Population Of Relativistic Electrons Research Articles

Transverse instability of electron-acoustic solitons in a relativistic degenerate astrophysical magnetoplasma

Abstract We study the nonlinear theory of small-amplitude electron-acoustic solitons (EASs) in a relativistic astrophysical magnetoplasma consisting of two-temperature electrons: a sparse population of relativistic nondegenerate classical electrons and a group of fully degenerate dense relativistic electrons (main constituent) immersed in a static magnetic field with a neutralizing stationary ion background. By using the multiple-scale reductive perturbation technique with the Lorentz transformation, the Zakharov-Kuznetsov (ZK) and the modified Zakharov-Kuznetsov (mZK) equations are derived to describe the evolution of EASs in two different regimes of relativistic degeneracy: $r_{d0}<50$ and $r_{d0}\gtrsim50$, where $r_{d0}=\left(n_{d0}/n_{\rm{cr}}\right)^{1/3}$ is the degeneracy parameter in which $n_{d0}$ is the unperturbed number density of degenerate electrons and $n_{\rm{cr}}\approx6\times10^{29}~\rm{cm}^{-3}$ the critical number density at which $p_F=m_ec$ and which defines the emergence of the relativistic regime. The characteristics of the plane soliton solutions of ZK and mZK equations and the soliton energy are studied. We show that the solitons moving at an angle $\alpha$ to the external magnetic field can be unstable under transverse long-wavelength perturbations. The growth rates of instabilities are obtained and analyzed with the effects of the relativity parameter $\beta_{\rm{cl}}=k_BT_{\rm{cl}}/m_ec^2$ and the degeneracy parameter $r_{d0}$, where $k_B$ is the Boltzmann constant and $T_{\rm{cl}}$ is the temperature of classical electrons. Interestingly, the ZK solitons, even if it is stable for the first-order perturbations, can be unstable in the second-order correction. Furthermore, while the first-order growth rates of perturbations for ZK solitons tend to vanish as $\alpha\rightarrow 38^\circ$, that for the mZK soliton goes to zero as $\alpha\rightarrow 90^\circ$. However, depending on the angle $\alpha$, the growth rates are found to be reduced either by increasing the values of $\beta_{\rm{cl}}$ or by decreasing the values of $r_{d0}$. The applications of our results to astrophysical plasmas, such as those in the environments of white dwarfs are discussed.

Suppression of Nonlinear Chorus Wave Growth by Active Control of Gyroresonant Interactions With Electron Hole Deformation

AbstractNonlinear gyroresonant acceleration is an essential mechanism for the formation of relativistic electrons in Earth's radiation belts, yet we still have only limited knowledge of how electron motion interacts nonlinearly with plasma waves. Here we demonstrate the compelling suppression of whistler‐mode chorus emissions as a resonant wave with energetic electrons by tuning an external transmitter signal in space plasmas. Through forcible modification of electron motions via phase trapping by the external waves, the nonlinear gyroresonant current is suppressed along with the electron hole deformation. Moreover, this can decrease the population of relativistic electrons due to the lack of coherent resonant waves seen by the phase‐organized electrons. Understanding such nonlinear wave suppression effects is crucial for a potential active control of Earth's plasma environment.

Ups and downs in the X-ray emission of the colliding wind binaries HD 168112 and HD 167971

Context. The long-period O-star binary system HD 168112 and the triple O-star system HD 167971 are well-known sources of non-thermal radio emission that arises from a colliding wind interaction. The wind-wind collisions in these systems should result in phase-dependent X-ray emissions. The presence of a population of relativistic electrons in the wind interaction zone could affect the properties of the X-ray emission and make it deviate from the behaviour expected for adiabatic shocks. Aims. We investigate the X-ray emission of these systems with the goals of quantifying the fraction of the X-ray flux arising from wind interactions and determining whether these emissions follow the predictions for adiabatic wind-wind collisions. Methods. Six X-ray observations were collected with XMM-Newton. Three observations were scheduled around the most recent peri-astron passage of HD 168112. Spectra and light curves were analysed and compared with simple predictions of model calculations for X-ray emission from colliding wind systems. Results. The X-ray emission of HD 168112 varies as the inverse of the orbital separation, as expected for an adiabatic wind interaction zone. The relative contribution of intrinsic X-ray emission from wind-embedded shocks varies between 38% at periastron to 81% at apastron. The wind-wind collision zone remains adiabatic even around periastron passage. The X-ray emission of HD 167971 displays variations on the orbital timescale of the inner eclipsing binary. The existing data of this system do not allow us to probe variations on the timescale of the outer orbit. Conclusions. Shock modification due to the action of relativistic electrons does not seem to be efficiently operating in the HD 168112 system. In the existing observations, a significant part of the emission of HD 167971 must arise in the inner eclipsing binary. The origin of this emission is as yet unclear.

Generation and Impacts of Whistler‐Mode Waves During Energetic Electron Injections in Jupiter's Outer Radiation Belt

AbstractEnergetic particle injections are commonly observed in Jupiter's magnetosphere and have important impacts on the radiation belts. We evaluate the roles of electron injections in the dynamics of whistler‐mode waves and relativistic electrons using Juno measurements and wave‐particle interaction modeling. The Juno spacecraft observed injected electron flux bursts at energies up to 300 keV at M shell ∼11 near the magnetic equator during perijove‐31. The electron injections are related to chorus wave bursts at 0.05–0.5 fce frequencies, where fce is the electron gyrofrequency. The electron pitch angle distributions are anisotropic, peaking near 90° pitch angle, and the fluxes are high during injections. We calculate the whistler‐mode wave growth rates using the observed electron distributions and linear theory. The frequency spectrum of the wave growth rate is consistent with that of the observed chorus magnetic intensity, suggesting that the observed electron injections provide free energy to generate whistler‐mode chorus waves. We further use quasilinear theory to model the impacts of chorus waves on 0.1–10 MeV electrons. Our modeling shows that the chorus waves could cause the pitch angle scattering loss of electrons at <1 MeV energies and accelerate relativistic electrons at multiple MeV energies in Jupiter's outer radiation belt. The electron injections also provide an important seed population at several hundred keV energies to support the acceleration to higher energies. Our wave‐particle interaction modeling demonstrates the energy flow from the electron injections to the relativistic electron population through the medium of whistler‐mode waves in Jupiter's outer radiation belt.

Relativistic electron ([formula omitted]) flux driving parameters over geostationary orbit using 30 years of data

The relativistic electron population at MeV energy in the Earth’s outer radiation belt at geostationary (GOE) orbit largely varies in association with solar wind disturbances. Although more researches have been done to understand the influence of different mechanisms that cause the variations in the relativistic electron flux in the region, but still the underlying physics is often complex and unclear. The present study uses relativistic electron (E>2MeV) fluxes data of Los Alamos National Laboratory (LANL) geosynchronous satellites to understand the variations and its relationship with the driving parameters. The study examines the relativistic electron fluxes variations associated with the interplanetary magnetic field (IMF) and solar wind parameters using daily averaged data from 01 January 1990 to 31 December 2018. The results indicated that the information transfer from solar wind and IMF parameters into relativistic electrons flux is dependent on the kind of driving parameter. The result identifies the rank of driving parameters based on their maximum values in the cross correlation and information transfer analysis. Thus, AE index and vsw are the dominant driving parameters for the dynamics of relativistic electrons flux at GEO orbit. The peak values of coefficient (correlation coefficients, mutual information and entropy transfer) are observed at similar lag times (τpeak). For instance, in the case of AE and Je, we have observed the maximum coefficient (associated with lag time) as follows; maximum correlation coefficient; CCmax=0.846(5days), maximum mutual information; MImax=0.803(5days), maximum transfer entropy; TEmax=0.892(5days). Such dependency analysis can help to select the model parameters that influence the relativistic electrons fluxes and improve the existed models of the dynamical system.

Contributions to Loss Across the Magnetopause During an Electron Dropout Event.

Dropout events are dramatic decreases in radiation belt electron populations that can occur in as little as 30 minutes. Loss to magnetopause due to a combination of magnetopause shadowing and outward radial transport plays a significant role in these events. We examine the dropout of relativistic electron populations during the October 2012 geomagnetic storm using simulated electron phase space density, evaluating the contribution of different processes to losses across the magnetopause. We compare loss contribution from outward transport calculated using a standard empirical radial diffusion model that assumes a dipolar geomagnetic field to an event-specific radial diffusion model evaluated with a non-dipolar geomagnetic field. We additionally evaluate the contribution of Shabansky type 1 particles, which bounce along magnetic field lines with local equatorial maxima, to the loss calculated during this event. We find that the empirical radial diffusion model with a dipolar background field underestimates the contribution of radial diffusion to this dropout event by up to 10% when compared to the event-specific, non-dipolar radial diffusion model. We additionally find that including Shabansky type 1 particles in the initial electron phase space density, that is, allowing some magnetic field lines distorted from the typical single-minima configuration in drift shell construction, increases the calculated loss by an average of 0.75%. This shows that the treatment of the geomagnetic field significantly impacts the calculation of electron losses to the magnetopause during dropout events, with the non-dipolar treatment of radial diffusion being essential to accurately quantify the loss of outer radiation belt populations.

Electromagnetic solitons and their stability in relativistic degenerate dense plasmas with two electron species

The evolution of electromagnetic (EM) solitons due to nonlinear coupling of circularly polarized intense laser pulses with low-frequency electron-acoustic perturbations is studied in relativistic degenerate dense astrophysical plasmas with two groups of electrons: a sparse population of classical relativistic electrons and a dense population of relativistic degenerate electron gas. Different forms of localized stationary solutions are obtained and their properties are analyzed. Using the Vakhitov-Kolokolov stability criterion, the conditions for the existence and stability of a moving EM soliton are also studied. It is noted that the stable and unstable regions shift around the plane of soliton eigenfrequency and the soliton velocity due to the effects of relativistic degeneracy, the fraction of classical to degenerate electrons and the EM wave frequency. Furthermore, while the standing solitons exhibit stable profiles for a longer time, the moving solitons, however, can be stable or unstable depending on the degree of electron degeneracy, the soliton eigenfrequency and the soliton velocity. The latter with an enhanced value can eventually lead to a soliton collapse. The results should be useful for understanding the formation of solitons in the coupling of highly intense laser pulses with slow response of degenerate dense plasmas in the next generation laser-plasma interaction experiments as well as the novel features of x-ray and γ-ray pulses that originate from compact astrophysical objects.

An event of extreme relativistic and ultra-relativistic electron enhancements following the arrival of consecutive corotating interaction regions: Coordinated observations by Van Allen Probes, Arase, THEMIS and Galileo satellites

During July to October of 2019, a sequence of isolated Corotating Interaction Regions (CIRs) impacted the magnetosphere, for four consecutive solar rotations, without any interposed Interplanetary Coronal Mass Ejections. Even though the series of CIRs resulted in relatively weak geomagnetic storms, the net effect of the outer radiation belt during each disturbance was different, depending on the electron energy. During the August-September CIR group, significant multi-MeV electron enhancements occurred, up to ultra-relativistic energies of 9.9 MeV in the heart of the outer Van Allen radiation belt. These characteristics deemed this time period a fine case for studying the different electron acceleration mechanisms. In order to do this, we exploited coordinated data from the Van Allen Probes, the Time History of Events and Macroscale Interactions during Substorms Mission (THEMIS), Arase and Galileo satellites, covering seed, relativistic and ultra-relativistic electron populations, investigating their Phase Space Density (PSD) profile dependence on the values of the second adiabatic invariant K, ranging from near-equatorial to off equatorial mirroring populations. Our results indicate that different acceleration mechanisms took place for different electron energies. The PSD profiles were dependent not only on the μ value, but also on the K value, with higher K values corresponding to more pronounced local acceleration by chorus waves. The 9.9 MeV electrons were enhanced prior to the 7.7 MeV, indicating that different mechanisms took effect on different populations. Finally, all ultra-relativistic enhancements took place below geosynchronous orbit, emphasizing the need for more Medium Earth Orbit (MEO) missions.

Discovery and insights from DSX mission’s high-power VLF wave transmission experiments in the radiation belts

Space weather phenomena can threaten space technologies. A hazard among these is the population of relativistic electrons in the Van Allen radiation belts. To reduce the threat, artificial processes can be introduced by transmitting very-low-frequency (VLF) waves into the belts. The resulting wave-particle interactions may deplete these harmful electrons. However, when transmitting VLF waves in space plasma, the antenna, plasma, and waves interact in a manner that is not well-understood. We conducted a series of VLF transmission experiments in the radiation belts and measured the power and radiation impedance under various frequencies and conditions. The results demonstrate the critical role played by the plasma-antenna-wave interaction around high-voltage space antennae and open the possibility to transmit high power in space. The physical insight obtained in this study can provide guidance to future high-power space-borne VLF transmitter developments, laboratory whistler-mode wave injection experiments, and the interpretation of various astrophysical and optical phenomena.

Flare echoes from relaxation shocks in perturbed relativistic jets

Context. One of the main scenarios to account for the multiwavelength flux variability observed in relativistic jets of active galactic nuclei (AGNs) is based on the diffusive shock acceleration of a population of relativistic electrons on internal shocks of various origins. Any complete AGN emission scenario has to be able to explain the wide range of observed variability timescales, which change over several orders of magnitude between the radio and gamma-ray band. In addition to observations of flux variability, constraints are also provided by very-long-baseline interferometry (VLBI), which shows a large variety of moving and standing emission zones with distinct behaviors. Aims. Combining dynamic hydrodynamic jet simulations with radiative transfer, we aim to characterize the evolution of stationary and moving emission zones in the jet and study their multiwavelength signatures through emission maps and light curves. We focus our study on flare events that occur during strong interactions between moving ejecta and stationary recollimation shocks. Such events are shown to lead to a significant perturbation of the stationary jet structure. Methods. We simulate relativistic jets with the magneto-hydrodynamic code MPI-AMRVAC and inject nonthermal particle distributions of electrons into shock regions. We follow the propagation of a moving shock and its interactions with a structure of standing recollimation shocks in the jet. Synchrotron emission and radiative transfer are calculated in the post-processing code RIPTIDE for given observation angles and frequencies, assuming a turbulent magnetic field and taking the light crossing effect into account. Results. In the case of strong shock–shock interactions, we demonstrate the appearance of trailing components behind the leading moving shock. The latter destabilizes the jet, causing the emergence of oscillating standing shocks and relaxation shocks. Emissions from these regions can dominate the overall flux or lead to “flare echoes” in the light curve. Another observational marker for the presence of relaxation shocks appears in time-distance plots of bright VLBI components of the jet. Our scenario provides a plausible explanation for radio VLBI observations of the radio galaxy 3C 111, where trailing components were observed during a radio outburst event in 1997, and may be applicable to other sources with similar features.

Sub-arcsecond LOFAR imaging of Arp 299 at 150 MHz

Context. Arp 299 is the brightest luminous infrared galaxy (LIRG) within 50 Mpc, with IR luminosity log(LIR∕L⊙) = 11.9. It provides a unique laboratory for testing physical processes in merging galaxies. Aims. We study for the first time the low-frequency (~150 MHz) radio brightness distribution of Arp 299 at subarcsecond resolution, tracing in both compact and extended emission regions the local spectral energy distribution (SED) in order to characterize the dominant emission and absorption processes. Methods. We analysed the spatially resolved emission of Arp 299 revealed by 150 MHz international baseline Low-Frequency Array (LOFAR) and 1.4, 5.0, and 8.4 GHz Very Large Array (VLA) observations. Results. We present the first subarcsecond (0.4″ ~ 100 pc) image of the whole Arp 299 system at 150 MHz. The high surface brightness sensitivity of our LOFAR observations (~100 μJy beam−1) allowed us to detect all of the nuclear components detected at higher frequencies, as well as the extended steep-spectrum emission surrounding the nuclei. We obtained spatially resolved, two-point spectral index maps for the whole galaxy: the compact nuclei show relatively flat spectra, while the extended, diffuse component shows a steep spectrum. We fitted the radio SED of the nuclear regions using two different models: a continuous free-free medium model and a clumpy model. The continuous model can explain the SED of the nuclei assuming a population of relativistic electrons subjected to synchrotron, bremsstrahlung, and ionization losses. The clumpy model fits assuming relativistic electrons with negligible energy losses, and thermal fractions that are more typical of star-forming galaxies than those required for the continuous model. Conclusions. Our results confirm the usefulness of combining spatially resolved radio imaging at both MHz and GHz frequencies to characterize in detail the radio emission properties of LIRGs from the central 100 pc out to the kiloparsec galaxy-wide scales.

Understanding how minority relativistic electron populations may dominate charge state balance and radiative cooling of a post-thermal quench tokamak plasma

Minority relativistic electron populations can occur in a range of complex plasmas. Of specific interest is when runaway electrons form among the presence of high-atomic-number ion species in a tokamak plasma discharge. It has been recently demonstrated that ion charge state distributions and radiation losses at low bulk electron temperatures can be dominated by relativistic electrons, even though their density is orders of magnitude lower. This was attributed to the relativistic enhancement of electron impact inelastic cross sections. In this work, we provide a closer inspection of the atomic physics underpinning this effect. We also demonstrate the consequences of runaway enhanced scattering on post-disruption tokamak fusion discharges with neon and argon impurities present. Effects on charge state distributions, radiation and spectral characteristics, and reduced-order modeling considerations are discussed.

MeerKAT discovery of radio emission from the Vela X-1 bow shock

ABSTRACT Vela X-1 is a runaway X-ray binary system hosting a massive donor star, whose strong stellar wind creates a bow shock as it interacts with the interstellar medium (ISM). This bow shock has previously been detected in H α and infrared, but, similar to all but one bow shock from a massive runaway star (BD+43o3654), has escaped detection in other wavebands. We report on the discovery of 1.3 GHz radio emission from the Vela X-1 bow shock with the MeerKAT telescope. The MeerKAT observations reveal how the radio emission closely traces the H α line emission, both in the bow shock and in the larger scale diffuse structures known from existing H α surveys. The Vela X-1 bow shock is the first stellar-wind-driven radio bow shock detected around an X-ray binary. In the absence of a radio spectral index measurement, we explore other avenues to constrain the radio emission mechanism. We find that thermal/free–free emission can account for the radio and H α properties, for a combination of electron temperature and density consistent with earlier estimates of ISM density and the shock enhancement. In this explanation, the presence of a local ISM overdensity is essential for the detection of radio emission. Alternatively, we consider a non-thermal/synchrotron scenario, evaluating the magnetic field and broad-band spectrum of the shock. However, we find that exceptionally high fractions (≳ 13 per cent) of the kinetic wind power would need to be injected into the relativistic electron population to explain the radio emission. Assuming lower fractions implies a hybrid scenario, dominated by free–free radio emission. Finally, we speculate about the detectability of radio bow shocks and whether it requires exceptional ISM or stellar wind properties.

Parsec‐scale view of CSS/GPS sources at 327 MHz

AbstractGlobal very long baseline interferometry (VLBI) observations at low frequencies are of great importance since they allow us to test the oldest part of the relativistic electron populations in young radio sources. At 327 MHz ( 92 cm), the most compact regions are self‐absorbed, and the number of cores detected is very small. This could provide useful measurements of the magnetic field in the relativistic plasma, to be compared with the equipartition field derived from minimum energy arguments. Partial self‐absorption takes place also in hot‐spots. However, disentangling the hot‐spot emission from that of the lobe turned out to be rather difficult. On the other hand, a clearer and more complete characterization of the volumes occupied by the magnetized relativistic plasma and its total energy content can be obtained at 327 MHz. In turn, an independent estimate of the source age can be inferred, on the basis of simple assumptions on the jet power. Here, we show that the ages derived in this way (between and year) are consistent with the radiative ages determined from the break frequency in the radio spectrum.

Geomagnetically Quiet Period Analysis of Relativistic Electrons, Auroral Precipitation, Joule Heating, and Ring Current During the Years of 1999, 2000 and 2004

This work presents the study of the quietest time variation in relativistic electrons, auroral precipitation, ring current, and joule heating during 1999, 2000, and 2004. Geostationary Operational Environmental Satellite (GOES) data on relativistic electrons with energies above 0.6 MeV, 2 MeV, and 4 MeV were analyzed. The time-series analysis of the relativistic electrons over a 24-hour averaged interval reveals a precise 24-hour modulation of the relativistic electron population during all seasons for energies above 0.6 MeV and 2 MeV, and during the winter season for higher energies above 4 MeV. In addition, relativistic electron fluxes at energies above 0.6 MeV and above 2 MeV were higher during the descending phase of the solar cycle compared to the ascending and solar-maximum phases. The cross-correlation analysis presented a strong correlation of Joule heating, ring current, and auroral precipitation with the relativistic electron population in three energy bands considered, as indicated by the zero-time lag. Studying the quiet time variation of relativistic electrons will lead to more complete ionospheric models, which were previously limited to the geomagnetically disturbed period.

On the Formation of Phantom Electron Phase Space Density Peaks in Single Spacecraft Radiation Belt Data

AbstractThis study examines the rapid losses and acceleration of trapped relativistic and ultrarelativistic electron populations in the Van Allen radiation belt during the September 7–9, 2017 geomagnetic storm. By analyzing the dynamics of the last closed drift shell (LCDS), and the electron flux and phase space density (PSD), we show that the electron dropouts are consistent with magnetopause shadowing and outward radial diffusion to the compressed LCDS. During the recovery phase, an in‐bound pass of Van Allen Probe A shows an apparent local peak in PSD, which does not exist in reality. A careful analysis of the multipoint measurements by the Van Allen Probes reveals instead how the apparent PSD peak arises from aliasing monotonic PSD profiles which are rapidly increasing due to acceleration from very fast inwards radial diffusion. In the absence of such multisatellite conjunctions during fast acceleration events, such peaks might otherwise be associated with local acceleration processes.

Diffuse radio emission from galaxy clusters in the LOFAR Two-metre Sky Survey Deep Fields

Low-frequency radio observations are revealing an increasing number of diffuse synchrotron sources from galaxy clusters, primarily in the form of radio halos or radio relics. The existence of this diffuse synchrotron emission indicates the presence of relativistic particles and magnetic fields. It is still an open question as to exactly what mechanisms are responsible for the population of relativistic electrons driving this synchrotron emission. The LOFAR Two-metre Sky Survey Deep Fields offer a unique view of this problem. Reaching noise levels below 30 μJy beam−1, these are the deepest images made at the low frequency of 144 MHz. This paper presents a search for diffuse emission in galaxy clusters in the first data release of the LOFAR Deep Fields. We detect a new high-redshift radio halo with a flux density of 8.9 ± 1.0 mJy and corresponding luminosity of P144MHz = (3.6 ± 0.6) × 1025 W Hz−1 in an X-ray detected cluster at z = 0.77 with a mass estimate of M500 = 3.3−1.7+1.1 × 1014 M⊙. Deep upper limits are placed on clusters with non-detections. We compare the results to the correlation between halo luminosity and cluster mass derived for radio halos found in the literature. This study is one of a few to find diffuse emission in low mass (M500 < 5 × 1014 M⊙) systems and shows that deep low-frequency observations of galaxy clusters are fundamental for opening up a new part of parameter space in the study of non-thermal phenomena in galaxy clusters.

Dynamic Mechanisms Associated With High‐Energy Electron Flux Dropout in the Earth's Outer Radiation Belt Under the Influence of a Coronal Mass Ejection Sheath Region

AbstractThe near‐Earth interplanetary environment conditions affect the dynamics of the relativistic electron population quasitrapped in the radiation belts. A complex chain of processes observed in the magnetosphere can contribute to the variability of these populations when interplanetary structures, such as the interplanetary counterpart of a solar coronal mass ejection (ICME), and high‐speed solar wind streams interact with the magnetosphere. However, as these processes can coexist, it is hard to untangle the relative contribution of each process to the loss of particles and the eventual repopulation. Here we show evidence that it is possible to distinguish the relative contribution of mechanisms related to the loss of the outer radiation belt electrons for an event observed on July 19 and 20, 2016. The interaction of an ICME's turbulent sheath with the Earth's magnetosphere resulted in a decrease in the outer radiation belt relativistic electron population. The ultralow frequency (ULF) and chorus wave activities are detected in the outer radiation belt during the time when the Earth's magnetosphere is under the influence of the ICME's sheath region, as well as the ICME's magnetic cloud region, while the electromagnetic ion cyclotron (EMIC) waves in the outer belt are observed only during the sheath region. Dynamic mechanisms such as magnetopause shadowing, outward radial diffusion driven by ULF waves, pitch‐angle scattering driven by both EMIC and chorus waves are quantitatively analyzed. Our results suggest that the structures of the ICMEs can trigger the drivers to generate the different dynamic mechanisms responsible for the radiation belt population variability.

Non-thermal filaments from the tidal destruction of clouds in the Galactic centre

ABSTRACT Synchrotron-emitting, non-thermal filaments (NTFs) have been observed near the Galactic centre for nearly four decades, yet their physical origin remains unclear. Here we investigate the possibility that NTFs are produced by the destruction of molecular clouds by the gravitational potential of the Galactic centre. We show that this model predicts the formation of a filamentary structure with length on the order of tens to hundreds of pc, a highly ordered magnetic field along the axis of the filament, and conditions conducive to magnetic reconnection that result in particle acceleration. This model therefore yields the observed magnetic properties of NTFs and a population of relativistic electrons, without the need to appeal to a dipolar, ∼mG, Galactic magnetic field. As the clouds can be both completely or partially disrupted, this model provides a means of establishing the connection between filamentary structures and molecular clouds that is observed in some, but not all, cases.

Background Microwave Emission and Microflares in Young Active Region 12635

Abstract The goal of this work is to study the atmospheric heating above isolated AR 12635 during a period in which a quasistationary increase in microwave and soft X-ray emission is combined with a series of B- and C-class microflares. Analysis of photospheric vector magnetograms showed that an increase in the SXR brightness lasting 14 hours was observed during the simplification of the magnetic structure and the growth of vertical currents in the head part of the active region. A long-term increase of SXR and microwave emission occurred when the total unsigned vertical current exceeded the critical value and coincided in time with the increased level of the emission measure calculated from the GOES X-ray data. Against the background of quasistationary emission, microflares lasting 6–10 minutes occurred. In the hard X-ray range, bursts are recorded with RHESSI channels up to 25 keV. In microwave emission, pulsed and smooth components are distinguished during microflares. The first component was recorded in the 4.5–7.5 GHz range at the beginning of microflares and it is generated by a small population of relativistic electrons by the gyrosychrotron mechanism. The smooth component of the bursts dominated at 17 GHz and was emitted by bremsstrahlung. It is shown that the sources of quasistationary emission and microflares coincide with each other and with the bremsstrahlung source calculated from the differential emission measure obtained from the EUV maps. Coronal magnetic field reconstruction shows that the release of energy on both timescales occurs in a stably existing bundle of magnetic field lines.