Model Of Electron Acceleration Research Articles

Electron surfing acceleration in a current sheet of flares

A model of electron acceleration in a current sheet of flares is studied by the analytical approximation solution and the test particle simulation. The electron can be trapped in a potential of propagating electrostatic wave. The trapped electron moving with the phase velocity v p of wave may be effectively accelerated by ev p/c × B z force along the outflow direction in the current sheet, if a criterion condition K > 0 for electron surfing acceleration is satisfied. The electron will be accelerated continuously until the electron de-trap from the wave potential at the turning point S.

RECONNECTION ELECTRIC FIELD AND HARDNESS OF X-RAY EMISSION OF SOLAR FLARES

Magnetic reconnection is believed to be the prime mechanism to trigger solar flares and accelerate electrons up to energies of MeV. In the classical two-dimensional reconnection model, the separation motion of chromospheric ribbons manifests the successive reconnection that takes place higher up in the corona. Meanwhile, downward traveling energetic electrons bombard the dense chromosphere and create hard X-ray (HXR) emissions, which provide a valuable diagnostic of electron acceleration. Analyses of ribbon dynamics and HXR spectrum have been carried out separately. In this Letter, we report a study of the comparison of reconnection electric field measured from ribbon motion and hardness (spectral index) of X-ray emission derived from X-ray spectrum. Our survey of the maximum average reconnection electric field and the minimum overall spectral index for 13 two-ribbon flares show that they are strongly anti-correlated. The former is also strongly correlated with flare magnitude measured using the peak flux of soft X-ray emissions. These provide strong support for electron acceleration models based on the electric field generated at reconnecting current sheet during flares.

Formation of anisotropic distributions of mildly relativistic electrons in flaring loops

AbstractIn this paper we show that different locations of acceleration/injection sites in flaring loops may produce very different types of pitch-angle distributions of accelerated electrons and, as a consequence, different spatial, spectral and polarization properties of the loop microwave emission. It is shown that these properties can be detected using spatially resolved microwave observations of specific flaring loops and be used to choose the most suitable electron acceleration model.

The shock-reprocessing model of electron acceleration in impulsive solar flares

We propose a new two-stage model for acceleration of electrons in solar flares. In the first stage, electrons are accelerated stochastically in a post-reconnection turbulent downflow. The second stage is the of a subset of these electrons as they pass through a weakly compressive fast above the apex of the closed flare loop on their way to the chromosphere. We call this the shock reprocessing model. The model reproduces the energy dependent arrival time delays observed for both the pulsed and smooth components of impulsive solar flare x-rays with physically reasonable parameters for the downflow region. The model also predicts an emission site above the loop-top, as seen in the Masuda flare. The loop-top source distinguishes the model from previous models. The model makes testable predictions for the energy dependence of footpoint pulse strengths and the location and spectrum of the loop-top emission, and can account for the observed soft-hard-soft trend in the spectral evolution of footpoint emission. Our model highlights the concept that reconnection is an acceleration environment rather than a single process. Which combination of processes operate may depend on the initial conditions that determine, for example, whether the reconnection downflow is turbulent. The model comprises one such combination.

Comment on “PIC simulations of circularly polarised Alfvén wave phase mixing: a new mechanism for electron acceleration in collisionless plasmas” by Tsiklauri et al.

Tsiklauri et al. recently published a theoretical model of electron acceleration by Alfv\'en waves in a nonuniform collisionless plasmas. We compare their work with a series of results published earlier by an another team, of which Tsiklauri et al. were probably unaware. We show that these two series of works, apparently conducted independently, lead to the same conclusions. This reinforces the theoretical consistency of the model.

Surface-plasmon-based electron acceleration

In this paper, we describe a surface-plasmon-based electron acceleration model, the results of which were supported by our recent experimental observations [S. E. Irvine et al., Phys. Rev. Lett. 93, 184801 (2004)]. The model incorporates femtosecond electromagnetic field dynamics and nonlinear electron photoemission characteristics of metallic surfaces. To account for the electromagnetic field structure in space and time, we numerically solve Maxwell's equations in two dimensions. Photoelectron emission is treated quasiclassically in accordance with empirical multiphoton statistics, whereas electron dynamics and energy gain are governed by the Lorentz force. Various aspects of the acceleration mechanism are discussed including surface plasmon coupling and evanescent decay, kinetic energy spectra, angular distributions, angle-resolved energy spectra, and the dependence of maximum energy on the surface electric field.

Electron acceleration by lower hybrid waves in magnetic reconnection regions

A new model for electron acceleration in magnetic reconnection regions is developed analytically, tested using quasilinear simulations, and the qualitative predictions compared with observations of reconnection in Earth’s magnetotail and the solar corona. The model involves lower hybrid (LH) waves, produced by a drift instability (LHDI) in reconnection regions, stochastically accelerating electrons parallel to the magnetic field by the Cherenkov resonance (LH drive or LHD). Analytic theory shows that LH waves produced by LHDI have the correct wave numbers to cause parallel electron acceleration from thermal to highly superthermal and even relativistic energies, for sufficiently low plasma β and long evolution times. Several previous Vlasov and particle-in-cell simulations show growth of LH waves by LHDI. Assuming that LHDI produces LH waves with the wave numbers and energy densities (≈50% that of the thermal ion plasma) found in the Vlasov simulations, quasilinear simulations with the correct mass ratio show creation of a superthermal electron tail resonant with the LH waves and parallel to B. The tail reaches about 20 electron thermal speeds in ≈600 lower hybrid times but extends to higher speeds with increasing time. When applied to Wind spacecraft data from Earth’s magnetotail, the model is qualitatively consistent with the occurrence of LH waves, primarily parallel electron acceleration, no ion acceleration, and overall time scale for the acceleration event. Similarly, it is not inconsistent with the primarily parallel electron acceleration, perpendicular ion heating, and time scales inferred from x-ray and radio observations of solar flares. Moreover, the conversion efficiencies from thermal and magnetic energies to semirelativistic electrons are estimated to be over 100 times lower for the magnetotail reconnection data than for the solar corona, not unexpected from the model since LH waves should be favored by the lower plasma beta conditions found in the corona.

Numerical modeling of electron energy‐time dispersions in the high‐latitude part of the cusp region

We have constructed a one‐dimensional numerical model of electron acceleration with dispersive Alfvén waves using realistic parameters based on a rocket observation in the high‐latitude part of the cusp region. This paper demonstrates that our model could quantitatively reproduce important characteristics of observed electron energy‐time dispersions, such as their fluxes, energy range, pitch angle distribution, and source altitudes. The energy‐time dispersion at the rocket altitude is produced by a time‐of‐flight effect of magnetosheath‐originated electrons accelerated by the resonant interaction with inertial Alfvén waves (IAWs) at altitudes of 2000 ∼ 6000 km. The higher‐energy electrons are generated at higher altitudes. The energy‐time dispersion changes according to three key parameters: wave power, perpendicular wavelength, and wave frequency. The wave power affects the flux and the maximum electron energy, and the perpendicular wavelength affects the source altitude calculated by the time‐of‐flight analysis, while the wave frequency (time period) affects the time width of the energy‐time dispersion. We summarize a reasonable set of these parameters for a typical energy‐time dispersion observed in the cusp/cleft region. Finally, we suggest that the IAWs are generated at the dayside magnetopause in association with magnetic reconnection and then propagate toward the ionosphere along the open field line connected to the magnetosheath. Their resonant acceleration of local electrons injected from the magnetosheath at altitudes of several thousand kilometers is the cause for multiple electron energy‐time dispersions observed in the high‐latitude part of the cusp region.

The spectral evolution of impulsive solar X-ray flares

We study the evolution of the spectral index and the normalization (flux) of the non-thermal component of the electron spectra observed by RHESSI during 24 solar hard X-ray flares. The quantitative evolution is confronted with the predictions of simple electron acceleration models featuring the soft-hard-soft behaviour. The comparison is general in scope and can be applied to different acceleration models, provided that they make predictions for the behavior of the spectral index as a function of the normalization. A simple stochastic acceleration model yields plausible best-fit model parameters for about 77% of the 141 events consisting of rise and decay phases of individual hard X-ray peaks. However, it implies unphysically high electron acceleration rates and total energies for the others. Other simple acceleration models such as constant rate of accelerated electrons or constant input power have a similar failure rate. The peaks inconsistent with the simple acceleration models have smaller variations in the spectral index. The cases compatible with a simple stochastic model require typically a few times 1036 electrons accelerated per second beyond a threshold energy of 18 keV in the rise phases and 24 keV in the decay phases of the flare peaks.

Narrowband dm-spikes in the frequency range above 1 GHz and hard X-ray emission

Narrowband dm-spikes observed in nine intervals during five solar flares in the 1–2 GHz range were analyzed together with the RHESSI and HXRS observations. It was found that the over-frequency integrated radio flux during the spike period is closely related with the hard X-ray bursts (the correlation coefficient was 0.7–0.9) and their time delays after X-rays were 2–14 s, with one exception (March 18, 2003) where the time delay was opposite −15 s. Association of spikes with X-ray spectral characteristics enabled us to divide the spikes into two groups: (a) those observed before the soft X-ray flare maximum and, (b) those observed after this maximum. While for the spikes observed after the flare maximum no systematic spectral characteristics were found, the spikes, observed before the flare maximum were at their beginning associated with relatively hard X-ray spectra and their hardness decreased with time. The RHESSI X-ray sources were compact, only in the March 18, 2003 event an additional X-ray source appeared just at the time of the dm-spikes observation. Fourier transformation of the dynamic spectra of spikes was done to compare their dynamics with the X-ray spectral indices. No correlation between power-law spike and X-ray indices were found. It indicates that the MHD turbulence, if it plays a role, does not represent a strong connection between the spectral characteristics of the dm-spikes and associated X-ray bursts. Furthermore, the results were compared with those obtained by (Aschwanden, M.J., Güdel, M. The coevolution of decimetric millisecond spikes and hard X-ray emission during solar flares. Astrophys. J. 401, 736–753, 1992) for spikes observed on lower radio frequencies. Contrary to their results, no monotonic dependence between time delays and X-ray intensities were found. Finally, the results were discussed using the model of the narrowband dm-spikes and model of electron acceleration in the collapsing magnetic trap.

Test particle simulation of the Electron Firehose instability

In the course of the energization of electrons to energies of some tens of keV during the impulsive phase of a solar flare, the velocity distribution function of the electrons is predicted to become anisotropic with (Here, and denote directions with respect to the background magnetic field). Such a configuration can become unstable to the so-called Electron Firehose instability (EFI). Left hand circularly polarized electromagnetic waves propagating along the magnetic field are excited via a non-resonant mechanism: electrons non-resonantly excite the waves while the protons are in resonance and carry the wave. The non-resonant nature of the instability raises the question of the response of the electron population to the growing waves. Test particle simulations are carried out to investigate the pitch-angle development of electrons injected to single waves and wave spectra. To interpret the simulation results, a drift kinetic approach is developed. The findings in the case of single wave simulations show the scattering to larger pitch-angles in excellent agreement with the theory. The situation dramatically changes when assuming a spectrum of waves. Stochasticity is detected at small initial parallel velocities resulting in significant deviations from drift kinetic theory. It enhances the scattering rate of electrons with initial parallel velocity below to the mean thermal perpendicular velocity. Increased scattering is also noticed for electrons having initial parallel velocity within an order of magnitude of the resonance velocity. The resulting pitch-angle scattering is proposed to be an important ingredient in Fermi-type electron acceleration models, particularly transit-time acceleration by compressional MHD waves.

On electron acceleration at CIR related shock waves

The interaction of fast and slow speed solar wind streams leads to the formation of so-called corotating interaction regions (CIRs) in the heliosphere. These CIRs are often associated with shock waves, at which electrons are accelerated as observed by the Ulysses spacecraft. A correlation between the ratio of energetic electron fluxes at the crossing of CIR related shocks to those in the far upstream region of these shocks and the magnetic field compression of the associated shocks has been revealed by analysing the data of the HISCALE instrument aboard Ulysses. This result can be explained by a model of electron acceleration at shock waves, where the electrons gain energy due to multiple reflections at large amplitude magnetic field fluctuations occurring in the vicinity of the shock transition.

Electron excitation by a multi passage laser beam in a plasma

The limits put by optical guiding, and channel guiding mechanisms on the Laser Wakefield Acceleration (LWFA) technique are imposed on the Resonant Laser Wakefield Acceleration (RLWFA) scheme. Energy gained by the electrons in both schemes are calculated and compared. It has presented that in the RLWFA case, the electrons gain more and more energy after each traversal of the laser pulse and the electrons in a plasma gain about 3 GeV after 10 passages of the laser pulse. They gain 13 GeV when the laser light makes 50 passages and 26 GeV after the laser beam traverses the plasma 100 times. Moreover, the channel guiding mechanism is integrated to the RLWFA scheme and together with diffraction guiding a model for electron acceleration is proposed.

Electron Acceleration and Time Variability of High‐Energy Emission from Blazars

Blazars are known to emit a broadband emission from radio to gamma rays with rapid time variations, particularly in X-rays and gamma rays. Synchrotron radiation and inverse Compton scattering are thought to play an important role in emission, and the time variations are likely related to the acceleration of nonthermal electrons. As simultaneous multiwavelength observations with continuous time spans have been recently available, some characteristics of electron acceleration can possibly be inferred from the spectral changes of high-energy emission. In order to make such inferences, we solve the time-dependent kinetic equations of electrons and photons simultaneously, using a simple model for electron acceleration. We then show how the time variations of emission are dependent on electron acceleration. We also present a simple model for a flare in X-rays and TeV gamma rays by temporarily changing the acceleration timescale. Our model can be used to analyze observed data in detail to obtain information on electron acceleration in blazars.

A model for stochastic acceleration of electrons during geomagnetic storms

The theory of resonant diffusion is extended to fully relativistic plasmas, and we examine resonant interactions between electrons and electromagnetic R mode (whistler) and L-mode (EMIC) waves. Resonant diffusion curves are constructed for plasma parameters representative of the Earth's storm time magnetosphere, both inside and outside the plasmapause. EMIC waves can resonate with electrons > 1 MeV, but the energies remain nearly constant along the diffusion curves. Storm-time EMIC waves can induce rapid pitch—angle scattering, but the waves are ineffective for stochastic acceleration of elections. Substantial energy change can occur along the diffusion curves for interactions between resonant electrons and whistler—mode waves, especially in regions of low plasma density. Specifically, whistlers can accelerate electrons from energies near 100 keV to above 1 MeV outside the plasmapause. A model is proposed comprising energy diffusion by whistler-mode chorus and pitch-angle scattering by EMIC waves to account for the gradual acceleration of electrons over the region 4 ≤ L ≤ 6 during the recovery phase of a geomagnetic storm.

Shock Drift Electron Acceleration in a Wavy Shock Front

It is commonly believed that solar type II bursts are caused by accelerated electrons at a shock front. Holman and Pesses (1983) suggested that electrons creating type II bursts are accelerated by the shock drift mechanism. Zlobec et al. (1993) dealt with a fine structure of type II bursts (herringbones) and suggested a qualitative model where electrons are accelerated by a nearly perpendicular wavy shock front. Using this idea, we developed a model of electron acceleration by such a wavy shock front. Electrons are accelerated by the drift mechanism in the shock layer. Under simplifying assumptions it is possible to obtain an analytical solution of electron motion in the wavy shock front. The calculations show that electrons are rarely reflected more than once at the wavy shock front and that their final energy is mostly 1–3 times the initial one. Their acceleration does not depend significantly on shock spatial parameters. In the present model all electrons are eventually transmitted downstream where they form two downstream beams. Resulting spectral and angular distributions of accelerated electrons are presented and the relevance of the model to the herringbone beams is discussed.

Stochastic acceleration of electrons in solar flares

The generation of lower-hybrid waves by cross-field currents is applied to reconnection processes proposed for solar flares. Recent observations on fragmentation of energy release and acceleration, and on hard X-ray (HXR) spectra are taken into account to develop a model for electron acceleration by resonant stochastic interactions with lower-hybrid turbulence. The continuity of the velocity distribution is solved including collisions and escape from the turbulence region. It describes acceleration as a diffusion process in velocity space. The result indicates two regimes that are determined by the energy of the accelerating electrons which may explain the double power-law often observed in HXR spectra. The model further predicts an anticorrelation between HXR flux and spectral index in agreement with observations.

Electron cyclotron wave acceleration outside a flaring loop

view Abstract Citations (48) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Electron cyclotron wave acceleration outside a flaring loop Sprangle, P. ; Vlahos, L. Abstract A model for the secondary acceleration of electrons outside a flaring loop is proposed. The results suggest that the narrow bandwidth radiation emitted by the unstable electron distribution inside a flaring loop can become the driver for secondary electron acceleration outside the loop. It is shown that a system of electrons gyrating about and streaming along an adiabatically spatially varying, static magnetic field can be efficiently accelerated to high energies by an electromagnetic wave propagating along and polarized transverse to the static magnetic field. The predictions from our model appear to be in general agreement with existing observations. Publication: The Astrophysical Journal Pub Date: October 1983 DOI: 10.1086/184137 Bibcode: 1983ApJ...273L..95S Keywords: Coronal Loops; Electromagnetic Radiation; Electron Acceleration; Solar Electrons; Solar Flares; Solar Radio Bursts; Cyclotron Frequency; Electron Distribution; Magnetically Trapped Particles; Plasma-Electromagnetic Interaction; Secondary Emission; Solar X-Rays; Solar Physics full text sources ADS |

Electron acceleration in an array of auroral arcs

Energy spectra of electrons encountered on a rocket flight across an array of auroral arcs are employed to test three related models of electron acceleration. All three are based on a potential difference existing between the source plasma in the magnetosphere and the observation point in the ionosphere. One of the models provides a satisfactory fit to the observed spectra. Two alternative mechanisms are suggested to explain this model. The first possibility is a time-varying potential difference, which results in the accelerated electrons being observed with a statistical distribution of energy gain. The second possibility, which results in the same energy gain distribution, is a constant potential difference operating in conjunction with plasma instabilities generated by the accelerated beam. The energy gain distribution in the second case is therefore a consequence of a constant potential difference and a variable energy loss. In addition it is suggested that electrostatic waves generated by the instabilities could accelerate ambient plasma to suprathermal energies. Application of the model to the complete data set yields a continuous record of the parameters defining the acceleration and source plasma across the array of arcs. Reference is also made to an acceleration mechanism involving resonance with electrostatic waves.