Nonthermal Electrons Research Articles

Radiometry for the vertical electron cyclotron emission from the runaway electrons at the COMPASS tokamak.

Due to an increased interest in runaway electron (RE) phenomena in tokamak research, the need for diagnostics of runaway electron population in plasma has emerged. A novel diagnostic of the nonthermal electron cyclotron emission from runaway electrons can be utilized for this purpose. It was designed and installed at the COMPASS tokamak based on the available heterodyne radiometer. The vertical ECE (V-ECE) system uses a 16-channel heterodyne radiometer with a vertically placed E2-band horn antenna with a 76.5-90 GHz frequency range front-end. Simulations with the ray-tracing SPECE code have shown a measurement feasibility of the runaway electrons with energies up to 1 MeV. Due to a low optical depth of the plasma in COMPASS during RE discharges, reflected waves from the tokamak wall can be detected as well. First results show strong connection with other RE diagnostics at COMPASS. The V-ECE can obtain important information about RE population evolution and primary generation mechanism.

Solar Fast-drifting Radio Bursts in an X1.3 Flare on 2014 April 25

Abstract One of the most important products of solar flares is nonthermal energetic particles, which may carry up to 50% of the energy released in the flaring processes. In radio observations, nonthermal particles generally manifest as spectral fine structures with fast frequency-drifting rates, named as solar fast-drifting radio bursts (FDRBs). This work demonstrated three types of FDRBs, including type III pair bursts, narrowband stochastic spike bursts following the type III bursts, and spike-like bursts superimposed on a type II burst in an X1.3 flare on 2014 April 25. We find that although all of them have fast frequency-drifting rates, they are intrinsically different from each other in frequency bandwidth, drifting rate, and statistical distribution. We suggest that they are possibly generated from different accelerating mechanisms. The type III pair bursts may be triggered by high-energy electron beams accelerated by the flaring magnetic reconnection, spike bursts are produced by the energetic electrons accelerated by a termination shock wave triggered by the fast reconnecting plasma outflows impacting the flaring loop top, and spike-like bursts are possibly generated by nonthermal electrons accelerated by moving magnetic reconnection triggered by interaction between coronal mass ejection and the background magnetized plasma. These results may help us to understand the generation mechanism of nonthermal particles and energy release in solar flares.

Submillimeter Radiation as the Thermal Component of the Neupert Effect

The Neupert effect is the empirical observation that the time evolution of non-thermal emission (e.g. hard X-rays) is frequently proportional to the time derivative of the thermal emission flux (soft X-rays), or, vice versa, that time integrated non-thermal flux is proportional to thermal flux. We analyzed the GOES M2.2 event SOL2011-02-14T17:25, and found that the 212 GHz emission plays quite well the role of the thermal component of the Neupert effect. We show that the maximum of the hard X-ray flux for energies above 50 keV is coincident in time with the time-derivative of the 212 GHz flux, within the uncertainties. The microwave flux density at 15.4 GHz, produced by optically thin gyrosynchrotron mechanism, and hard-X rays above 25 keV mark the typical impulsive phase, and have similar time evolution. On the other hand, the 12 GHz emission is delayed by about 25 seconds with respect of the microwave and hard X-ray peak. We argue that this delay cannot be explained by magnetic trapping of non-thermal electrons. With all the observational evidence, we suggest that the 212 GHz emission is produced by thermal bremsstrahlung, initially in the chromosphere, and shifting to optically thin emission from thehot coronal loops at the end of the gradual phase.

Kinetic Simulations of Nonrelativistic Perpendicular Shocks of Young Supernova Remnants. II. Influence of Shock-surfing Acceleration on Downstream Electron Spectra

Abstract We explore electron preacceleration at high-Mach-number nonrelativistic perpendicular shocks at, e.g., young supernova remnants, which are a prerequisite of further acceleration to very high energies via diffusive shock acceleration. Using fully kinetic particle-in-cell simulations of shocks and electron dynamics in them, we investigate the influence of shock-surfing acceleration (SSA) at the shock foot on the nonthermal population of electrons downstream of the shock. The SSA is followed by further energization at the shock ramp where the Weibel instability spawns a type of second-order Fermi acceleration. The combination of these two processes leads to the formation of a nonthermal electron population, but the importance of SSA becomes smaller for larger ion-to-electron mass ratios in the simulation. We discuss the resulting electron spectra and the relevance of our results to the physics of systems with real ion-to-electron mass ratios and fully three-dimensional behavior.

Resolving accretion flows in nearby active galactic nuclei with the Event Horizon Telescope

ABSTRACTThe Event Horizon Telescope (EHT), now with its first ever image of the photon ring around the supermassive black hole of M87, provides a unique opportunity to probe the physics of supermassive black holes through Very Long Baseline Interferometry (VLBI), such as the existence of the event horizon, the accretion processes as well as jet formation in low-luminosity AGNs (LLAGNs). We build a theoretical model that includes an advection dominated accretion flow (ADAF) with emission from thermal and non-thermal electrons in the flow and a simple radio jet outflow. The predicted spectral energy distribution (SED) of this model is compared to sub-arcsec resolution observations to get the best estimates of the model parameters. The model-predicted radial emission profiles at different frequency bands are used to predict whether the inflow can be resolved by the EHT or with telescopes such as the Global 3-mm VLBI array (GMVA). In this work the model is initially tested with high-resolution SED data of M87 and then applied to our sample of five galaxies (Cen A, M84, NGC 4594, NGC 3998, and NGC 4278). The model then allows us to predict if one can detect and resolve the inflow for any of these galaxies using the EHT or GMVA within an 8 h integration time.

Relativistic electron impact ionization cross sections of carbon ions and application to an optically thin plasma

Context.Ionization through electron impact is a fundamental process associated with the evolution of the ionic structure and emissivity of astrophysical plasmas. Over several decades substantial efforts have been made to measure and calculate the ionization cross sections of ionization through electron impact of different ions shell by shell, in particular, of carbon ions. Spectral emission codes use electron-impact ionization cross sections and/or rates taken from different experimental and theoretical sources. The theoretical cross sections are determined numerically and include a diversity of quantum mechanical methods. The electron-impact ionization database therefore is not uniform in the methods, which makes it hard to determine the reason for the deviations with regard to experimental data. In many cases only total ionization rates for Maxwell–Boltzmann plasmas are available, which makes calculating inner-shell ionization in collisional-radiative models using thermal and nonthermal electron distribution functions difficult. A solution of this problem is the capability of generating the cross sections with an analytical method using the minimum number of atomic parameters. In this way, uniformity in the database is guaranteed, and thus deviations from experiments are easily identified and traced to the root of the method.Aims.The modified relativistic binary encounter Bethe (MRBEB) method is such a simple analytical scheme based on one atomic parameter that allows determining electron-impact ionization cross sections. This work aims the determination of K- and L-shell cross sections of the carbon atom and ions using the MRBEB method and show their quality by: (i) comparing them with those obtained with the general ionization processes in the presence of electrons and radiation (GIPPER) code and the flexible atomic code (FAC), and (ii) determining their effects on the ionic structure and cooling of an optically thin plasma.Methods.The MRBEB method was used to calculate the inner-shells cross sections, while the plasma calculations were carried out with the collisional+photo ionization plasma emission software (CPIPES). The mathematical methods used in this work comprise a modified version of the double-exponential over a semi-finite interval method for numerical integrations, Gauss-elimination method with scaled partial pivoting for the solution of systems of linear equations, and an iterative least-squares method to determine the fits of ionization cross sections.Results.The three sets of cross sections show deviations among each other in different energy regions. The largest deviations occur near and in the peak maximum. Ion fractions and plasma emissivities of an optically thin plasma that evolves under collisional ionization equilibrium, derived using each set of cross sections, show deviations that decrease with increase in temperature and ionization degree. In spite of these differences, the calculations using the three sets of cross sections agree overall.Conclusions.A simple model like the MRBEB is capable of providing cross sections similar to those calculated with more sophisticated quantum mechanical methods in the GIPPER and FAC codes.

Three-dimensional modeling of chromospheric spectral lines in a simulated active region

Context. Because of the complex physics that governs the formation of chromospheric lines, interpretation of solar chromospheric observations is difficult. The origin and characteristics of many chromospheric features are, because of this, unresolved. Aims. We focus on studying two prominent features: long fibrils and flare ribbons. To model these features, we use a 3D magnetohydrodynamic simulation of an active region, which self-consistently reproduces both of these features. Methods. We modeled the Hα, Mg II k, Ca II K, and Ca II 8542 Å lines using the 3D non-LTE radiative transfer code Multi3D. To obtain non-LTE electron densities, we solved the statistical equilibrium equations for hydrogen simultaneously with the charge conservation equation. We treated the Ca II K and Mg II k lines with partially coherent scattering. Results. This simulation reproduces long fibrils that span between the opposite-polarity sunspots and go up to 4 Mm in height. They can be traced in all lines owing to density corrugation. In contrast to previous studies, Hα, Mg II h&k, and Ca II H&K are formed at similar height in this model. Although some of the high fibrils are also visible in the Ca II 8542 Å line, this line tends to sample loops and shocks lower in the chromosphere. Magnetic field lines are aligned with the Hα fibrils, but the latter holds to a lesser extent for the Ca II 8542 Å line. The simulation shows structures in the Hα line core that look like flare ribbons. The emission in the ribbons is caused by a dense chromosphere and a transition region at high column mass. The ribbons are visible in all chromospheric lines, but least prominent in Ca II 8542 Å line. In some pixels, broad asymmetric profiles with a single emission peak are produced similar to the profiles observed in flare ribbons. They are caused by a deep onset of the chromospheric temperature rise and large velocity gradients. Conclusions. The simulation produces long fibrils similar to what is seen in observations. It also produces structures similar to flare ribbons despite the lack of nonthermal electrons in the simulation. The latter suggests that thermal conduction might be a significant agent in transporting flare energy to the chromosphere in addition to nonthermal electrons.

Modulation instability of obliquely propagating ion acoustic waves in a collisionless magnetized plasma consisting of nonthermal and isothermal electrons

We have studied the modulation instability of obliquely propagating ion acoustic waves in a collisionless magnetized warm plasma consisting of warm adiabatic ions and two different species of electrons at different temperatures. We have derived a nonlinear Schr{\"o}dinger equation using the standard reductive perturbation method to describe the nonlinear amplitude modulation of ion acoustic wave satisfying the dispersion relation of ion acoustic wave propagating at an arbitrary angle to the direction of the external uniform static magnetic field. We have investigated the correspondence between two nonlinear Schr{\"o}dinger equations $-$ one describes the amplitude modulation of ion acoustic waves propagating along any arbitrary direction to the direction of the magnetic field and other describes the amplitude modulation of ion acoustic waves propagating along the direction of the magnetic field. We have derived the instability condition and the maximum growth rate of instability of the modulated ion acoustic wave. We have seen that the region of existence of maximum growth rate of instability decreases with increasing values of the magnetic field intensity whereas the region of existence of the maximum growth rate of instability increases with increasing $\cos \theta$, where $\theta$ is the angle of propagation of the ion acoustic wave with the external uniform static magnetic field. Again, the maximum growth rate of instability increases with increasing $\cos \theta$ and also this maximum growth rate of instability increases with increasing $\beta_{e}$ upto a critical value of the wave number, where $\beta_{e}$ is the parameter associated with the nonthermal distribution of hotter electron species.

Bifurcation of ion-acoustic superperiodic waves in auroral zone of Earth’s magnetosphere

Ion-acoustic (IA) superperiodic waves are explored in an unmagnetized plasma composing of mobile ions, Maxwell distributed cold electrons and non-thermal hot electrons. Implementing the phase portrait concept of the Hamiltonian systems, all distinct phase plots containing superperiodic and superhomoclinic trajectories are displayed for distinct data values of the physical parameters, such as, nonthermal parameter ($\beta $), number density ratio ($\mu $) of cold electrons and ions at equilibrium, temperature ratio ($\sigma $) of cold electrons to hot electrons and of traveling wave velocity ($v$). The IA superperiodic wave features are examined for various conditions of physical parameters by symbolic computations. The outcomes of this study may be beneficial in discerning IA superperiodic wave in auroral zone of the Earth’s magnetosphere.

Electron trapping in the coma of a weakly outgassing comet

Measurements from the Rosetta mission have shown a multitude of nonthermal electron distributions in the cometary environment, challenging the previously assumed plasma interaction mechanisms near a cometary nucleus. In this paper, we discuss electron trapping near a weakly outgassing comet from a fully kinetic (particle-in-cell) perspective. Using the electromagnetic fields derived from the simulation, we characterize the trajectories of trapped electrons in the potential well surrounding the cometary nucleus and identify the distinguishing features in their respective velocity and pitch angle distributions. Our analysis allows us to define a clear boundary in velocity phase space between the distributions of trapped and passing electrons.

Secondary CMB anisotropies from magnetized haloes

Magnetized plasmas within haloes of galaxies leave their footprint on the polarized anisotropies of the cosmic microwave background. The two dominant effects of astrophysical haloes are Faraday rotation, which generates rotation of the plane of linear polarization, and Faraday conversion, which induces a leakage from linear polarization to circular polarization. We revisit these sources of secondary anisotropies by computing the angular power spectra of the Faraday rotation angle and the Faraday conversion rate by the large-scale structures. To this end, we use the halo model and we pay special attention to the impact of magnetic field projections. Assuming magnetic fields of haloes to be uncorrelated, we found a vanishing two-halo term, and angular power spectra peaking at multipoles ℓ ∼ 104. The Faraday rotation angle is dominated by the contribution of thermal electrons. For the Faraday conversion rate, we found that both thermal electrons and relativistic, non-thermal electrons contribute equally in the most optimistic case for the density and Lorentz factor of relativistic electrons, while in more pessimistic cases the thermal electrons give the dominant contribution. Assuming the magnetic field to be independent of the halo mass, the angular power spectra for both effects roughly scale with the amplitude of matter perturbations as ∼σ38, and with a very mild dependence with the density of cold dark matter. Introducing a dependence of the magnetic field strength with the halo mass leads to an increase of the scaling at large angular scales (above a degree) with the amplitude of matter fluctuations up to ∼σ9.58 for Faraday rotation and ∼σ158 for Faraday conversion for a magnetic field strength scaling linearly with the halo mass. Introducing higher values of the magnetic field for galaxies, as compared to clusters, instead leads to a decrease of such a scaling at arcminute scales down to ∼σ0.98 for Faraday rotation.

Ultrafast electric field controlled spin correlations in the Hubbard model

Highly intense electric field pulses can move the electronic momentum occupation in correlated metals over large portions of the Brillouin zone, leading to phenomena such as dynamic Bloch oscillations. Using the nonequilibrium fluctuation-exchange approximation for the two-dimensional Hubbard model, we study how such nonthermal electron distributions drive collective spin and charge fluctuations. Suitable pulses can induce a highly anisotropic modification of the occupied momenta, and the corresponding spin dynamics results in a transient change from antiferromagnetic to anisotropic ferromagnetic correlations. To good approximation this behavior is understood in terms of an instantaneous response of the spin correlations to the single-particle properties.

MAXI J1820+070 with NuSTAR I. An increase in variability frequency but a stable reflection spectrum: coronal properties and implications for the inner disc in black hole binaries

ABSTRACT MAXI J1820+070 (optical counterpart ASASSN-18ey) is a black hole candidate discovered through its recent very bright outburst. The low extinction column and long duration at high flux allow detailed measurements of the accretion process to be made. In this work, we compare the evolution of X-ray spectral and timing properties through the initial hard state of the outburst. We show that the inner accretion disc, as measured by relativistic reflection, remains steady throughout this period of the outburst. Nevertheless, subtle spectral variability is observed, which is well explained by a change in coronal geometry. However, characteristic features of the temporal variability – low-frequency roll-over and quasi-periodic oscillation frequency – increase drastically in frequency, as the outburst proceeds. This suggests that the variability time-scales are governed by coronal conditions rather than solely by the inner disc radius. We also find a strong correlation between X-ray luminosity and coronal temperature. This can be explained by electron pair production with a changing effective radius and a non-thermal electron fraction of $\sim 20$ per cent.

Energy Partition in Two M-class Circular-ribbon Flares

Abstract In this paper, we investigate the energy partition of two homologous M1.1 circular-ribbon flares (CRFs) in active region (AR) 12434. They were observed by SDO, GOES, and RHESSI on 2015 October 15 and 16, respectively. The peak thermal energy, nonthermal energy of flare-accelerated electrons, total radiative loss of hot plasma, and radiant energies in 1–8 Å and 1–70 Å of the flares are calculated. The two flares have similar energetics. The peak thermal energies are (1.94 ± 0.13) × 1030 erg. The nonthermal energies in flare-accelerated electrons are (3.9 ± 0.7) × 1030 erg. The radiative outputs of the flare loops in 1–70 Å, which are ∼200 times greater than the outputs in 1–8 Å, account for ∼62.5% of the peak thermal energies. The radiative losses of SXR-emitting plasma are one order of magnitude lower than the peak thermal energies. Therefore, the total heating requirements of flare loops including radiative loss are (2.1 ± 0.1) × 1030 erg, which could sufficiently be supplied by nonthermal electrons.

Precursor Wave Amplification by Ion–Electron Coupling through Wakefield in Relativistic Shocks

Abstract We investigated electromagnetic precursor wave emission in relativistic shocks by using two-dimensional particle-in-cell simulations. We found that the wave amplitude is significantly enhanced by a positive feedback process associated with ion–electron coupling through the wakefields for high magnetization. The wakefields collapse during the nonlinear process of the parametric decay instability in the near-upstream region, where nonthermal electrons and ions are generated. The intense coherent emission and the particle acceleration may operate in high-energy astrophysical objects.

Rapid Variability in the SOL2011-08-04 Flare: Implications for Electron Acceleration

Abstract Particle acceleration in solar flares remains an outstanding problem in solar physics. It is currently unclear which of the acceleration mechanisms dominates and how exactly the excessive magnetic energy is transferred to nonthermal and other forms of energy. We emphasize that the ultimate acceleration mechanism must be capable of efficiently working in the most extreme conditions, such as the shortest detected timescales and the highest acceleration efficiency. Here we focus on a detailed multiwavelength analysis of the initial phase of the SOL2011-08-04 flare, which demonstrated prominent short subpeaks of nonthermal emission during filament eruption associated with the flare. We demonstrate that the three-dimensional configuration of the flare, combined with timing and spectral behavior of the rapidly varying component, put very stringent constraints on the acceleration regime. Specifically, the rapid subpeaks are generated by short injections of nonthermal electrons with a reasonably hard, single power-law spectrum and a relatively narrow spread of pitch-angles along the mean magnetic field. The acceleration site is a compact volume located near the top of the extended coronal loop(s). The electrons are promptly accelerated up to several hundreds of keV, with the characteristic acceleration time shorter than 50 ms. We show that these properties are difficult to reconcile with widely adopted stochastic acceleration models, while the data inescapably require acceleration by a super-Dreicer electric field, whether regular or random.

Head-on Collision of Ion-Acoustic Solitary Waves with Two Negative Ion Species in Electron–Positron–Ion Plasmas and Production of Rogue Waves

The interactions between ion acoustic solitary waves (IASWs) are investigated considering completely ionized electron–positron–ion (e–p–i) plasmas consisting of ions with positive and two negative species, nonthermal electrons, and positrons. Two-sided Korteweg–de Vries (KdV) and modified KdV (mKdV) equations are derived using extended Poincare–Lighthill–Kuo (ePLK) reductive perturbation method. To investigate the production of ion-acoustic rogue waves (IARWs), the rational solution of nonlinear Schrodinger equation (NLSE) is derived from the mKdV equation. Two types of plasmas containing $${\text{A}}{{{\text{r}}}^{ + }}$$ , $${{{\text{F}}}^{ - }}$$ , and $${\text{SF}}_{{\text{5}}}^{ - }$$ species, as well as $${\text{SF}}_{{\text{5}}}^{ + }$$ , $${{{\text{F}}}^{ - }}$$ , and $${\text{SF}}_{{\text{5}}}^{ - }$$ species, are taken into account to study their effects on the amplitudes and phase shifts after collision, as well as the production and properties of rogue waves (RWs). It is observed during collision that a high-amplitude wave is produced in the interaction region depending on the type and parameters of plasmas. The nonthermality of electrons and positrons, electron-to-positron temperature ratio, and the density of negative ions modify the phase shift and amplitude of the waves produced during the collision of the two solitons. The amplitude of the RW for the $${\text{A}}{{{\text{r}}}^{ + }}$$ , $${{{\text{F}}}^{ - }}$$ , and $${\text{SF}}_{{\text{5}}}^{ - }$$ plasmas is found to be larger than that for the $${\text{SF}}_{{\text{5}}}^{ + }$$ , $${{{\text{F}}}^{ - }}$$ , and $${\text{SF}}_{{\text{5}}}^{ - }$$ plasmas.

(Invited) Ultrafast Optical Spectroscopy of Plasmonic and Energy Conversion Materials

Ultrafast photoprocesses can play a leading role in determining the efficiency of optically induced catalysis and energy conversion. As a result, a rigorous understanding of the ultrafast radiative and nonradiative processes in relevant materials, such as plasmonic and semiconductor materials, and learning how to amplify the magnitude of the favorable photoprocesses, is important for ultimately improving their performance. For example, in nanostructured plasmonic materials the creation of energetic electrons before thermalization has been proposed for a wide number of applications in optical energy conversion and ultrafast nanophotonics. However, the use of "nonthermal" electrons is primarily limited by both a low generation efficiency and their ultrafast decay. We report experimental and theoretical results on the use of broadband plasmonic nanopatch metasurfaces comprising a gold substrate coupled to silver nanocubes that produce large concentrations of hot electrons, which we measure using transient absorption spectroscopy. The metasurfaces bear resemblance to perfect absorber type structures. We further find evidence for three subpopulations of nonthermal carriers, which we propose are due to localization of energetic carriers near the three band crossings of the Fermi surface. The bimetallic character of the metasurface strongly impacts the physics, with dissipation occurring primarily in the gold whereas the quantum process of hot electron generation takes place in both components. Our calculations show that the choice of geometry and materials is crucial for producing strong ultrafast nonthermal electron components and may help to guide future efforts in this area. Additional ultrafast studies of light harvesting and energy conversion materials are described, particularly in using assemblies of chromophore-functionalized biomolecules and also in perovskite materials. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, and supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.

(Invited) Tunable and Efficient Hot Carrier Generation from Plasmonic Au-Pd Nanoalloy Photocatalyst

Noble metals are the most studied plasmonic materials due to their chemical stability, small ohmic losses and high DC conductivity. However, the majority of electronic states in the valence band of these metals require at least 2 eV to excite. Therefore, even Au, which is often used for near-infrared hot carrier applications, has a band structure that is not ideal for this low energy wavelength regime. One possible route to improving the efficiency of hot carrier generation in noble metals at near-infrared wavelengths, is to shift their electronic density of states (EDOS) closer to the Fermi level via alloying with a transition metal. Here, we present the tuning of optical and electronic properties of co-evaporated Au-Pd alloy thin films at different compositions by taking ultraviolet photoelectron spectroscopy (UPS) measurements to represent the EDOS, ellipsometry measurements to show the modulated dielectric function and the calculated plasmonic quality factors, and infrared-pump/THz-probe spectroscopy measurements for studying the dynamics of the hot carrier generation in the near-infrared regime. For comparison, calculated EDOS and hot carrier distributions for these alloys are also shown. Furthermore, we characterized the alloy thin film structural properties by grazing incidence x-ray diffraction (GIXRD). Our GIRXD results show that all Au-Pd alloy thin films are true alloys (single phase). UPS data supports our initial hypothesis that adding Pd to Au considerably increases the EDOS in the near-infrared region. The calculated hot carrier distribution show that adding Pd to Au increases the number of carriers generated, and that the Au-Pd alloys thin films generate hotter holes than electrons. Finally, time-dependent changes in ∆E/E from 1550 nm-pump/ THz-probe spectroscopy illustrate that for Pd and Au-Pd alloys there is a change in the conductivity attributed to contributions arising from both thermal and nonthermal electron distribution, while for Au there is no evidence of transitions under 1550 nm pump-excitation. We hypothesize this is due to a lower generation rate of hot carriers in Au than for Pd and Au-Pd alloys in the near-infrared regime.

Impulsive Coronal Heating from Large-scale Magnetic Rearrangements: From IRIS to SDO/AIA

Abstract The Interface Region Imaging Spectrograph (IRIS) has observed bright spots at the transition region footpoints associated with heating in the overlying loops, as observed by coronal imagers. Some of these brightenings show significant blueshifts in the Si iv line at 1402.77 Å ( ). Such blueshifts cannot be reproduced by coronal loop models assuming heating by thermal conduction only, but are consistent with electron beam heating, highlighting for the first time the possible importance of nonthermal electrons in the heating of nonflaring active regions. Here we report on the coronal counterparts of these brightenings observed in the hot channels of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. We show that the IRIS bright spots are the footpoints of very hot and transient coronal loops that clearly experience strong magnetic interactions and rearrangements, thus confirming the impulsive nature of the heating and providing important constraints for their physical interpretation.