Structure Of Flares Research Articles

Flare heating of the chromosphere: Observations of flare continuum from GREGOR and IRIS

Context. On 2022 May 4, an M5.7 flare erupted in the active region NOAA 13004, which was the target of a coordinated campaign between GREGOR, IRIS, Hinode, and ground-based instruments at the Ondřejov observatory. A flare kernel located at the edge of a pore was co-observed by the IRIS slit and GREGOR HiFI+ imagers. Aims. We investigated the flare continuum enhancement at different wavelength ranges in order to derive the temperature of the chromospheric layer heated during the flare. Methods. All datasets were aligned to IRIS slit-jaw images. We selected a pixel along the IRIS slit where the flare kernel was captured and evaluated multi-wavelength light curves within it. We defined a narrow IRIS near-UV band that comprises only continuum emission. The method, which assumes that the flare continuum enhancement is due to optically thin emission from hydrogen recombination processes, was applied to obtain a lower limit on the temperature in the layer where the continuum enhancement was formed. Results. We determined a lower limit for the temperature and its time evolution in the chromospheric layer heated during the flare in the range of (3–15) ×103 K. The mean electron density in that layer was estimated to be ∼1 × 1013 cm−3. Conlcusions. Multi-wavelength flare co-observations are a rich source of diagnostics. Due to the rapidly evolving nature of flares, the sit-and-stare mode is key to achieving a high temporal cadence that allows one to thoroughly analyse the same flare structure.

Heating manifestations at the onset of the 29 June 2012 flare

Analysis of GOES data for the SOL2012-06-29T04:09 flare, class C4.6, shows a thermal character of the energy release for several minutes before the impulsive stage. Plasma heating to temperatures above 10 MK leads to the appearance of plasma jets along open field lines and in large loops. This work examines the relationship between the heated plasma and the flare structure and its dynamics, using observations in the X-ray, extreme ultraviolet (EUV), and radio-wave ranges. Particular attention is drawn to the detection of narrow-band fine temporal structures of radio emission before and after the impulsive stage of the flare in dynamic spectra. In the initial stage, broadband pulses in the decimeter range are observed which can be associated with the formation of thermal fronts in the jets. A series of super-bright drifting bursts in the centimeter range occurs after the end of the impulsive energy release in the flare kernel. Using data from the Siberian Solar Radio Telescope (5.7 GHz), we managed to localize the position of the source of the fine structure of drifting bursts at the remote footpoint of the large-scale flare loop.

Проявления нагрева в начале вспышки 29 июня 2012 г.

Analysis of GOES data for the SOL2012-06-29T04:09 flare, class C4.6, shows a thermal character of the energy release for several minutes before the impulsive stage. Plasma heating to temperatures above 10 MK leads to the appearance of plasma jets along open field lines and in large loops. This work examines the relationship between the heated plasma and the flare structure and its dynamics, using observations in the X-ray, extreme ultraviolet (EUV), and radio-wave ranges. Particular attention is drawn to the detection of narrow-band fine temporal structures of radio emission before and after the impulsive stage of the flare in dynamic spectra. In the initial stage, broadband pulses in the decimeter range are observed which can be associated with the formation of thermal fronts in the jets. A series of super-bright drifting bursts in the centimeter range occurs after the end of the impulsive energy release in the flare kernel. Using data from the Siberian Solar Radio Telescope (5.7 GHz), we managed to localize the position of the source of the fine structure of drifting bursts at the remote footpoint of the large-scale flare loop.

Systematic Search for and Study of Short-timescale Flare Structures in BL Lac Object Gamma-Ray Emission

We present here the first systematic search of short-timescale γ-ray flares from 29 high Galactic latitude BL Lac objects over 14 yr of Fermi Large Area Telescope data. Using a combined Bayesian Blocks and HOP algorithm, we identified seven high-quality orbital timescale flare segments from three sources and quantified 24 short-timescale flare structures. We then performed a comprehensive analysis of flare symmetry, power spectral density (PSD) of variability, and flux–photon index relation. The main results are as follows. (1) The flare symmetry parameter A shows a “U-shaped” distribution. Short-timescale flares are symmetric while long-timescale flares are asymmetric. The number of fast-rise, slow-decay and slow-rise, fast-decay type flares are equal. No correlation is found between A and peak/integral flux. No parameter evolution is seen between consecutive flares, either. The observations support a scenario where longer-timescale flares originate from the superposition of short, symmetric subhour flares. (2) PSD from yearly to hourly timescales is modeled using the CARMA process. At lower frequencies, the PSD follows the typical broken power-law form. The high-frequency region of the PSD exhibits a continuous power-law shape, indicating that γ-ray variability originates from a single physical process across all probed timescales. (3) The flux–photon index distribution shows a pattern of “harder-when-brighter” or “softer-when-brighter,” but becomes flat above a certain critical flux, with Γ ≈ 2. This behavior cannot be simply explained by a two-component or blazar sequence model, and we speculate it may be related to complex interplay between electron acceleration and cooling.

Polarization of Circumstellar Debris Disk Light Echoes

Light echoes of debris disks around active stars can reveal disk structure and composition even when disks are not spatially resolved. Unfortunately, distinguishing reflected light from quiescent starlight and unexpected post-peak flare structure is challenging, especially for edge-on geometries where the time delay between observed flare photons and light scattered from the near side of the disk is short. Here, we take advantage of the fact that scattered light from a dusty disk is polarized, depending on the location of the scattering site and the orientation of the disk relative to a distant observer. Filtering reflected light into its polarized components allows echoes to stand out in predictable ways. We test this idea with a simple model for a disk around an active M dwarf. Our results demonstrate that the use of polarimetric data of flaring stars can significantly enhance echo signals relative to starlight and yield more robust and accurate fits to disk parameters compared to analyses based on the total intensity alone.

A Possible Mechanism for the “Late Phase” in Stellar White-light Flares

M dwarf flares observed by the Transiting Exoplanet Survey Satellite (TESS) sometimes exhibit a peak-bump light-curve morphology, characterized by a secondary, gradual peak well after the main, impulsive peak. A similar late phase is frequently detected in solar flares observed in the extreme ultraviolet from longer hot coronal loops distinct from the impulsive flare structures. White-light emission has also been observed in off-limb solar flare loops. Here, we perform a suite of one-dimensional hydrodynamic loop simulations for M dwarf flares inspired by these solar examples. Our results suggest that coronal plasma condensation following impulsive flare heating can yield high electron number density in the loop, allowing it to contribute significantly to the optical light curves via free-bound and free–free emission mechanisms. Our simulation results qualitatively agree with TESS observations: the longer evolutionary timescale of coronal loops produces a distinct, secondary emission peak; its intensity increases with the injected flare energy. We argue that coronal plasma condensation is a possible mechanism for the TESS late-phase flares.

Early Planet Formation in Embedded Disks (eDisk). VII. Keplerian Disk, Disk Substructure, and Accretion Streamers in the Class 0 Protostar IRAS 16544–1604 in CB 68

We present observations of the Class 0 protostar IRAS 16544–1604 in CB 68 from the “Early Planet Formation in Embedded Disks (eDisk)” ALMA Large program. The ALMA observations target continuum and lines at 1.3 mm with an angular resolution of ∼5 au. The continuum image reveals a dusty protostellar disk with a radius of ∼30 au seen close to edge-on and asymmetric structures along both the major and minor axes. While the asymmetry along the minor axis can be interpreted as the effect of the dust flaring, the asymmetry along the major axis comes from a real nonaxisymmetric structure. The C18O image cubes clearly show the gas in the disk that follows a Keplerian rotation pattern around a ∼0.14 M ⊙ central protostar. Furthermore, there are ∼1500 au scale streamer-like features of gas connecting from northeast, north–northwest, and northwest to the disk, as well as the bending outflow as seen in the 12CO (2–1) emission. At the apparent landing point of the NE streamer, there is SO (65–54) and SiO (5–4) emission detected. The spatial and velocity structure of the NE streamer can be interpreted as a free-falling gas with a conserved specific angular momentum, and the detection of the SO and SiO emission at the tip of the streamer implies the presence of accretion shocks. Our eDisk observations have unveiled that the Class 0 protostar in CB 68 has a Keplerian-rotating disk with a flaring and nonaxisymmetric structure associated with accretion streamers and outflows.

Tidal Disruption Events from Eccentric Orbits and Lessons Learned from the Noteworthy ASASSN-14ko

Stars grazing supermassive black holes (SMBHs) on bound orbits may survive tidal disruption, causing periodic flares. Inspired by the recent discovery of the periodic nuclear transient ASASSN-14ko, a promising candidate for a repeating tidal disruption event (TDE), we study the tidal deformation of stars approaching SMBHs on eccentric orbits. With both analytical and hydrodynamic methods, we show the overall tidal deformation of a star is similar to that in a parabolic orbit provided that the eccentricity is above a critical value. This allows one to make use of existing simulation libraries from parabolic encounters to calculate the mass fallback rate in eccentric TDEs. We find the flare structures of eccentric TDEs show a complicated dependence on both the SMBH mass and the orbital period. For stars orbiting SMBHs with relatively short periods, we predict significantly shorter-lived duration flares than those in parabolic TDEs, which can be used to predict repeating events if the mass of the SMBH can be independently measured. Using an adiabatic mass-loss model, we study the flare evolution over multiple passages, and show the evolved stars can survive many more passages than main-sequence stars. We apply this theoretical framework to the repeating TDE candidate ASASSN-14ko and suggest that its recurrent flares originate from a moderately massive (M ≳ 1 M ⊙), extended (likely ≈10 R ⊙), evolved star on a grazing, bound orbit around the SMBH. Future hydrodynamic simulations of multiple tidal interactions will enable realistic models on the individual flare structure and the evolution over multiple flares.

Scale invariance in x-ray flares of gamma-ray bursts

X-ray flares are generally believed to be produced by the reactivation of the central engine, and may have the same energy dissipation mechanism as the prompt emission of gamma-ray bursts (GRBs). X-ray flares can therefore provide important clues to understanding the nature of the central engines of GRBs. In this work, we study for the first time the physical connection between differential size and return distributions of X-ray flares of GRBs with known redshifts. We find that the differential distributions of duration, energy, and waiting time can be well fitted by a power-law function. In particular, the distributions for the differences of durations, energies, and waiting times at different times (i.e., the return distributions) well follow a $q$-Gaussian form. The $q$ values in the $q$-Gaussian distributions remain nearly steady for different temporal interval scales, implying a scale-invariant structure of GRB X-ray flares. Moreover, we verify that the $q$ parameters are related to the power-law indices $\alpha$ of the differential size distributions, characterized as $q=(\alpha+2)/\alpha$. These statistical features can be well explained within the physical framework of a self-organizing criticality system.

SIMULATION OF RING CONNECTION MODIFICATION ON FLARE STACK STRUCTURE

The flare stack serves to protect process equipment facilities against unexpected overpressure. This study aims to follow up on previous research that recommends improvements to the flare stack structure based on corrosion findings that occur in the area of the riser stack around the ring connection for the guy wire. By replacing rusty material, it is also necessary to consider modifications to prevent the risk of re-corrosion. Based on current design observations in the connection ring area for guy wire, side cutting is required which can result in water being trapped. This research aims to find out whether by modifying the connection ring for guy wire, this flare structure still has structural reliability when receiving critical outer loads or not. Based on the results of the study, it shows that modifying the geometry of the connection ring guy wire still provides reliable structural performance when receiving critical outer loads. The maximum stress that occurs in the area around the pad eye is 71.2 Mpa with a maximum stress ratio of 0.434 it shows that the value is still quite far from the maximum allowable voltage ratio of 1.0. So it can be concluded by modifying the connection ring for guy wire to prevent the risk of corrosion can be done without causing a decrease in the performance of the structure in withstanding critical outer loads.

Temperature and differential emission measure evolution of a limb flare on 13 January 2015

Context. Spatially unresolved observations show that the cooling phase in solar flares can be much longer than theoretical models predict. It has not yet been determined whether this is also the case for different subregions within the flare structure. Aims. We aim to investigate whether or not the cooling times, which are observed separately in coronal loops and the supra-arcade fan (SAF), are in accordance with the existing cooling models, and whether the temperature and emission measure of supra-arcade downflows (SADs) are different from their surroundings. Methods. We analysed the M5.6 limb flare on 13 January 2015 using SDO/AIA observations. We applied a differential emission measure (DEM) reconstruction code to derive spatially resolved temperature and emission measure maps, and used the output to investigate the thermal evolution of coronal loops, the SAF, and the SADs. Results. In the event of 13 January 2015, the observed cooling times of the loop arcade and the SAF are significantly longer than predicted by the Cargill model, even with suppressed plasma heat conduction. The observed SADs show different temperature characteristics, and in all cases a lower density than their surroundings. Conclusions. In the limb flare event studied here, continuous heating likely occurs in both loops and SAF during the gradual flare phase and leads to an extended cooling phase.

A wide‐angle scanning Luneburg lens antenna

The Luneburg lens is an effective way to implement multibeam antennas. However, the existing planar Luneburg lens configuration usually can only provide a relatively narrow scanning range. To deal with this problem, a Luneburg lens consisting of one dielectric cylinder and nine dielectric rings is proposed. It can be easily fabricated using low cost ABS material through 3D printing technology. The lens is placed between two metal plates which have an additional flare structure to enhance the antenna gain. To effectively excite the lens for multibeam applications, 9 magneto electric dipoles are utilized. For validation, a wide-angle scanning planar Luneburg lens antenna operating at 35 GHz was designed, fabricated and measured. A good agreement between simulation and measurement can be observed. At the center frequency, a wide scanning coverage of 172° can be obtained with a gain variation less than 1.6 dB.

Plasma dynamics in the flaring loop observed by RHESSI

Context. Hard X-rays (HXRs) contain the most direct information about the non-thermal electron population in solar flares. The approximation of the HXR emission mechanism (bremsstrahlung), known as the thick-target model, is well developed. It allows one to diagnose the physical conditions within a flaring structure. The thick-target model predicts that in flare foot points, we should observe lowering of HXR sources’ altitude with increasing energy. Aims. The foot point of HXR sources result from the direct interaction of non-thermal electron beams with plasma in the lower part of the solar atmosphere, where the density increases rapidly. Therefore, we can estimate the plasma density distribution along the non-thermal electron beam directly from the observations of the altitude-energy relation obtained for the HXR foot point sources. However, the relation is not only density-dependent. Its shape is also determined by the power-law distribution of non-thermal electrons. Additionally, during the impulsive phase, the plasma density and a degree of ionisation within foot points may change dramatically due to heating and chromospheric evaporation. For this reason, the interpretation of observed HXR foot point sources’ altitudes is not straightforward and needs detailed numerical modelling of the electron precipitation process. Methods. We present the results of numerical modelling of one well-observed solar flare. We used HXR observations obtained by RHESSI. The numerical model was calculated using the hydrodynamic 1D model with an application of the Fokker-Planck formalism for non-thermal beam precipitation. Results. HXR data were used to trace chromospheric density changes during a non-thermal emission burst, in detail. We have found that the amount of mass that evaporated from the chromosphere is in the range of 2.7 × 1013 − 4.0 × 1014 g. This is in good agreement with the ranges obtained from hydrodynamical modelling of a flaring loop (2.3 × 1013 − 3.3 × 1013 g), and from an analysis of observed emission measure in the loop top (3.9 × 1013 − 5.3 × 1013 g). Additionally, we used specific scaling laws which gave another estimation of the evaporated mass around 2 × 1014 g. Conclusions. Consistency between the obtained values shows that HXR images may provide an important constraint for models – a mass of plasma that evaporated due to a non-thermal electron beam depositing energy in the chromosphere. High-energy, non-thermal sources’ (above 20 keV in this case) positions fit the column density changes obtained from the hydrodynamical model perfectly. Density changes seem to be less affected by the electrons’ spectral index. The obtained results significantly improve our understanding of non-thermal electron beam precipitation and allow us to refine the energy balance in solar flare foot points during the impulsive phase.

The Flare and Warp of the Young Stellar Disk Traced with LAMOST DR5 OB-type Stars

Abstract We present an analysis of the spatial density structure for the outer disk from 8–14 kpc with the LAMOST DR5 13,534 OB-type stars and observe similar flaring on the north and south sides of the disk, implying that the flaring structure is symmetrical about the Galactic plane, for which the scale height at different Galactocentric distances is from 0.14 to 0.5 kpc. By using the average slope to characterize the flaring strength, we find that the thickness of the OB stellar disk is similar but that flaring is slightly stronger compared to the thin disk as traced by red giant branch stars, possibly implying that secular evolution is not the main contributor to the flaring but rather perturbation scenarios such as interactions with passing dwarf galaxies could be possible. When comparing the scale height of the OB stellar disk on the north and south sides with the gas disk, the former one is slightly thicker than the latter one by ≈33 and 9 pc, meaning that one could tentatively use young OB-type stars to trace the gas properties. Meanwhile, we determine that the radial scale length of the young OB stellar disk is 1.17 ± 0.05 kpc, which is shorter than that of the gas disk, confirming that the gas disk is more extended than the stellar disk. What is more, by considering the midplane displacements (Z 0) in our density model we find that almost all values of Z 0 are within 100 pc, with an increasing trend as Galactocentric distance increases.

Is Flare Ribbon Fine Structure Related to Tearing in the Flare Current Sheet?

Abstract Observations of solar flare ribbons show significant fine structure in the form of breaking wavelike perturbations and spirals. The origin of this structure is not well understood, but one possibility is that it is related to the tearing instability in the flare current sheet. Here we study this connection by constructing an analytical 3D magnetic field representative of an erupting flux rope with a flare current sheet below it. We introduce small-scale flux ropes representative of those formed during a tearing instability in the current layer, and use the squashing factor on the solar surface to identify the shape of the presumed flare ribbons and fine structure. Our analysis suggests there is a direct link between flare ribbon fine structure and flare current sheet tearing, with the majority of the ribbon fine structure related to oblique tearing modes. Depending upon the size, location, and twist of the small-scale flux ropes, breaking wavelike and spiral features within the hooks and straight sections of the flare ribbon can be formed that are qualitatively similar to observations. We also show that the handedness of the spirals/waves must be the same as the handedness of the hooks of the main ribbon. We conclude that tearing in the flare current layer is a likely explanation for spirals and wavelike features in flare ribbons.

Subsecond X-ray Spikes in Solar Flares: Kinematics of Acceleration and Transport of Accelerated Electrons

Hard X-rays time structure of solar flares can be caused by the timing of electrons acceleration, the kinematics of their propagation in the flaring plasma, and possible changes in plasma parameters and the loop magnetic field. In this paper the subsecond hard X-ray emission features were studied in a model of collapsing magnetic trap. The injection of 50-millisecond pulses of accelerated electrons in looptop with specified distributions of plasma density and magnetic field structure was considered. The magnetic field fluctuations and ion-acoustic turbulence influence on the fine time structure of hard X-ray emission was shown. The simulation results that it is possible to observe a super-thin time structure in the hard X-ray emission from accelerated electrons only if: ion-acoustic turbulence develops in magnetic loops and/or the pitch-angular distribution of electrons is sharply anisotropic. Moreover, the amplitude of individual spike is greater for harder electron spectra.

Structure and Evolution of Powerful H2O Maser Flares in the Protostellar Object IRAS 18316–0602 (G25.65+1.05)

The structure and evolution of powerful H2O maser flares in the source IRAS 18316–0602 are studied using the results of observations on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory. The main origin of the last powerful, short-term flare in 2017 may be the superposition of two maser condensations with very similar radial velocities along the line of sight. All powerful flares occurring in IRAS 18316–0602 since 2002 can be associated with a cluster of maser condensations whose radial velocities are in the interval 41.0–43.5 km/s. It is suggested that this may be related to turbulent, possibly vortical, motions of material in this region.

A Wide-Angle Broadband Converter: From Odd-Mode Spoof Surface Plasmon Polaritons to Spatial Waves

A wide-angle broadband converter from odd-mode spoof surface plasmon polaritons (SPPs) to spatial waves is proposed using spatially gradient Vivaldi-shaped flaring structure. We first show the dispersion, Eigen mode, and effective circuit topology of the designed SPP structure and show that it can support a propagating odd mode. A conversion structure between odd-mode spoof SPPs and spatial mode has been designed to realize smooth impedance and wavenumber transition. Through the full-wave simulation method, we obtained the optimized geometric parameters, from which we fabricate the converter using the print circuit board technology. The measured results from both far field and near field show that the proposed structure has successfully excited odd-mode spoof SPPs coming from an extremely wide-angle spatial radiation. The allowed azimuthal and pitched angle ranges are larger than 72°.

On Solar Recurrent Coronal Jets: Coronal Geysers as Sources of Electron Beams and Interplanetary Type-III Radio Bursts

Abstract Coronal jets are transitory small-scale eruptions that are omnipresent in solar observations. Active regions jets produce significant perturbations on the ambient solar atmosphere and are believed to be generated by microflare reconnection. Multiple sets of recurrent jets are identified in extreme-ultraviolet filter imaging. In this work we analyze the long timescale recurrence of coronal jets originating from a unique footpoint structure observed in the lower corona. We report the detection of penumbral magnetic structures in the lower corona. These structures, which we call “coronal geysers,” persist through multiple reconnection events that trigger recurrent jets in a quasi-periodical trend. Recurrent jet eruptions have been associated with Type-III radio bursts that are manifestations of traveling non-thermal electron beams. We examine the assumed link, as the coronal sources of interplanetary Type-III bursts are still open for debate. We scrutinized the hypothesized association by temporally correlating a statistically significant sample of six Geyser structures that released at least 50 recurrent jets, with correspondent Type-III radio bursts detected in the interplanetary medium. Data analysis of these phenomena provides new information on small-scale reconnection, non-thermal electron beam acceleration, and energy release. We find that the penumbral Geyser-like flaring structures produce recurring jets. They can be long-lived, quasi-stable, and act as coronal sources for Type-III bursts, and, implicitly, upward accelerated electron beams.

Ultimate Longitudinal Strength of Ships' Hull Girder under Combined Loading — 3rd Report: Introduction of the effect of bulkheads and side structures to torsional analysis with using 1 dimensional finite element method —

In recent years, container ships become larger and larger to increase the transport efficiency. The torsional rigidity of a container ship is lower than other type of ships, because the container ship has the large deck opening and large flare structure. Therefore, it is important to calculate the longitudinal ultimate strength with considering the torsional moment for the design of ships.