Electrons In Jet Research Articles

Probing the Magnetic Field Structure in on Black Hole Horizon Scales with Polarized Radiative Transfer Simulations

Abstract Magnetic fields are believed to drive accretion and relativistic jets in black hole accretion systems, but the magnetic field structure that controls these phenomena remains uncertain. We perform general relativistic (GR) polarized radiative transfer of time-dependent three-dimensional GR magnetohydrodynamical simulations to model thermal synchrotron emission from the Galactic Center source Sagittarius A* (Sgr A*). We compare our results to new polarimetry measurements by the Event Horizon Telescope (EHT) and show how polarization in the visibility (Fourier) domain distinguishes and constrains accretion flow models with different magnetic field structures. These include models with small-scale fields in disks driven by the magnetorotational instability as well as models with large-scale ordered fields in magnetically arrested disks. We also consider different electron temperature and jet mass-loading prescriptions that control the brightness of the disk, funnel-wall jet, and Blandford–Znajek-driven funnel jet. Our comparisons between the simulations and observations favor models with ordered magnetic fields near the black hole event horizon in Sgr A*, though both disk- and jet-dominated emission can satisfactorily explain most of the current EHT data. We also discuss how the black hole shadow can be filled-in by jet emission or mimicked by the absence of funnel jet emission. We show that stronger model constraints should be possible with upcoming circular polarization and higher frequency (349 GHz) measurements.

Improving LHC searches for dark photons using lepton-jet substructure

Collider signals of dark photons are an exciting probe for new gauge forces and are characterized by events with boosted lepton jets. Existing techniques are efficient in searching for muonic lepton jets but due to substantial backgrounds have difficulty constraining lepton jets containing only electrons. This is unfortunate since upcoming intensity frontier experiments are sensitive to dark photon masses which only allow electron decays. Analyzing a recently proposed model of kinetic mixing, with new scalar particles decaying into dark photons, we find that existing techniques for electron jets can be substantially improved. We show that using lepton-jet-substructure variables, in association with a boosted decision tree, improves background rejection, significantly increasing the LHC's reach for dark photons in this region of parameter space.

Magnetospheric Multiscale mission observations of the outer electron diffusion region

AbstractThis paper presents Magnetospheric Multiscale mission (MMS) observations of the exhaust region in the vicinity of the central reconnection site in Earth's magnetopause current sheet. High‐time‐resolution measurements of field and particle distributions enable us to explore the fine structure of the diffusion region near the X line. Ions are decoupled from the magnetic field throughout the entire current sheet crossing. Electron jets flow downstream from the X line at speeds greater than the E × B drift velocity. At/around the magnetospheric separatrix, large‐amplitude electric fields containing field‐aligned components accelerate electrons along the magnetic field toward the X line. Near the neutral sheet, crescent‐shaped electron distributions appear coincident with (1) an out‐of‐plane electric field whose polarity is opposite to that of the reconnection electric field and (2) the energy transfer from bulk kinetic to field energy. The observations indicate that MMS passed through the edge of an elongated electron diffusion region (EDR) or the outer EDR in the exhaust region.

Measurement of the mathrm{t}overline{mathrm{t}} production cross section using events in the mathrm {e}mu final state in pp collisions at sqrt{s}=13,text {TeV}

The cross section of top quark–antiquark pair production in proton–proton collisions at {sqrt{s}} = 13,text{TeV} is measured by the CMS experiment at the LHC, using data corresponding to an integrated luminosity of 2.2,text{fb}^{-1}. The measurement is performed by analyzing events in which the final state includes one electron, one muon, and two or more jets, at least one of which is identified as originating from hadronization of a b quark. The measured cross section is 815 pm 9,text{(stat)} pm 38,text{(syst)} pm 19,text{(lumi) pb} , in agreement with the expectation from the standard model.

Sub-picosecond snapshots of fast electrons from high intensity laser-matter interactions

The interaction of a high-intensity short-pulse laser with thin solid targets produces electron jets that escape the target and positively charge it, leading to the formation of the electrostatic potential that in turn governs the ion acceleration. The typical timescale of such phenomena is on the sub-picosecond level. Here we show, for the first time, temporally-resolved measurements of the first released electrons that escaped from the target, so-called fast electrons. Their total charge, energy and temporal profile are provided by means of a diagnostics based on Electro-Optical Sampling with temporal resolution below 100 fs.

Particle dynamics in the electron current layer in collisionless magnetic reconnection

Particle dynamics in the electron current layer in collisionless magnetic reconnection is investigated by using a particle-in-cell simulation. The electron motion and velocity distribution functions are studied by tracking self-consistent trajectories. New classes of electron orbits are discovered: figure-eight-shaped regular orbits inside the electron jet, noncrossing regular orbits on the jet flanks, noncrossing Speiser orbits, and nongyrotropic electrons in the downstream of the jet termination region. The properties of a super-Alfvénic outflow jet are attributed to an ensemble of electrons traveling through the Speiser orbits. The noncrossing orbits are mediated by the polarization electric field near the electron current layer. The noncrossing electrons are found to be non-negligible in number density. The impact of these new orbits to electron mixing, spatial distribution of energetic electrons, and observational signatures is presented.

MMS observations of electron‐scale filamentary currents in the reconnection exhaust and near the X line

AbstractWe report Magnetospheric Multiscale observations of macroscopic and electron‐scale current layers in asymmetric reconnection. By intercomparing plasma, magnetic, and electric field data at multiple crossings of a reconnecting magnetopause on 22 October 2015, when the average interspacecraft separation was ~10 km, we demonstrate that the ion and electron moments are sufficiently accurate to provide reliable current density measurements at 30 ms cadence. These measurements, which resolve current layers narrower than the interspacecraft separation, reveal electron‐scale filamentary Hall currents and electron vorticity within the reconnection exhaust far downstream of the X line and even in the magnetosheath. Slightly downstream of the X line, intense (up to 3 μA/m2) electron currents, a super‐Alfvénic outflowing electron jet, and nongyrotropic crescent shape electron distributions were observed deep inside the ion‐scale magnetopause current sheet and embedded in the ion diffusion region. These characteristics are similar to those attributed to the electron dissipation/diffusion region around the X line.

Electron scale structures and magnetic reconnection signatures in the turbulent magnetosheath

AbstractCollisionless space plasma turbulence can generate reconnecting thin current sheets as suggested by recent results of numerical magnetohydrodynamic simulations. The Magnetospheric Multiscale (MMS) mission provides the first serious opportunity to verify whether small ion‐electron‐scale reconnection, generated by turbulence, resembles the reconnection events frequently observed in the magnetotail or at the magnetopause. Here we investigate field and particle observations obtained by the MMS fleet in the turbulent terrestrial magnetosheath behind quasi‐parallel bow shock geometry. We observe multiple small‐scale current sheets during the event and present a detailed look of one of the detected structures. The emergence of thin current sheets can lead to electron scale structures. Within these structures, we see signatures of ion demagnetization, electron jets, electron heating, and agyrotropy suggesting that MMS spacecraft observe reconnection at these scales.

Electron jet of asymmetric reconnection

AbstractWe present Magnetospheric Multiscale observations of an electron‐scale current sheet and electron outflow jet for asymmetric reconnection with guide field at the subsolar magnetopause. The electron jet observed within the reconnection region has an electron Mach number of 0.35 and is associated with electron agyrotropy. The jet is unstable to an electrostatic instability which generates intense waves with E∥ amplitudes reaching up to 300 mV m−1 and potentials up to 20% of the electron thermal energy. We see evidence of interaction between the waves and the electron beam, leading to quick thermalization of the beam and stabilization of the instability. The wave phase speed is comparable to the ion thermal speed, suggesting that the instability is of Buneman type, and therefore introduces electron‐ion drag and leads to braking of the electron flow. Our observations demonstrate that electrostatic turbulence plays an important role in the electron‐scale physics of asymmetric reconnection.

Particle-in-cell simulations of electron energization in laser-driven magnetic reconnection

Electrons can be energized during laser-driven magnetic reconnection, and the energized electrons form three super-Alfvénic electron jets in the outflow region (Lu et al 2014 New J. Phys. 16 083021). In this paper, by performing two-dimensional particle-in-cell simulations, we find that the electrons can also be significantly energized before magnetic reconnection occurs. When two plasma bubbles with toroidal magnetic fields expand and squeeze each other, the electrons in the magnetic ribbons are energized through betatron acceleration due to the enhancement of the magnetic field, and an electron temperature anisotropy develops. Meanwhile, some electrons are trapped and bounced repeatedly between the two expanding/approaching bubbles and get energized through a Fermi-like process. The energization before magnetic reconnection is more significant (or important) than that during magnetic reconnection.

Time delay and extended halo for constraints on the intergalactic magnetic field

Primary gamma rays emitted from extragalactic very-high-energy (VHE) sources, such as blazars, will generate cascade radiation in intergalactic space with a scale of ∼ 100 Mpc, for z ∼ 0.1 and Eγ ∼ 1TeV. These cascades proceed through electron-positron pair production and inverse Compton (IC) scattering in the cosmic background radiation fields, mainly cosmic microwave background (CMB) radiation and extragalactic background light in the voids of the universe. The existence of an intergalactic magnetic field (IGMF) would deflect paths of electron-positron pairs that scatter CMB photons, causing some observable effects, such as time delay, an extended halo, and a spectral change. Here we reanalyze the diffusion of an electron jet deflected by IGMF and propose a unified semi-analytical model. By using publicly available data from the Fermi/LAT detector and contemporaneous TeV observations, we find that the cascade photon flux is not significantly affected by the IGMF strength for non-variable blazars when the IGMF is weaker than ∼ 10−16 G. This result is clearly different from previous works that analyzed the extended halo and time delay separately for non-variable blazars and flaring blazars. By applying our model to two extreme blazars (1ES 0229+200 and 1ES 1218+304), we obtain the IGMF lower limit of order ≳ 10−13 ∼ 10−14 G in the non-variable case, which is a stronger constraint on the IGMF strength than previous works (≳ 10−16 ∼ 10−18 G), and ≳ 10−18 ∼ 10−19 G in the case of flaring blazars. Furthermore, we study the light curves and extended halo of the cascade photons by considering the effects of the IGMF.

Physical processes of driven magnetic reconnection in collisionless plasmas: Zero guide field case

The key physical processes of the electron and ion dynamics, the structure of the electric and magnetic fields, and how particles gain energy in the driven magnetic reconnection in collisionless plasmas for the zero guide field case are presented. The key kinetic physics is the decoupling of electron and ion dynamics around the magnetic reconnection region, where the magnetic field is reversed and the electron and ion orbits are meandering, and around the separatrix region, where electrons move mainly along the field line and ions move mainly across the field line. The decoupling of the electron and ion dynamics causes charge separation to produce a pair of in-plane bipolar converging electrostatic electric field (E→es) pointing toward the neutral sheet in the magnetic field reversal region and the monopolar E→es around the separatrix region. A pair of electron jets emanating from the reconnection current layer generate the quadrupole out-of-plane magnetic field, which causes the parallel electric field (E→||) from E→ind to accelerate particles along the magnetic field. We explain the electron and ion dynamics and their velocity distributions and flow structures during the time-dependent driven reconnection as they move from the upstream to the downstream. In particular, we address the following key physics issues: (1) the decoupling of electron and ion dynamics due to meandering orbits around the field reversal region and the generation of a pair of converging bipolar electrostatic electric field (E→es) around the reconnection region; (2) the slowdown of electron and ion inflow velocities due to acceleration/deceleration of electrons and ions by E→es as they move across the neutral sheet; (3) how the reconnection current layer is enhanced and how the orbit meandering particles are accelerated inside the reconnection region by E→ind; (4) why the electron outflow velocity from the reconnection region reaches super-Alfvenic speed and the ion outflow velocity reaches Alfvenic speed; (5) how the quadrupole magnetic field is produced and how E→|| is produced; (6) how electrons and ions are accelerated by E→|| around the separatrix region; (7) why electrons have a flat-top parallel velocity distribution in the upstream just outside the reconnection region as observed in the magnetotail; (8) how electron and ion dynamics decouple and how the monopolar electrostatic electric field is produced around the separatrix region; (9) how ions gain energy as they move across the separatrix region into the downstream and how the ion velocity distribution is thermalized in the far downstream; and (10) how electrons move across the separatrix region and in the downstream and how the electron velocity distribution is thermalized in the far downstream. Finally, the main energy source for driving magnetic reconnection and particle acceleration/heating is the inductive electric field, which accelerates both electrons and ions around the reconnection current layer and separatrix regions.

Slowing of Magnetic Reconnection Concurrent with Weakening Plasma Inflows and Increasing Collisionality in Strongly Driven Laser-Plasma Experiments.

An evolution of magnetic reconnection behavior, from fast jets to the slowing of reconnection and the establishment of a stable current sheet, has been observed in strongly driven, β≲20 laser-produced plasma experiments. This process has been inferred to occur alongside a slowing of plasma inflows carrying the oppositely directed magnetic fields as well as the evolution of plasma conditions from collisionless to collisional. High-resolution proton radiography has revealed unprecedented detail of the forced interaction of magnetic fields and super-Alfvénic electron jets (V_{jet}∼20V_{A}) ejected from the reconnection region, indicating that two-fluid or collisionless magnetic reconnection occurs early in time. The absence of jets and the persistence of strong, stable magnetic fields at late times indicates that the reconnection process slows down, while plasma flows stagnate and plasma conditions evolve to a cooler, denser, more collisional state. These results demonstrate that powerful initial plasma flows are not sufficient to force a complete reconnection of magnetic fields, even in the strongly driven regime.

Collisionless absorption, hot electron generation, and energy scaling in intense laser-target interaction

Among the various attempts to understand collisionless absorption of intense and superintense ultrashort laser pulses, a whole variety of models and hypotheses has been invented to describe the laser beam target interaction. In terms of basic physics, collisionless absorption is understood now as the interplay of the oscillating laser field with the space charge field produced by it in the plasma. A first approach to this idea is realized in Brunel's model the essence of which consists in the formation of an oscillating charge cloud in the vacuum in front of the target, therefore frequently addressed by the vague term “vacuum heating.” The investigation of statistical ensembles of orbits shows that the absorption process is localized at the ion-vacuum interface and in the skin layer: Single electrons enter into resonance with the laser field thereby undergoing a phase shift which causes orbit crossing and braking of Brunel's laminar flow. This anharmonic resonance acts like an attractor for the electrons and leads to the formation of a Maxwellian tail in the electron energy spectrum. Most remarkable results of our investigations are the Brunel like spectral hot electron distribution at the relativistic threshold, the minimum of absorption at Iλ2≅(0.3−1.2)×1021 Wcm−2μm2 in the plasma target with the electron density of neλ2∼1023cm−3μm2, the drastic reduction of the number of hot electrons in this domain and their reappearance in the highly relativistic domain, and strong coupling, beyond expectation, of the fast electron jets with the return current through Cherenkov emission of plasmons. The hot electron energy scaling shows a strong dependence on intensity in the moderately relativistic domain Iλ2≅(1018−1020) Wcm−2μm2, a scaling in vague accordance with current published estimates in the range Iλ2≅(0.14−3.5)×1021 Wcm−2μm2, and again a distinct power increase beyond I=3.5×1021 Wcm−2μm2. The low energy electrons penetrate normally to the target surface, the energetic electrons propagate in laser beam direction.

High-energy-density electron jet generation from an opening gold cone filled with near-critical-density plasma

By using two-dimensional particle-in-cell simulations, we propose a scheme for strong coupling of a petawatt laser with an opening gold cone filled with near-critical-density plasmas. When relevant parameters are properly chosen, most laser energy can be fully deposited inside the cone with only 10% leaving the tip opening. Due to the asymmetric ponderomotive acceleration by the strongly decayed laser pulse, high-energy-density electrons with net laser energy gain are accumulated inside the cone, which then stream out of the tip opening continuously, like a jet. The jet electrons are fully relativistic, with speeds around 0.98−0.998 c and densities at 1020/cm3 level. The jet can keep for a long time over 200 fs, which may have diverse applications in practice.

S0510303 Couette流れと干渉する衝突噴流の熱伝達に関する研究

Impinging jet is widely used in industrial fields, such as thermal management of electronic devices or jet washing. In addition, we can see many both of numerical and experimental studies on interfering flows between mainstream or Hagen-Poiseuille and impinging jet. These kinds of flows are well appearing at T-shape merging pipe flows or film cooling on turbine blades. However, there are few studies on heat transfer field interfering Couette flow and impinging jet, in spite of typical flow such as cooling rotary inner-cylinder of bearing, cooling rolled steel in its production process and carrier gas flow of Additive Manufacturing (A.M.). Especially for industrial using of A.M., it becomes a huge obstacle due to its much complicated multi-physical mechanism between the moving grinding table and the jet including carrier gas and additive particle. Therefore, we have studied numerically on the heat transfer of impinging jet interfering with Couette flow, and compared with several different jet inlet angles. In this study, we used DNS which employing secondary conservative second order central difference scheme for convective terms of Navier-Stokes equations and heat transport equation to obtain unsteady flow and temperature scalar fields. Then, we confirmed this DNS results agree qualitatively with those of Quasi-DNS which had been done in our former studies.

Ultrashort Pulsed Neutron Source

We report on a novel compact laser-driven neutron source with an unprecedented short pulse duration (<50 ps) and high peak flux (>10(18) n/cm(2)/s), an order of magnitude higher than any existing source. In our experiments, high-energy electron jets are generated from thin (<3 μm) plastic targets irradiated by a petawatt laser. These intense electron beams are employed to generate neutrons from a metal converter. Our method opens venues for enhancing neutron radiography contrast and for creating astrophysical conditions of heavy element synthesis in the laboratory.

Formation of super-Alfvénic electron jets during laser-driven magnetic reconnection at the Shenguang-II facility: particle-in-cell simulations

Magnetic reconnection experiments in high-energy-density (HED) laser-produced plasmas have recently been conducted at the Shenguang-II (SG-II) facility. Two plasma bubbles and a ‘frozen-in’ magnetic field are generated by irradiating an Al foil using two laser beams. As the two bubbles with opposing magnetic fields expand and squeeze each other, magnetic reconnection occurs. In the experiments, three well-collimated high-speed electron jets are observed in the fanlike outflow region of the laser-driven magnetic reconnection. Based on two-dimensional (2D) particle-in-cell (PIC) simulations, we demonstrate that the three electron jets in the outflow region of laser-driven magnetic reconnection are super-Alfvénic, and their formation mechanism is also revealed in this paper. The two super-Alfvénic jets at the edge are formed by the outflow electrons, which move along magnetic field lines after they are accelerated in the vicinity of the X-line by the reconnection electric field. The super-Alfvénic jet at the center is formed by the electrons that come from the outside of the plasma bubbles. These electrons are reflected by the magnetic field in the pileup region and are meanwhile accelerated by the resulting electric field.

An Element in a Paradigm Shift? – Could Population II Stars Actually be Younger than Population I stars?

Recent astronomical observations have indicated the existence of supermassive black holes at the centres of most, perhaps all, galaxies, and also that immensely powerful relativistic jets of protons and electrons may occasionally emanate from some such supermassive black holes. Together these observations suggest a possible mechanism for alternative star formation, which may require a revision of the traditional classification of stars into a more recent Population I and a more ancient Population II. Rather than assumedly being members of an older generation of stars preceding Population I, maybe many Population II stars could perhaps actually be younger than Population I stars, with their lower metallicity being an effect of their much more recent creation from the pristine hydrogen left by such relativistic jets.

Evidence of deflected super‐Alfvénic electron jet in a reconnection region with weak guide field

AbstractRecent numerical simulations demonstrated that electron diffusion region develops into two‐scale structure, i.e., the inner electron diffusion region and the outer electron diffusion region. The outer diffusion region is manifested as super‐Alfvénic electron jet embedded in central current sheet. However, the electron jets are deflected from neutral sheet with a weak guide field. In this paper we present the in situ evidence of deflected super‐Alfvénic electron jet in a reconnection region with a weak guide field in the Earth's magnetotail. The electron‐scale jet was detected at about 37 ion inertial lengths from the X line. There was a strong electric field at the jet. The strong electric field at the jet was primarily balanced by Hall electric field, as the intense current was mainly carried by magnetized electrons. Another event in the magnetosheath also supports our conclusion that guide field deflects the electron jet away the neutral sheet.