Fermi Acceleration Research Articles

ГАММА-ВИПРОМІНЮВАННЯ ТЕВНОГО ДІАПАЗОНУ ВІД ЗАЛИШКУ НАДНОВОЇ ВІТРИЛА

Sources and mechanisms of acceleration of the cosmic rays (CR) – a stream of nuclei, electrons and, to a lesser extent, antiparticles with a nonthermal power law energy spectrum extending to energies above 1020 eV, are still unknown. Theoretical calculations and experimental data show that the diffusive acceleration of the CR at the shock wave fronts (first order Fermi mechanism) in Galactic sources, first of all in the Supernova remnants (SNR), may be responsible for the observed CR flux with energies up to E ≤ 1018 eV. In this case, the lepton component of the CR (electrons, positrons) is confidently manifested in the SNR due to synchrotron radiation in a turbulent magnetic field inside the SNR. At the same time, the proton and nuclear component is not effective in synchrotron emission under typical CR parameters and magnetic fields in the SNR. Its presence in the SNR can be revealed due to the hadron mechanism of gamma-ray generation – the decay of neutral pions generated by inelastic collisions of the CR with particles-targets – atoms of interstellar medium inside and in the vicinity of SNR. Now only a few SNRs with a detectable gamma-ray flux of a hadron origin are known, in which the interaction of SNR with molecular clouds is observed, which provides the necessary concentration of particles-targets for CR. In our work we calculate the expected fluxes of such gamma-radiation from the Vela SNR, one of the closest SNR to the Earth, which evolves in a cloudy interstellar medium and is a potential candidate for the hadron mechanism gamma-ray source. The possibility of detecting Vela SNR with modern (Magic, HAWC, H.E.S.S.) and future (CTA) gamma-ray detectors is analyzed. It is shown that the sensitivity of CTA will be sufficient to detect the gamma-radiation generated by the hadron mechanism. Also we analyzed the possibility of registration of localized sources of gamma-radiation within the Vela SNR due to the presence of high gas density clouds. The fluxes from the two most massive clouds within the Vela SNR are calculated and compared with the sensitivity of the modern and future gamma-ray detectors. It is shown that sufficient sensitivity to detect the most massive clouds within the SNR will have the CTA facility if the spectrum of the gamma-rays generated by the hadron mechanism is without cut-off on the energy 10 TeV.

Energetic Electron Acceleration and Injection During Dipolarization Events in Mercury's Magnetotail

AbstractEnergetic particle bursts associated with dipolarization events within Mercury's magnetosphere were first observed by Mariner 10. The events appear analogous to particle injections accompanying dipolarization events at Earth. The Energetic Particle Spectrometer (3 s resolution) aboard MESSENGER determined the particle bursts are composed entirely of electrons with energies ≳ 300 keV. Here we use the Gamma‐Ray Spectrometer high‐time‐resolution (10 ms) energetic electron measurements to examine the relationship between energetic electron injections and magnetic field dipolarization in Mercury's magnetotail. Between March 2013 and April 2015, we identify 2,976 electron burst events within Mercury's magnetotail, 538 of which are closely associated with dipolarization events. These dipolarizations are detected on the basis of their rapid (~2 s) increase in the northward component of the tail magnetic field (ΔBz ~30 nT), which typically persists for ~10 s. Similar to those at Earth, we find that these dipolarizations appear to be low‐entropy, depleted flux tubes convecting planetward following the collapse of the inner magnetotail. We find that electrons experience brief, yet intense, betatron and Fermi acceleration during these dipolarizations, reaching energies ~130 keV and contributing to nightside precipitation. Thermal protons experience only modest betatron acceleration. While only ~25% of energetic electron events in Mercury's magnetotail are directly associated with dipolarization, the remaining events are consistent with the Near‐Mercury Neutral Line model of magnetotail injection and eastward drift about Mercury, finding that electrons may participate in Shabansky‐like closed drifts about the planet. Magnetotail dipolarization may be the dominant source of energetic electron acceleration in Mercury's magnetosphere.

The Chaplygin Sleigh with Parametric Excitation: Chaotic Dynamics and Nonholonomic Acceleration

This paper is concerned with the Chaplygin sleigh with timevarying mass distribution (parametric excitation). The focus is on the case where excitation is induced by a material point that executes periodic oscillations in a direction transverse to the plane of the knife edge of the sleigh. In this case, the problem reduces to investigating a reduced system of two first-order equations with periodic coefficients, which is similar to various nonlinear parametric oscillators. Depending on the parameters in the reduced system, one can observe different types of motion, including those accompanied by strange attractors leading to a chaotic (diffusion) trajectory of the sleigh on the plane. The problem of unbounded acceleration (an analog of Fermi acceleration) of the sleigh is examined in detail. It is shown that such an acceleration arises due to the position of the moving point relative to the line of action of the nonholonomic constraint and the center of mass of the platform. Various special cases of existence of tensor invariants are found.

Electron Pre-acceleration at Nonrelativistic High-Mach-number Perpendicular Shocks

Abstract We perform particle-in-cell simulations of perpendicular nonrelativistic collisionless shocks to study electron heating and pre-acceleration for parameters that permit the extrapolation to the conditions at young supernova remnants. Our high-resolution large-scale numerical experiments sample a representative portion of the shock surface and demonstrate that the efficiency of electron injection is strongly modulated with the phase of the shock reformation. For plasmas with low and moderate temperature (plasma beta and ), we explore the nonlinear shock structure and electron pre-acceleration for various orientations of the large-scale magnetic field with respect to the simulation plane, while keeping it at 90° to the shock normal. Ion reflection off of the shock leads to the formation of magnetic filaments in the shock ramp, resulting from Weibel-type instabilities, and electrostatic Buneman modes in the shock foot. In all of the cases under study, the latter provides first-stage electron energization through the shock-surfing acceleration mechanism. The subsequent energization strongly depends on the field orientation and proceeds through adiabatic or second-order Fermi acceleration processes for configurations with the out-of-plane and in-plane field components, respectively. For strictly out-of-plane field, the fraction of suprathermal electrons is much higher than for other configurations, because only in this case are the Buneman modes fully captured by the 2D simulation grid. Shocks in plasma with moderate provide more efficient pre-acceleration. The relevance of our results to the physics of fully 3D systems is discussed.

Formation of electron energy spectra during magnetic reconnection in laser-produced plasma

Energetic electron spectra formed during magnetic reconnection between two laser-produced plasma bubbles are investigated by the use of two-dimensional particle-in-cell simulations. It is found that the evolution of such an interaction between the two plasma bubbles can be separated into two distinct stages: squeezing and reconnection stages. In the squeezing stage, when the two plasma bubbles expand quickly and collide with each other, the magnetic field in the inflow region is greatly enhanced. In the second stage, a thin current sheet is formed between the two plasma bubbles, and then, magnetic reconnection occurs therein. During the squeezing stage, electrons are heated in the perpendicular direction by betatron acceleration due to the enhancement of the magnetic field around the plasma bubbles. Meanwhile, non-thermal electrons are generated by the Fermi mechanism when these electrons bounce between the two plasma bubbles approaching quickly and get accelerated mainly by the convective electric field associated with the plasma bubbles. During the reconnection stage, electrons get further accelerated mainly by the reconnection electric field in the vicinity of the X line. When the expanding speed of the plasma bubbles is sufficiently large, the formed electron energy spectra have a kappa distribution, where the lower energy part satisfies a Maxwellian function and the higher energy part is a power-law distribution. Moreover, the increase in the expanding speed will result in the hardening of formed power-law spectra in both the squeezing and reconnection stages.

Adiabatic Non-resonant Acceleration in Magnetic Turbulence and Hard Spectra of Gamma-Ray Bursts

Abstract We introduce a non-resonant acceleration mechanism arising from the second adiabatic invariant in magnetic turbulence and apply it to study the prompt emission spectra of gamma-ray bursts (GRBs). The mechanism contains both the first- and second-order Fermi acceleration, originating from the interacting turbulent reconnection and dynamo processes. It leads to a hard electron energy distribution up to a cutoff energy at the balance between the acceleration and synchrotron cooling. The sufficient acceleration rate ensures a rapid hardening of any initial energy distribution to a power-law distribution with the index , which naturally produces a low-energy photon index via the synchrotron radiation. For typical GRB parameters, the synchrotron emission can extend to a characteristic photon energy on the order of ∼100 keV.

Corotating Magnetic Reconnection Site in Saturn’s Magnetosphere

Using measurements from the Cassini spacecraft in Saturn's magnetosphere, we propose a 3D physical picture of a corotating reconnection site, which can only be driven by an internally generated source. Our results demonstrate that the corotating magnetic reconnection can drive an expansion of the current sheet in Saturn's magnetosphere and, consequently, can produce Fermi acceleration of electrons. This reconnection site lasted for longer than one of Saturn's rotation period. The long-lasting and corotating natures of the magnetic reconnection site at Saturn suggest fundamentally different roles of magnetic reconnection in driving magnetospheric dynamics (e.g., the auroral precipitation) from the Earth. Our corotating reconnection picture could also potentially shed light on the fast rotating magnetized plasma environments in the solar system and beyond.

3D kinetic plasma simulations reveal the Fermi mechanism for magnetic reconnection

Researchers use 3D particle-in-cell simulations to explain how electrons continuously accelerate during magnetic reconnection.

Fermi acceleration of electrons inside foreshock transient cores

AbstractForeshock transients upstream of Earth's bow shock have been recently observed to accelerate electrons to many times their thermal energy. How such acceleration occurs is unknown, however. Using Time History of Events and Macroscale Interactions during Substorms (THEMIS) case studies, we examine a subset of acceleration events (31 of 247 events) in foreshock transients with cores that exhibit gradual electron energy increases accompanied by low background magnetic field strength and large‐amplitude magnetic fluctuations. Using the evolution of electron distributions and the energy increase rates at multiple spacecraft, we suggest that Fermi acceleration between a converging foreshock transient's compressional boundary and the bow shock is responsible for the observed electron acceleration. We then show that a one‐dimensional test particle simulation of an ideal Fermi acceleration model in fluctuating fields prescribed by the observations can reproduce the observed evolution of electron distributions, energy increase rate, and pitch angle isotropy, providing further support for our hypothesis. Thus, Fermi acceleration is likely the principal electron acceleration mechanism in at least this subset of foreshock transient cores.

Electron acceleration in solar-flare magnetic traps: Model properties and their observational confirmations

Using an analytical solution of the kinetic equation, we have investigated the model properties of the coronal and chromospheric hard X-ray sources in the limb flare of July 19, 2012. We calculated the emission spectrum at the flare loop footpoints in the thick-target approximation with a reverse current and showed it to be consistent with the observed one. The spectrum of the coronal source located above the flare loop was calculated in the thin-target approximation. In this case, the slope of the hard X-ray spectrum is reproduced very accurately, but the intensity of the coronal emission is lower than the observed one by several times. Previously, we showed that this contradiction is completely removed if the additional (relative to the primary acceleration in the reconnecting current layer) electron acceleration in the coronal magnetic trap that contracts in the transverse direction and decreases in length during the impulsive flare phase is taken into account. In this paper we study in detail this effect in the context of a more realistic flare scenario, where a whole ensemble of traps existed in the hard X-ray burst time, each of which was at different stages of its evolution: formation, collapse, destruction. Our results point not only to the existence of first-order Fermi acceleration and betatron electron heating in solar flares but also to their high efficiency. Highly accurate observations of a specific flare are used as an example to show that the previously predicted theoretical features of the model find convincing confirmations.

Astrophysical particle acceleration mechanisms in colliding magnetized laser-produced plasmas

Significant particle energization is observed to occur in numerous astrophysical environments, and in the standard models, this acceleration occurs alongside energy conversion processes including collisionless shocks or magnetic reconnection. Recent platforms for laboratory experiments using magnetized laser-produced plasmas have opened opportunities to study these particle acceleration processes in the laboratory. Through fully kinetic particle-in-cell simulations, we investigate acceleration mechanisms in experiments with colliding magnetized laser-produced plasmas, with geometry and parameters matched to recent high-Mach number reconnection experiments with externally controlled magnetic fields. 2-D simulations demonstrate significant particle acceleration with three phases of energization: first, a “direct” Fermi acceleration driven by approaching magnetized plumes; second, x-line acceleration during magnetic reconnection of anti-parallel fields; and finally, an additional Fermi energization of partic...

The role of three-dimensional transport in driving enhanced electron acceleration during magnetic reconnection

Magnetic reconnection is an important driver of energetic particles in many astrophysical phenomena. Using kinetic particle-in-cell simulations, we explore the impact of three-dimensional reconnection dynamics on the efficiency of particle acceleration. In two-dimensional systems, Alfvénic outflows expel energetic electrons into flux ropes where they become trapped and disconnected from acceleration regions. However, in three-dimensional systems these flux ropes develop an axial structure that enables particles to leak out and return to acceleration regions. This requires a finite guide field so that particles may move quickly along the flux rope axis. We show that greatest energetic electron production occurs when the guide field is of the same order as the reconnecting component: large enough to facilitate strong transport, but not so large as to throttle the dominant Fermi mechanism responsible for efficient electron acceleration. This suggests a natural explanation for the envelope of electron acceleration during the impulsive phase of eruptive flares.

An explanation of auroral intensification during the substorm expansion phase

AbstractA multiple auroral onset substorm on 28 March 2010 provides an opportunity to understand the physical mechanism in generating auroral intensifications during a substorm expansion phase. Conjugate observations of magnetic fields and plasma from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, of field‐aligned currents (FACs) from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellites, and from ground‐based magnetometers and aurora are all available. The comprehensive measurements allow us to further our understanding of the complicated causalities among dipolarization, FAC generation, particle acceleration, and auroral intensification. During the substorm expansion phase, the plasma sheet expanded and was perturbed leading to the generation of a slow mode wave, which modulated electron flux in the outer plasma sheet. During this current sheet expansion, field‐aligned currents formed, and geomagnetic perturbations were simultaneously detected by ground‐based instruments. However, a magnetic dipolarization did not occur until about 3 min later in the outer plasma sheet observed by THEMIS‐A spacecraft (THA). We believe that this dipolarization led to an efficient Fermi acceleration to electrons and consequently the cause of a significant auroral intensification during the expansion phase as observed by the All‐Sky Imagers (ASIs). This Fermi acceleration mechanism operating efficiently in the outer plasma sheet during the expansion phase could be a common explanation of the poleward auroral development after substorm onset. These results also show a good agreement between the upward FAC derived from AMPERE measurements and the auroral brightening observed by the ASIs.

Explaining the rolling‐pin distribution of suprathermal electrons behind dipolarization fronts

AbstractThe rolling‐pin distribution of suprathermal electrons (40–200 keV), showing electron pitch angles primarily at 0°, 90°, and 180°, has recently been reported behind dipolarization fronts (DFs) both in observations and simulations. The formation of such type of distribution, however, has been unclear so far. In this study, we present an observation of such type of distribution by Cluster in the magnetotail behind a DF. We interpret the formation of such distribution using the global‐scale Fermi acceleration together with local‐scale betatron acceleration. We quantitatively reproduce these two processes and therefore the rolling‐pin distribution of suprathermal electrons using an analytical model. We further reveal that only at energies higher than 26 keV can such distribution be formed. This study, quantitatively explaining the formation of rolling‐pin distribution, can improve the understanding of electron dynamics behind DFs.

Fermi acceleration along the orbit of η Carinae

The {\eta} Carinae binary system hosts the most massive stars with the highest known mass-loss rate. Its dense wind encounters the faster wind expelled by the companion, dissipating mechanical energy in the shock, accelerating particles up to relativistic energies and producing high-energy (HE) {\gamma}-rays. We used the first 7-year data of the Fermi LAT which span two passages of {\eta} Carinae at periastron. We extracted low and HE light curves and spectra in different orbital phase bins using the new PASS8 pipeline. We used particle acceleration in hydrodynamic simulations of the system in a multi-cell geometry and compared the prediction with the observations. The {\gamma}-ray emission location is compatible with {\eta} Carinae. Two emission components are distinguished. The low-energy (LE) one cuts off below 10 GeV and its flux, modulated by the orbital motion, varies by a factor < 2. Short-term variability occurs at periastron. The HE component flux varies by a factor 3-4 but differently during the two periastrons. The variabilities observed at LE and HE during the first half of the observations, match the prediction of the simulation, assuming a surface magnetic field of 500 G. The HE component and the thermal X-ray emission were weaker than expected around the second periastron suggesting a modification of the wind density in the inner wind collision region (WCR). Diffuse shock acceleration in the WCR provides a convincing match to the observations and new diagnostic tools to probe the geometry and energetics of the system. Further observations are required to explain the periastron-to-periastron HE variability and to associate it firmly with hadronic origin. {\eta} Carinae is a pevatron at periastron. Its $\nu$ flux can be detected by IceCube after many years of observations. Orbital modulations of the HE component can be distinguished from those of photo absorption by CTA.

Particle Acceleration in Mildly Relativistic Shearing Flows: The Interplay of Systematic and Stochastic Effects, and the Origin of the Extended High-energy Emission in AGN Jets

Abstract The origin of the extended X-ray emission in the large-scale jets of active galactic nuclei (AGNs) poses challenges to conventional models of acceleration and emission. Although electron synchrotron radiation is considered the most feasible radiation mechanism, the formation of the continuous large-scale X-ray structure remains an open issue. As astrophysical jets are expected to exhibit some turbulence and shearing motion, we here investigate the potential of shearing flows to facilitate an extended acceleration of particles and evaluate its impact on the resultant particle distribution. Our treatment incorporates systematic shear and stochastic second-order Fermi effects. We show that for typical parameters applicable to large-scale AGN jets, stochastic second-order Fermi acceleration, which always accompanies shear particle acceleration, can play an important role in facilitating the whole process of particle energization. We study the time-dependent evolution of the resultant particle distribution in the presence of second-order Fermi acceleration, shear acceleration, and synchrotron losses using a simple Fokker–Planck approach and provide illustrations for the possible emergence of a complex (multicomponent) particle energy distribution with different spectral branches. We present examples for typical parameters applicable to large-scale AGN jets, indicating the relevance of the underlying processes for understanding the extended X-ray emission and the origin of ultrahigh-energy cosmic rays.

Impact of compressibility and a guide field on Fermi acceleration during magnetic island coalescence

Previous work has shown that Fermi acceleration can be an effective heating mechanism during magnetic island coalescence, where electrons may undergo repeated reflections as the magnetic field lines contract. This energization has the potential to account for the power-law distributions of particle energy inferred from observations of solar flares. Here, we develop a generalized framework for the analysis of Fermi acceleration that can incorporate the effects of compressibility and non-uniformity along field lines, which have commonly been neglected in previous treatments of the problem. Applying this framework to the simplified case of the uniform flux tube allows us to find both the power-law scaling of the distribution function and the rate at which the power-law behavior develops. We find that a guide magnetic field of order unity effectively suppresses the development of power-law distributions.

Electron acceleration in a secondary magnetic island formed during magnetic reconnection with a guide field

Secondary magnetic islands may be generated in the vicinity of an X line during magnetic reconnection. In this paper, by performing two-dimensional (2-D) particle-in-cell simulations, we investigate the role of a secondary magnetic island in electron acceleration during magnetic reconnection with a guide field. The electron motions are found to be adiabatic, and we analyze the contributions of the parallel electric field and Fermi and betatron mechanisms to electron acceleration in the secondary island during the evolution of magnetic reconnection. When the secondary island is formed, electrons are accelerated by the parallel electric field due to the existence of the reconnection electric field in the electron current sheet. Electrons can be accelerated by both the parallel electric field and Fermi mechanism when the secondary island begins to merge with the primary magnetic island, which is formed simultaneously with the appearance of X lines. With the increase in the guide field, the contributions of the Fermi mechanism to electron acceleration become less and less important. When the guide field is sufficiently large, the contribution of the Fermi mechanism is almost negligible.

ALMA polarization observations of the particle accelerators in the hotspot of the radio galaxy 3C 445

Abstract We present Atacama Large Millimeter Array polarization observations at 97.5 GHz of the southern hotspot of the radio galaxy 3C 445. The hotspot structure is dominated by two bright components enshrouded by diffuse emission. Both components show fractional polarization between 30 and 40 per cent, suggesting the presence of shocks. The polarized emission of the western component has a displacement of about 0.5 kpc outward with respect to the total intensity emission and may trace the surface of a front shock. Strong polarization is observed in a thin strip marking the ridge of the hotspot structure visible from radio to optical. No significant polarization is detected in the diffuse emission between the main components, suggesting a highly disordered magnetic field likely produced by turbulence and instabilities in the downstream region that may be at the origin of the extended optical emission observed in this hotspot. The polarization properties support a scenario in which a combination of both multiple and intermittent shock fronts due to jet dithering and spatially distributed stochastic second-order Fermi acceleration processes are present in the hotspot complex.

Electron Acceleration at Pulsar Wind Termination Shocks

Abstract We study the acceleration of electrons and positrons at an electromagnetically modified, ultrarelativistic shock in the context of pulsar wind nebulae. We simulate the outflow produced by an obliquely rotating pulsar in proximity of its termination shock with a two-fluid code that uses a magnetic shear wave to mimic the properties of the wind. We integrate electron trajectories in the test-particle limit in the resulting background electromagnetic fields to analyze the injection mechanism. We find that the shock-precursor structure energizes and reflects a sizable fraction of particles, which becomes available for further acceleration. We investigate the subsequent first-order Fermi process sustained by small-scale magnetic fluctuations with a Monte Carlo code. We find that the acceleration proceeds in two distinct regimes: when the gyroradius exceeds the wavelength of the shear λ, the process is remarkably similar to first-order Fermi acceleration at relativistic, parallel shocks. This regime corresponds to a low-density wind that allows the propagation of superluminal waves. When which corresponds to the scenario of driven reconnection, the spectrum is softer.