Subrelativistic Electrons Research Articles

The Energetic Problem of X-Ray Emission from the Coma Cluster

The stochastic acceleration of subrelativistic electrons from a background plasma is studied to find an explanation of the hard X-ray emission detected from the Coma cluster. We show that the temporal evolution of the electron distribution functions has, at its final stationary stage, a rather specific form determined by the interactions with charged background particles and electromagnetic fluctuations. These distribution functions cannot be described by simple exponential or power-law expressions. A broad transfer region is formed by Coulomb collisions at energies between the Maxwellian and power-law parts of the distribution functions. In this region the radiative lifetime of a single quasi-thermal electron differs greatly from the lifetime of the distribution function as a whole. This solves the problem of rapid cluster overheating by nonthermal electrons (Petrosian, 2001): while Petrosian's estimates are correct for nonthermal particles they are inapplicable in the quasi-thermal range.

Sub-fs electron pulses for ultrafast electron diffraction

We present a new concept for an electron gun generating subrelativistic electron pulses with a duration down to the attosecond range. It is based on a cylindrical RF cavity (a so-called pill-box cavity) oscillating in its TM010 eigenmode with a photocathode triggered by a fs-laser pulse. Injecting electrons at an appropriate phase of the RF cycle compensates for their initial velocities and time delays and makes the electrons arrive at a target in a sub-fs temporal window. Such electron pulses will allow nuclear motion and electronic dynamics to be studied on an attosecond time scale.

In-situ acceleration of subrelativistic electrons in the Coma halo and the halo's influence on the Sunyaev-Zeldovich effect

Aims.The stochastic acceleration of subrelativistic electrons from a background plasma is studied in order to find a possible explanation of the hard X-ray emission detected from the Coma cluster.

Observations and modeling of energetic electron dynamics during the October 2001 storm

We examined and simulated the dynamics of energetic electrons during the October 2001 magnetic storm with the relativistic RAM electron model for a wide range of energies. The storm had a rapid main phase followed by a day of strong geomagnetic activity that produced a second Dst minimum and then a very quiet recovery phase. During the main phase and the period of intense activity, the observed hot electron flux (E = 30 keV) increased at low L while decreasing at large L and then decayed abruptly at the beginning of the recovery phase when activity subsided. The flux of subrelativistic (E = 100–300 keV) electrons also increased at low L and decreased at large L during the main phase and the period of intense activity but remained high throughout the recovery phase. In contrast, the relativistic (E = 300–1200 keV) electron flux decreased during the main phase, remained low throughout the period of intense activity, and then increased above prestorm values during the recovery phase in spite of the low activity. The highest energy electron flux (E > 1200 keV) decreased during the main phase and never recovered to prestorm levels. The numerical simulation was compared with observations. We identified the physical processes which produce the flux variations at the different energies. In the simulation, the hot electrons were convected inward during the main phase, reproducing the observed local time flux asymmetry. The higher‐energy electrons, on the other hand, were predominantly transported inward by radial diffusion and not convective motion. The simulation was not able to reproduce the subrelativistic and relativistic electron flux enhancement and spatial expansion as observed during the recovery phase. In the simulation, most of the energization occurred around the main phase and the period of intense activity with negligible transport or flux enhancement during the recovery phase. The discrepancy between the observed and simulated high energy electron flux suggests that only convective transport and radial diffusion cannot fully explain the electron dynamics. An additional mechanism may be necessary to explain enhancements of high energy electron flux during the recovery phase of the storm.

Dense Gas Discharge With Runaway Electrons as a New Plasma Source for Surface Modification and Treatment

Experimental results on runaway electron generation with an energy of 20-350 keV at nanosecond discharge in air under normal conditions are presented. Physical conditions are specified for the efficient generation of runaway electrons in dense gas using this type of discharge. A subrelativistic electron beam is obtained in air under normal conditions with an output aperture of 4times33 cm2 and a current amplitude of 2 kA. Experimental results on atmospheric discharge with runaway electrons (ADRE) application for bacteria decontamination on the surface of solid states including the inside of tight polymeric packings are described. Essentially, a higher surface sterilization capacity by the ADRE (with energy up to 300 keV and current amplitude up to 700 A) in comparison with other types of discharges is obtained. The basic biophysical mechanisms determining the ADRE sterilization futures are discussed

Relativistic nucleon and electron production in the 2003 October 28 solar event

A flare on 2003 October 28 produced a relativistic particle event at Earth, although the active region AR 10486 was located to the east of the central meridian of the Sun. The paper considers features related to the acceleration at the Sun and the propagation to the Earth of energetic particles in this event, which occurred on a disturbed interplanetary background caused by preceding activity on the Sun and a corotating high‐speed solar wind stream. From a study of the onset times of the event at different neutron monitors, we conclude that the earliest arriving solar particles were neutrons. The first relativistic protons arrived a few minutes later. Among relativistic solar protons (RSP), two populations could clearly be distinguished: prompt and delayed ones. The prompt solar protons caused an impulse‐like increase at a few neutron monitor stations. The delayed solar protons arrived at Earth 0.5 hours later. Both prompt and delayed relativistic protons arrived at Earth from the antisunward direction. On the other hand, subrelativistic electrons that were traced by their radio emission from meter waves (Nançay Radioheliograph and Decametric Array) to kilometer waves (Wind/WAVES) are accompanied by metric radio emission in the western solar hemisphere, far from the flaring active region. We propose a scenario that reconciles the unusual features of energetic particles at the Earth with the observed structure of the interplanetary magnetic field, which suggests the Earth is at the interface between an interplanetary coronal mass ejection (ICME) and a corotating stream during the event. In this scenario the high‐energy protons and electrons are accelerated in the flaring active region, injected into the eastern leg of an ICME loop rooted in the active region, and reach the Earth from the antisunward direction after passing through the summit of the loop. We attribute the promptly escaping subrelativistic electrons to acceleration in the western solar hemisphere and propagation along the nominal Parker spiral.

Stochastic Fermi acceleration of subrelativistic electrons and its role in impulsive solar flares

We re-examine stochastic Fermi acceleration (STFA) in the low-energy (Newtonian) regime in the context of solar flares. The particle energization rate depends on a dispersive term and on a coherent gain term. The energy dependence of pitch angle scattering is important for determining the electron energy spectrum. For scattering by whistler wave turbulence, STFA produces a quasi-thermal spectrum. A second well-constrained scattering mechanism is needed for STFA to match the observed 10‐100 keV non-thermal spectrum. We suggest that STFA most plausibly acts as phase one of a two-phase particle acceleration engine in impulsive flares: STFA can match the thermal spectrum below 10 keV, and possibly the power-law spectrum between 10 and 100 keV, given the proper pitch angle scattering. However, a second phase, such as shock acceleration at loop tops, is probably required to match the spectrum above the observed knee at 100 keV. Understanding this knee, if it survives further observations, is tricky.

Nonthermal hard X-ray emission from the Galactic Ridge

We investigate the origin of the nonthermal X-ray emission from the Galactic ridge in the range 10-200 keV. We consider bremsstrahlung of subrelativistic cosmic ray protons and electrons as production processes. From the solution of the kinetic equations describing the processes of particle in-situ acceleration and spatial propagation we derive parameters of the spectra for protons and electrons. For in-situ acceleration the flux of accelerated particles consists mainly of protons since the ratio of the accelerated protons to electrons is large and the flux of nuclei with charges Z>1 is strongly suppressed. Our analysis has shown that the proton bremsstrahlung model is free from the previously discussed problems of the high gamma-ray flux at MeV or at hundreds of MeV energy regions. However, the model with proton bremsstrahlung requires specific restrictions on the proton spectrum.The real problem of the proton bremsstrahlung model is the large pressure of the accelerated protons in the disk. For the electron origin of the ridge flux we do not have the problems of the gamma-ray line and continuous fluxes. On the other hand the electron bremsstrahlung model requires more effective acceleration of background particles.

Creating and using of superdense micro-beams of relativistic electrons

The problem of creation and the results of experimental investigation of superdense subrelativistic electrons micro-beams with current density 10 10 A/cm 2 ⩽j⩽10 14 A/cm 2 , duration Δ t=1–10 ns and energy about 0.01–1 MeV are discussed. The interaction of these micro-beams with condensed target and ions beams are investigated. The problems of isomers excitation and active medium for both X-ray laser and gamma-laser creation by use of super-dense electrons micro-beams are also studied.

Gyro-TWT with a helical operating waveguide: new possibilities to enhance efficiency and frequency bandwidth

A helical corrugation of the inner surface of an oversized cylindrical waveguide provides, for certain parameters, an almost constant value of group velocity and close to zero longitudinal wavenumber of an eigenwave for a very broad frequency band. The use of such a helical waveguide as an operating section of a gyrotron traveling wave tube (gyro-TWT) allows significant widening of its bandwidth and an increase in the efficiency at very large particle velocity spreads. In this paper, the new concept is confirmed by theoretical analysis and "cold" measurements of the helical waveguide dispersion. Results of a linear and nonlinear theory of the helical gyro-TWT as well as two designs for subrelativistic (80 keV, 20 A) and relativistic (300 keV, 80 A) electron beams are also presented. For both designs, parameters providing a very broad frequency band (about 20%) and high efficiency (above 30%) have been found. When the transverse velocity spread is increased from zero up to a very high value of 40%, simulations showed only a 20%-30% narrowing in the frequency band and a 20% decrease in electron efficiency. The theoretical analysis demonstrates important advantages of the helical gyro-TWT over the "smooth" one in frequency bandwidth, sensitivity to electron velocity spread, and stability to parasitic self-excitation.

Disk-Corona Models and X-Ray Emission from Seyfert Galaxies

AbstractThe current status of understanding of the X-ray emission from Seyfert galaxies involves Comptonization of soft photons by hot subrelativistic electrons. After briefly reviewing the early theoretical basis for the presence of hot optically thin plasma in or around accretion disks and the key observations that led to develop the presently popular model of an accretion disk with a hot corona, we summarize recent progress in accretion models that take into account energy dissipation and/or angular momentum transport in the corona. Finally, adopting the simple scheme of a homogeneous plane parallel corona, we discuss in detail the dependence of the X-ray spectrum on the coronal parameters. Despite the strong coupling between optical depth and temperature which determines a spectral shape insensitive to their precise values, moderate spectral changes are possible. The spectral variability patterns can be used as diagnostics for coronal physics and should allow to determine whether the optical depth of the corona is dominated bye+e−pairs.

The nonequilibrium beam plasma

The kinetics and the heating of nonequilibrium plasma, formed as a result of injection of intensive relativistic (or subrelativistic) electron or proton beams in the atomic-molecular gas (nitrogen or air with water vapor) at different pressures (from 0.1–10 atm) have been investigated numerically by using the mathematical model “SKIF” which was developed by the author 15 years ago.More than 200 plasma-chemical reactions and elementary processes are taken into consideration, including the action of “non-Maxwell” electrons of the cascade, formed in the ionization of the molecules by the beam particles. For the description of the deviation of energy distribution of such electrons from the equilibrium distribution, a simplified analytical model was used, with the help of which one can calculate the energy spectrum of the cascade electrons with satisfactory precision. This essentially reduces the calculating expenditure.

Electron injection in collisionless shocks

Electron acceleration in collisionless shocks is considered. It is shown that injection of thermal electrons may be controlled by self-generated whistlers, provided that the Alfven Mach number ahead of the shock exceeds 43/(beta<SUB>e)</SUB> exp 1/2, where beta<SUB>e</SUB> is the ratio of thermal electron pressure to magnetic pressure inside the shock. Acceleration of subrelativistic electrons by whistlers at lower Mach number shocks may still be possible if the thermal electrons are preinjected by some other mechanism. The confinement of the upstreaming electrons to the background fluid is accomplished by off-axis whistlers. Using quasi-linear theory the diffusion coefficient is calculated self-consistently and appears to increase with increasing electron energy and with decreasing efficiency. Wave damping due to induced scattering and whistler mode coupling is considered. Mode coupling is shown to be unimportant when M(A) is greater than 13, whereas induced scattering is even less important, except maybe for waves having frequencies near the ion gyrofrequency.

Radio evidence for a magnetic mirror effect on beams of subrelativistic electrons in the solar corona

view Abstract Citations (28) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radio Evidence for a Magnetic Mirror Effect on Beams of Subrelativistic Electrons in the Solar Corona Caroubalos, C. ; Poquerusse, M. ; Bougeret, J. -L. ; Crepel, R. Abstract New radio spectral observations of the sun are presented which reveal a magnetic mirror effect on impulsively injected beams of subrelativistic electrons in coronal arch structures. A third branch following the descending branch of some U-bursts was detected, called 'N-bursts'. A detailed analysis of the digital time profiles shows that the duration of the beam emission regularly increased with time along the three successive branches of the N-bursts. This duration is interpreted as the dispersion of the beam along its path. This continuity strongly suggests that the three successive branches were produced by the same electron beam, demonstrating that the N-bursts actually trace reflections on magnetic mirrors near the feet of magnetic arches. Nineteen N-bursts were analyzed in detail. The information thus obtained allows a detailed probing of the magnetic mirror effect on beams of subrelativistic electrons in astrophysical conditions. Publication: The Astrophysical Journal Pub Date: August 1987 DOI: 10.1086/165473 Bibcode: 1987ApJ...319..503C Keywords: Electron Beams; High Energy Electrons; Magnetic Mirrors; Solar Corona; Solar Magnetic Field; Solar Radio Bursts; Magnetic Field Configurations; Radio Spectra; Solar Spectra; Temporal Distribution; Solar Physics; SUN: CORONA; SUN: MAGNETIC FIELDS; SUN: RADIO RADIATION full text sources ADS |

Fast solar hard X-ray bursts and large-scale coronal structures

view Abstract Citations (9) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Fast solar hard X-ray bursts and large-scale coronal structures Simnett, G. M. Abstract The conditions at the sun at the times corresponding to a selected set of 22 fast impulsive hard X-ray bursts reported by Crannell et al. (1978) are examined. It is suggested that one of the bursts must arise from a precipitating beam of subrelativistic electrons; the source of the electrons is postulated to be in a region very remote from the X-ray site on the basis of type III and other radio data. The connection is via a coronal magnetic loop extending to approximately 3 solar radii above the photosphere. The energy in the electron beam is estimated at 3 x 10 to the 27th ergs. Intense soft X-ray and/or microwave radio storms at times corresponding to many of the impulsive X-ray bursts lead to the conclusion that 14, and possibly 18, of the 22 bursts could have the same interpretation. The energy in such an electron beam could be important when considering the trigger phase of some flares. Publication: The Astrophysical Journal Pub Date: April 1982 DOI: 10.1086/159871 Bibcode: 1982ApJ...255..721S Keywords: Solar Corona; Solar X-Rays; Type 3 Bursts; Coronal Loops; Electron Acceleration; Electron Precipitation; Energetic Particles; Noise Storms; Relativistic Electron Beams; Solar Flares; Solar Storms; X Ray Spectra; Solar Physics full text sources ADS |

Solar activity and its forecast

An essential problem for solar activity forecasting is understanding the physical basis of solar activity phenomena. During the last decade, interest shifted from a global evaluation of the active centers to a detailed study of their microscopic magnetic components. This allows for the possibility of an identification of the flare sites but we do not understand the physical process of the triggering itself. The X-ray data give us some evidence that during a flare the particles were accelerated in two distinct phases. Obviously other processes of particle acceleration are at work on the sun: transient perturbations of the chromosphere linked to type-III bursts are thought to be connected to an acceleration of subrelativistic electrons which may or may not trigger an optical flare. High time resolution data show that many events are so complex that in the future they will have to be taken into account. Since forecasting centers need a worldwide network, they cooperate through the International Ursigram and World Day Service (IUWDS) : They agree on a permanent program of data exchanges and daily-forecasts and on temporary projects. Several such projects are briefly reported in order to illustrate the variety of the requests that a warning center has to fulfill.