observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager. We discuss the general picture of the evolution of the nonthermal electron population in this flare by incorporating observations at the two complementary wavelengths. We also make an estimate of the total energy of the nonthermal electrons contained in the observed coronal low-frequency MW source and discuss its temporal evolution.</p>