Resonant Acceleration Research Articles

Parallel electric fields associated with inertial Alfvén waves

The parallel electric fields associated with inertial Alfvén waves are investigated using a self-consistent drift-kinetic simulation code. The simulation code allows for non-linear effects and we show that when the wave energy is large in comparison to the plasma thermal energy, the amplitude and form of the parallel electric field can evolve significantly as the wave propagates throughout the plasma. In this paper, we discuss the effects of grid resolution on the amplitude of the parallel electric field and show that the integral of positive parallel electric field with respect to time is a more useful measure of the downward electric field force available for electron acceleration than the maximum of the parallel electric field. Although the parallel electric field theoretically maximises when the product of perpendicular wavenumber ( k ⊥ ) and electron skin depth ( δ e ) is equal to one, the amount of resonant electron acceleration maximises for k ⊥ δ e > 1 , since the electron acceleration mechanism is sensitive to the local phase velocity of the wave. We determine the phase velocity of the idealised pulses studied by our simulation code and show that it is well approximated by the solutions of the linear dispersion relation. Our results suggest that electron acceleration occurs mainly for k ⊥ δ e > 1 , which in turn implies very short perpendicular scale lengths for waves in the auroral acceleration region.

Resonant Acceleration of Electrons in Combined Self-ConsistentQuasistatic Electromagnetic Fields and Intense Laser Fields

Using the single electron model, the acceleration of electrons incombined circularly polarized intense laser fields and the spontaneousquasistatic fields (including axial and azimuthal magnetic fields, theaxial and transverse electric fields) produced in intense laser plasmainteraction is investigated analytically and numerically by fitting theproper parameters of the quasistatic fields based on the data from theexperiment and numerical calculation. A new resonant condition isgiven. It is found that the resonance acceleration of electron dependsnot only on laser field, but also on the bounce frequency oscillatingin the quasistatic magnetic field and electric field. The net energygained by electron does not increase monotonously with axial electricfield, but there are some optimal axial electric fields.

Additional acceleration and collimation of relativistic electron beams by magnetic field resonance at very high intensity laser interaction

For the interpretation of experiments for acceleration of electrons at interaction up to nearly GeV energy in laser produced plasmas, we present a new model using interaction magnetic fields. In addition to the ponderomotive acceleration of highly relativistic electrons at the interaction of very short and very intense laser pulses, a further acceleration is derived from the interaction of these electron beams with the spontaneous magnetic fields of about 100 MG. This additional acceleration is the result of a laser-magnetic resonance acceleration (LMRA) around the peak of the azimuthal magnetic field. This causes the electrons to gain energy within a laser period. Using a Gaussian laser pulse, the LMRA acceleration of the electrons depends on the laser polarization. Since this is in the resonance regime, the strong magnetic fields affect the electron acceleration considerably. The mechanism results in good collimated high energetic electrons propagating along the center axis of the laser beam as has been observed by experiments and is reproduced by our numerical simulations.

Auroral electron dispersion below inverted‐V energies: Resonant deceleration and acceleration by Alfvén waves

Rocket observations of electron time dispersion which occurs at energies below the primary inverted‐V electron precipitation exhibit a variety of characteristics in the dispersion timescale and flux intensity distribution. We present simulation results that illustrate how different dispersion signatures can be produced by resonant acceleration and deceleration of auroral electrons by inertial Alfvén waves. We investigate the individual effects of relevant parameters such as the altitude of the inverted‐V potential, Alfvén wave amplitude, wave velocity, and source electron population. The energies to which Alfvén waves can accelerate electrons are primarily determined by the peak Alfvén speed below the DC potential drop and to a lesser extent by the amplitude of the wave parallel electric field. The observed electron signatures can be used to obtain information about the peak Alfvén speed, the wave parallel electric field, the DC electric field, and plasma environment through which the waves propagate.

Self‐consistent electron acceleration due to inertial Alfvén wave pulses

We present self‐consistent kinetic simulations of the electron response to finite duration shear Alfvén wave pulses in a magnetized plasma. In Earth's magnetosphere, the evidence suggests that parallel electric fields in inertial scale shear Alfvén waves can accelerate electrons in the geomagnetic field‐aligned direction. Here, we study large‐amplitude wave forms as they travel through ambient plasma at phase velocities which are consistent with resonant electron acceleration predicted by linear kinetic theory. Our self‐consistent simulations reveal that the wave potential evolves nonlinearly as shear Alfvén wave pulses travel through the simulation domain. The evolution of the wave pulse from a symmetrical to a nonsymmetrical potential structure, and the large perturbation in the distribution function required to carry the parallel current of the pulse, leads to nonresonant acceleration of electrons (i.e., acceleration of electrons which are not traveling at the same velocity as the wave). We compare the signature of resonant and nonresonant electron acceleration with data from a low‐altitude spacecraft and suggest an explanation for features often referred to as field‐aligned suprathermal electron bursts. Finally, we discuss how resonant and nonresonant acceleration of electrons is affected by the perpendicular wavelength and amplitude of shear Alfvén wave pulses.

Stochastic Acceleration of Electrons by the Right-HandExtraordinary Mode in the High Density Plasma Region

The resonant acceleration of electrons by the superluminous R-X modeis evaluated in the high density plasma region of Earth, specificallyaround the geostationary orbit. The corresponding resonantfrequency range together with the harmonic N required for producing asignificant acceleration is studied in detail. It is found that thestochastic acceleration is basically controlled by the harmonic N anda dimensionless parameter α = |Ωe2|/ωpe2(where |Ωe| and ωpe are the electron gyrofrequencyand plasma frequency respectively). For α = 0.1 (around thegeostationary orbit), there are not gyroresonances occurringbetween electrons and right-handextraordinary-mode electromagnetic waves until higher harmonicsN⩾4; while for α = 0.5, the gyroresonance begins at N = 2.Substantial acceleration of electrons by the right-handextraordinary mode is possible forthose higher harmonic (N>1) resonances over a wide range of wavenormal angles θ. This indicates that higher harmonic resonancecan play an important role on the electron acceleration in the highdensity plasma region (α<1).

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.

Resonant Acceleration of Magnetospheric Electrons Driven bythe R–X Mode

An extended relativistic model is developed to evaluate thesuperluminous R–X-mode resonance especially the second-order andthird-order resonances with electrons in the Earth's magnetosphere. Thepotential for stochastic electron acceleration driven by the R–X modeis determined by the dispersive properties of the R–X mode andspecifically the resonant harmonic N. In contrast to the limitedacceleration at the first harmonic (N = 1) resonance, for the higherharmonic (N>1) resonances, the R–X mode is capable of acceleratingelectrons from ~10 keV to ~ MeV energies, over a wide rangeof wave normal angles, in spatial regions extending from the auroralcavity to the latitude (>30 degrees) outer radiation belt. Thisindicates that higher-order resonance is essentially important forthe electron acceleration for the oblique wave propagation.

High-Altitude CUSP: The Extremely Dynamic Region in Geospace

The high-altitude dayside cusps (both northern and southern) are extremely dynamic regions in geospace. Large diamagnetic cavities with significant fluctuations of the local magnetic field strength have been observed there. These cusp diamagnetic cavities are always there day after day and are as large as 6 R E. Associated with these cavities are charged particles with energies from 20 keV up to 10 MeV. The intensities of the cusp energetic ions have been observed to increase by as much as four orders of the magnitude when compared with regions adjacent to the cusp which includes the magnetosheath. Their seed populations are a mixture of ionospheric and solar wind particles. The measured energetic ion fluxes in the high-altitude cusp are higher than that in both the regions upstream and downstream from the bow shock. Turbulent electric fields with an amplitude of about 10 mV/m are also present in the cusp, and a cusp resonant acceleration mechanism is suggested. The observations indicate that the dayside high-altitude cusp is a key region for transferring the solar wind mass, momentum, and energy into the Earth’s magnetosphere.

Spatial-temporal ion structures in the earth’s magnetotail: Beamlets as a result of nonadiabatic impulse acceleration of the plasma

The properties of high-energy ion beams (beamlets) observed in the boundary layer of the plasma sheet of the Earth’s magnetotail during short time intervals (1–2 min) have been considered. Beamlets are induced by nonlinear impulse accelerating processes occurring in the current sheet of the far regions of the geomagnetic tail. Then, moving toward the Earth along the magnetic field lines, they are detected in the magnetotail (in the plasma sheet boundary layer) and in the high-latitude part of the auroral zone in the form of short bursts of high-energy ions (with energies of several tens of keVs). The size of the localization region of the beamlets in the magnetotail and auroral zone has been determined by the epoch-superposition method, and it has been shown that beamlets are concentrated in a narrow region near the plasma sheet boundary, whose latitude size is no more than 0.8δ. This conclusion corroborates the theoretical prediction that the nonadiabatic resonant acceleration of ions occurs in a spatially localized region near the separatrix separating the open magnetic field lines and closed field lines, which contain the hot and isotropic plasmas of the plasma sheet. Based on the CLUSTER multisatellite measurements, the spatial structure of beamlets is analyzed and it has been found that the Alfven wave arises due to the excitation of fire-hose instability at the instant of the exit of the ion beam from the current sheet to the high-latitude region of the far tail of the Earth’s magnetosphere. The longitudinal (along the magnetic field) and transverse sizes of a beamlet are estimated. It has been found that the beamlet is a dynamic plasma structure whose longitudinal size is several hundred times larger than its transverse size.

Charged particle acceleration by an intense wake-field excited in plasmas by either laser pulse or relativistic electron bunch

The results from theoretical and experimental studies, as well as from 2.5-dimensional (2.5-D) numerical simulation of plasma wake field excitation, by either relativistic electron bunch, laser pulse, and the charged particle wake field acceleration are discussed. The results of these investigations make it possible to evaluate the potentialities of the wake field acceleration method and to analyze whether it can serve as a basis for creating a new generation of devices capable of charged particle accelerating at substantially higher (on the order of two to three magnitudes) rates in comparison with those achievable in classical linear high-frequency (resonant) accelerators.

Multiple ions resonant heating and acceleration by Alfvén/cyclotron fluctuations in the corona and the solar wind

We study the interaction between protons, multiple minor ions (O5+, He++), and a given Alfvén/cyclotron fluctuation spectra in the corona and the solar wind. One‐dimensional hybrid simulations are performed in initially homogeneous, collisionless, magnetized plasma with waves propagating parallel to the background magnetic field. The self‐consistent hybrid simulations and linear Vlasov theory are used to study the effect of the driving spectrum, ion drift velocity, and microinstabilities on the coronal plasma and solar wind heating. The simulation results provide a clear picture of wave‐particle interaction and successfully explain (1) how the driving spectrum frequency range and the power law index affect the resonant heating, (2) how ion double‐peak distribution is formed and how differential flows affect the resonant interaction via the change of the resonant wave dispersion relation, and (3) how local microinstabilities affect minor ion heating and constraint the values of plasma parameters observed in the solar wind.

Resonance acceleration of electrons in combined strong magnetic fields and intense laser fields.

The acceleration mechanism of electrons in combined strong axial magnetic fields and circularly polarized laser pulse fields is investigated by solving the dynamical equations for relativistic electrons both numerically and analytically. We find that the electron acceleration depends not only on the laser intensity, but also on the ratio between electron Larmor frequency and laser frequency. As the ratio approaches unity, a clear resonance peak is observed, corresponding to the laser-magnetic resonance acceleration. Away from the resonance regime, the strong magnetic fields still affect the electron acceleration dramatically. We derive an approximate analytical solution of the relativistic electron energy in adiabatic limit, which provides a full understanding of this phenomenon. Application of our theory to fast ignition of inertial confinement fusion is discussed.

Mechanism of resonant electron acceleration at half gyrofrequency in non-neutral plasmas

The experiments studying the electron heating in a magnetic trap by an incident wave at half gyrofrequency showed that there is an essential increase in electron energy and the intensity of the produced x-rays compared to the acceleration at gyrofrequency. The estimates of the influence of various non-linearities do not allow for a satisfactory explanation of this phenomenon. Proposed in this paper is the interpretation of the experimental results in the framework of a non-neutral plasma model. We show the existence of a strong resonance of an incident wave at half gyrofrequency in non-neutral plasma created by an excess of electron density. The analysis of confinement of a cylindrical electron layer is carried out both in radial and in axial directions; we have taken into account several factors such as mirror geometry, pulse mode and relativistic influence. We made separate estimates for each mechanism independently. They show that the proposed mechanisms are quite adequate for the explanation of the increase in x-ray energy and intensity.

Resonant acceleration and diffusion of outer zone electrons in an asymmetric geomagnetic field

The outer zone radiation belt consists of energetic electrons drifting in closed orbits encircling the Earth between ∼3 and 7 RE. Electron fluxes in the outer belt show a strong correlation with solar and magnetospheric activity, generally increasing during geomagnetic storms with associated high solar wind speeds, and increasing in the presence of magnetospheric ULF waves in the Pc‐5 frequency range. In this paper, we examine the influence of Pc‐5 ULF waves on energetic electrons drifting in an asymmetric, compressed dipole and find that such particles may be efficiently accelerated through a drift‐resonant interaction with the waves. We find that the efficiency of this acceleration increases with increasing magnetospheric distortion (such as may be attributed to increased solar wind pressure associated with high solar wind speeds) and with increasing ULF wave activity. A preponderance of ULF power in the dawn and dusk flanks is shown to be consistent with the proposed acceleration mechanism. Under a continuum of wave modes and frequencies, we find that the drift resonant acceleration process leads to additional modes of radial diffusion in the outer belts, with timescales that may be appropriate to those observed during geomagnetic storms.

Nonthermal Emissions from Particles Accelerated by Turbulence in Clusters of Galaxies

We consider nonthermal emission from clusters of galaxies produced by particle acceleration by resonant scattering of Alfv\'{e}n waves driven by fluid turbulence through the Lighthill mechanism in the intracluster medium. We assume that the turbulence is driven by cluster mergers. We find that the resonant Alfv\'{e}n waves can accelerate electrons up to \gamma ~10^5 through resonant scattering. We also find that the turbulent resonant acceleration can give enough energy to electrons to produce the observed diffuse radio relic emission from clusters if the clusters have a pool of electrons with \gamma ~10^3. This mechanism can also explain the observed hard X-ray emission from clusters if the magnetic field in a cluster is small enough (~< \mu G) or the fluid turbulence spectrum is flatter than the Kolmogorov law. The fluid turbulence could be observed with Astro-E2 in the regions where diffuse radio emission is observed. Although non-gravitational heating before cluster formation (preheating) steepens a relation between radio luminosity and X-ray temperature, our predicted relation is still flatter than the observed one.

Induced resonance processes accompanying interaction of the field of a tem trap with oscillating charges

We theoretically analyze resonance processes in an electromagnetic trap (TEM trap) formed by a circularly polarized high-frequency standing field of homogeneous plane waves and a uniform static magnetic field aligned with the direction of wave propagation. The regime of resonance amplification of the trap field by an ensemble of initially nonphased oscillators in the absence of a static magnetic field is described. The regime of resonance acceleration of charges from thermal to relativistic velocities for a bounded particle motion in the presence of a static magnetic field is considered. It is shown that charge oscillations in the trap are similar to flutter in mechanical systems. The efficient energy exchange is stipulated by an M-type interaction mechanism.

Particle Acceleration by a Time-Varying Electric Field in Merging Magnetic Fields

Traditional models for particle acceleration by magnetic reconnection in solar flares assumed a constant electric field in a steady reconnecting magnetic field. Although this assumption may be justified during the gradual phase of flares, the situation is different during the impulsive phase. Observed rapid variations in flare emissions imply that reconnection is non-steady and a time-varying electric field is present in a reconnecting current sheet. This paper describes exploratory calculations of charged particle orbits in an oscillating electric field present either at a neutral plane or a neutral line of two-dimensional magnetic field. A simple analytical model makes it possible to explain the effects of particle trapping and resonant acceleration previously noted by Petkaki and MacKinnon in a numerical simulation. As an application, electron acceleration to X-ray generating energies in impulsive solar flares is discussed within the context of the model.

Resonant heating and acceleration of ions in coronal holes driven by cyclotron resonant spectra

Recent observations and models suggest that the resonant absorption of ion cyclotron waves heats and accelerates the ions in the solar wind. Velocity distributions of minor ions derived from SOHO Ultraviolet Cronagraph Spectrometer (UVCS) observations in coronal holes indicate that the minor ion temperature anisotropy is T⟂/T∣∣ &gt; 10 and that outflow speeds are higher than those of the solar wind protons. Here one‐dimensional hybrid simulations of initially homogeneous, collisionless plasmas are used to study a model of coronal plasmas including kinetic protons, a tenuous component of oxygen ions, and massless fluid electrons. Spectra of ion cyclotron resonant Alfvén waves are imposed on the system to study the resultant heating of both ion species. We investigate the effects of various power spectra of the form f−1 or f−5/3 and vary the input frequency range. We find that the ion heating strongly depends on the power contained in the frequency range of the input spectrum that can resonate with the ions. The minor O5+ ions are easily heated and become anisotropic due to various forms of the spectra. The protons remain nearly isotropic and are weakly heated in most cases in this study. We investigate the self‐consistent fluctuation spectrum generated by the response of the ions and the non‐Maxwellian features in the velocity distribution.

Simulation of electron postacceleration in a two-stage laser wakefield accelerator

Electron bunches produced in self-modulated laser wakefield experiments usually have a broad energy distribution, with most electrons at low energy (1--3 MeV) and only a small fraction at high energy. We propose and investigate further acceleration of such bunches in a channel-guided resonant laser wakefield accelerator. Two-dimensional simulations with and without the effects of self-consistent beam loading are performed and compared. These results indicate that it is possible to trap about $40%$ of the injected bunch charge and accelerate this fraction to an average energy of about 50 MeV in a plasma channel of a few mm.