Coronal Loops Research Articles

Effect of magnetically dependent heating on the behaviour of magnetoacoustic waves in coronal plasma with thermal misbalance

ABSTRACT The magnetic nature of coronal heating has been actively investigated within the framework of theoretical models and statistical analysis of observational data for decades. At present, a rather wide range of possible mechanisms has been proposed in the literature that requires additional verification. In this paper, we investigate the possibility of analysing the magnetic nature of coronal heating by means of magnetoacoustic (MA) waves propagating in coronal structures. To address this issue, we perform the analysis of fast and slow waves using a magnetic slab geometry. Applying the assumption of strong magnetic structuring, we derive the dispersion relation, which allows us to study the properties of MA waves. To analyse the dependence of phase velocity and wave decrement/increment on wavenumber, we numerically solved the obtained equations using the parameters corresponding to ‘warm’ coronal loop. It is shown that oscillations on the fundamental harmonic in a plasma with a weak magnetic field, where the effect of phase velocity dispersion is most pronounced, are best suited for diagnostics of magnetic heating using slow MA waves. In turn, the geometry remains the primary source for fast MA wave dispersion. Magnetic heating can either suppress or increase the damping of fast and slow MA waves. Moreover, the amplification of fast MA waves accompanied by damping of slow MA waves can be achieved. This issue is of interest in the context of the excitation of the decayless kink oscillations in the solar coronal loops.

Transverse oscillations of two parallel magnetic tubes with slowly changing density

ABSTRACT We study kink oscillations of the system of two parallel magnetic tubes in the presence of plasma cooling. We assume that the characteristic cooling time is much larger than the characteristic time of kink oscillations. Using the ratio of two characteristic times as a small parameter, we derive the expression for the adiabatic invariant, which is a quantity that remains constant during the cooling process. Then, we study in detail a particular case where the plasma densities in the two tubes are the same, the plasma temperature outside of the tube does not change, and the plasma temperature inside the tubes decreases exponentially. We found that cooling causes the increase of the oscillation frequencies and amplitudes. These results are the generalization of similar results previously obtained for a single magnetic tube. We compared the efficiency of amplification of kink oscillations caused by cooling in counteracting the damping of oscillations due to resonant absorption in two models of coronal magnetic loops: monolithic and consisting of two parallel filaments.

Investigating coronal loop morphology and dynamics from two vantage points

Coronal loops are the fundamental building blocks of the solar corona. Therefore, comprehending their properties is essential in unraveling the dynamics of the upper solar atmosphere. In this study, we conduct a comparative analysis of the morphology and dynamics of a coronal loop observed from two different spacecraft: the High Resolution Imager (HRIEUV) of the Extreme Ultraviolet Imager on board the Solar Orbiter, and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. These spacecraft were separated by 43° during this observation. The main findings of this study are that (1) the observed loop exhibits similar widths in both the HRIEUV and AIA data, suggesting that the cross-sectional shape of the loop is circular; (2) the loop maintains a uniform width along its entire length, supporting the notion that coronal loops do not exhibit expansion; and (3) notably, the loop undergoes unconventional dynamics, including thread separation and abrupt downward movement. Intriguingly, these dynamic features also appear similar in data from both spacecraft. Although based on observation of a single loop, these results raise questions about the validity of the coronal-veil hypothesis and underscore the intricate and diverse nature of the complexity within coronal loops.

High-frequency dissipative MHD waves in straight magnetic cylindrical plasma: Coronal loops heating application

The theoretical model for analyzing the waves and oscillatory behavior in the structured solar corona using straight magnetic cylindrical geometry filled with uniform low-β plasma has been recognized as the most preferable classical model for the last few decades. A number of observations, since the first observation of the transition region and coronal explorer to the latest ones, have been adequately explained by adopting this model. In order to analytically formulate the oscillatory characteristics of magnetohydrodynamic (MHD) waves, most of the studies have considered the nature of plasma as an ideal fluid, particularly in the context of solar physics. However, a departure from ideal plasma consideration to non-ideal may lead to a number of modifications in the characteristics of the MHD waves, including its damping too. In what follows, we derive a more general analytical dispersion relation by extending the classical dispersion relation of [Edwin and Roberts, “Wave propagation in a magnetic cylinder,” Sol. Phys. 88, 179–191 (1983)] taking into account the effect of plasma viscosity as a non-ideal term in the existing formulations of the classical model. Consequently, the effects of viscosity on the damping of sausage and kink modes are examined in detail. Multiple trapped body waves of different frequencies exist for both kink and sausage modes in which trapped sausage body wave of comparatively high frequency is damped potentially to generate enough energy to balance the radiative losses of the coronal loop regions. For the coronal loop's plasma parameters, it is found that trapped first radial overtone body wave of sausage type is able to balance the radiative losses of coronal loop structure provided magnetic field strength does not exceed its value of more than 20G.

Spectroscopic Observations of Coronal Rain Formation and Evolution Following an X2 Solar Flare

A significant impediment to solving the coronal heating problem is that we currently only observe active region loops in their cooling phase. Previous studies showed that the evolution of cooling loop densities and apex temperatures is insensitive to the magnitude, duration, and location of energy deposition. Still, potential clues to how energy is released are encoded in the properties of the cooling phase. The appearance of coronal rain, one of the most spectacular phenomena of the cooling phase, occurs when plasma has cooled below 1 MK, which sets constraints on the heating frequency, for example. Most observations of coronal rain have been made by imaging instruments. Here we report rare Hinode/EUV Imaging Spectrometer (EIS) observations of a loop arcade where coronal rain forms following an X2.1 limb flare. A bifurcation in plasma composition measurements between photospheric at 1.5 MK and coronal at 3.5 MK suggests that we are observing postflare-driven coronal rain. Increases in nonthermal velocities and densities with decreasing temperature (2.7–0.6 MK) suggest that we are observing the formation and subsequent evolution of the condensations. Doppler velocity measurements imply that a 10% correction of apparent flows in imaging data is reasonable. Emission measure analysis at 0.7 MK shows narrow temperature distributions, indicating coherent behavior reminiscent of that observed in coronal loops. The limitations on spatio-temporal resolution of EIS suggest that we are observing the largest features or rain showers. These observations provide insights into the heating rate, source, turbulence, and collective behavior of coronal rain from observations of the loop cooling phase.

Are Coronal Loops Projection Effects?

We report results of an in-depth numerical investigation of three-dimensional projection effects that could influence the observed loop-like structures in an optically thin solar corona. Several archetypal emitting geometries are tested, including collections of luminous structures with circular cross sections of fixed and random size, and light-emitting structures with highly anisotropic cross sections, as well as two-dimensional stochastic current density structures generated by fully developed magnetohydrodynamic turbulence. A comprehensive set of statistical signatures is used to compare the line-of-sight (LOS) integrated emission signals predicted by the constructed numerical models with the loop profiles observed by the extreme ultraviolet telescope on board the flight 2.1 of the High-Resolution Coronal Imager (Hi-C). The results suggest that typical cross-sectional emission envelopes of the Hi-C loops are unlikely to have high eccentricity, and that the observed loops cannot be attributed to randomly oriented quasi-two-dimensional emitting structures, some of which would produce anomalously strong optical signatures due to an accidental LOS alignment, as expected in the ''coronal veil“ scenario proposed recently by Malanushenko et al. The possibility of apparent loop-like projections of very small (close to the resolution limit) or very large (comparable with the size of an active region) light-emitting sheets remains open, but the intermediate range of scales commonly associated with observed loop systems is most likely filled with true quasi-one-dimensional (roughly axisymmetric) structures embedded into the three-dimensional coronal volume.

Identifying footpoints of pre-eruptive and coronal mass ejection flux ropes with sunspot scars

Context. The properties of pre-eruptive structures and coronal mass ejections (CMEs) are characterized by those of their footpoints, the latter of which attract a great deal of interest. However, the matter of how to identify the footpoints of pre-eruptive structures and how to do so with the use of ground-based instruments still remains elusive. Aims. In this work, we study an arc-shaped structure intruding in the sunspot umbra. It is located close to the (pre-)eruptive flux rope footpoint and it is expected to help identify the footpoint. Methods. We analyzed this arc-shaped structure, which we call a “sunspot scar”, in a CME event on July 12, 2012, and in two CME events from observationally inspired magnetohydrodynamic simulations performed by OHM and MPI-AMRVAC. Results. The sunspot scar displays a more inclined magnetic field with a weaker vertical component and a stronger horizontal component relative to that in the surrounding umbra and is manifested as a light bridge in the white light passband. The hot field lines anchored in the sunspot scar are spatially at the transition between the flux rope and the background coronal loops and temporally in the process of the slipping reconnection which builds up the flux rope. Conclusions. The sunspot scar and its related light bridge mark the edge of the CME flux rope footpoint and particularly indicate the edge of the pre-eruptive flux rope footpoint in the framework of “pre-eruptive structures being flux ropes”. Therefore, they provide a new perspective for the identification of pre-eruptive and CME flux rope footpoints, as well as new methods for studying the properties and evolution of pre-eruptive structures and CMEs with photospheric observations only.

The Maxillary Artery Loops Dangerously Approaching the Mandible's Lingula and Inferior Orbital Fissure.

The maxillary artery (MA) leaves from the external carotid artery and further courses through the infratemporal fossa to reach the pterygopalatine fossa. It typically has an ascending course either on the lateral or on the medial side of the inferior bundle of the lateral pterygoid muscle. While studying retrospectively, the computed tomography angiogram of an adult male case with peculiar tortuosities of the MA was documented. The left MA described a large inferior sagittal loop applied on the mandible's ramus's inner cortical in the pterygomandibular space's upper part. That loop reached 1.66 cm inferior to the mandible's notch and 0.5 cm superior to the mandibular foramen and was applied on the mandible's lingula (spine of Spix). That MA continued to the pterygomaxillary suture, and a second superior coronal loop was found applied on the posterior wall of the maxillary sinus, reaching the inferior orbital fissure. It is therefore mandatory to take good care in the decortication of the mandibular ramus or during osteotomies of the ramus. When an inferior loop of the MA approaches the spine of Spix, bleeding when an inferior alveolar nerve block is performed seems hardly avoidable.

Deep learning-based solar image captioning

Solar images are essential for identifying and predicting solar phenomena, and have been used as key information for analyzing space weather. In this paper, we propose a solar image captioning method that applies a transformer-based deep learning (DL) natural language processing method. In addition, we provide a new DeepSDO description dataset for training solar image captioning models. First, we develop the DeepSDO description dataset using solar image data from Korean Data Center for solar dynamics observatory (SDO) and scripts from the National Aeronautics and Space Administration (NASA) SDO gallery website. The DeepSDO description dataset includes nine solar events: sunspots, flares, prominences, prominent eruptions, coronal holes, coronal loops, filaments, active regions, and eclipses. Second, we train the DL-based image captioning model, the meshed-memory transformer, using the DeepSDO description dataset. The experimental results show that the proposed method outperforms other benchmark methods in terms of four evaluation metrics. This study demonstrates that DL-based image captioning can successfully generate solar image captions for multiple solar features, and could potentially be used in other themes of solar physics and space weather.

Transverse MHD Waves as Signatures of Braiding-induced Magnetic Reconnection in Coronal Loops

A major coronal heating theory based on magnetic reconnection relies on the existence of braided magnetic field structures in the corona. In this small-angle reconnection scenario, numerical simulations indicate that the reconnected magnetic field lines are driven sideways by magnetic tension and can overshoot from their new rest position, thereby leading to low-amplitude transverse MHD waves. This provides an efficient mechanism for transverse MHD wave generation, and the direct causality also constitutes substantial evidence of reconnection from braiding. However, this wave-generation mechanism has never been directly observed. Recently, the telltale signature of small-angle reconnection in a sheared coronal structure has been identified through nanojets, which are small, short-lived, and fast jetlike bursts in the nanoflare range transverse to the guide field. We present for the first time Interface Region Imaging Spectrograph and Solar Dynamics Observatory observations of transverse MHD waves in a coronal loop that directly result from braiding-induced reconnection. The reconnection is identified by the presence of nanojets at the loop apex that release nanoflare-range energy. We find that the oscillations have an energy flux on the order of 106–108 erg cm−2 s−1, which is within the budget to power active region loops. The estimated kinetic and thermal energy from the nanojets is also sufficient to power the transverse waves and sustain the observed heating at the loop apex. This discovery provides major support to (a) existing theories that transverse MHD waves can be a signature of reconnection, (b) the existence of braiding in coronal structures, and (c) the coronal reconnection scenario identified by nanojets.

Exploring Magnetic Coupling of Solar Atmosphere Through Frequency Modulations of 3-min Slow Magnetoacoustic Waves

Coronal fan loops rooted in sunspot umbra show outward propagating waves with subsonic phase speed and a period around 3 min. However, their source region in the lower atmosphere is still ambiguous. We performed multi-wavelength observations of a clean fan loop system rooted in a sunspot utilizing data from the Interface Region Imaging Spectrograph (IRIS) and the Solar Dynamics Observatory (SDO). We explored the less studied property of frequency modulation of these 3-min waves from the photosphere to the corona, and identified periodicities in the ranges of 14-20 min, and 24-35 min. Based on our findings, we interpret that the 3-min slow waves observed in the coronal fan loops are driven by 3-min oscillations observed at the photospheric footpoints of these fan loops in the umbral region. We also investigated possible connections between the 3-min and 5-min oscillations observed at the photosphere, which are still poorly understood. Our results provide clear evidence of magnetic coupling of the solar umbral atmosphere through the propagation of 3-min waves along the fan loops at different atmospheric heights.

Identifying Alfvén wave modes in the solar corona

Context. Oscillations are observed to be pervasive throughout the solar corona, but it remains challenging to positively identify different wave modes. Improving this identification would provide a powerful tool for investigating coronal wave heating and improving seismological inversions. Aims. We aim to establish whether theoretical methods used to identify magnetohydrodynamical wave modes in numerical simulations can be employed on observational datasets. Methods. We applied wave identifiers based on fundamental wave characteristics such as compressibility and direction of propagation to a fully 3D numerical simulation of a transversely oscillating coronal loop. The same wave identifiers were applied to the line-of-sight integrated synthetic emission derived from the numerical simulation data to investigate whether this method could feasibly be useful for observational studies. Results. We established that for particular line(s) of sight and assumptions about the magnetic field, we can correctly identify the properties of the Alfvén mode in synthetic observations of a transversely oscillating loop. Under suitable conditions, there is a strong agreement between the simulation and synthetic emission results. Conclusions. For the first time, we have provided a proof of concept that this theoretically derived classification of magnetohydrodynamic wave modes can be applied to observational data.

Observational Characteristics of Solar EUV Waves

Extreme-ultraviolet (EUV) waves are one of the large-scale phenomena on the Sun. They are defined as large propagating fronts in the low corona with speeds ranging from a few tens km s−1 to several 1000 km s−1. They are often associated with solar filament eruptions, flares, and/or coronal mass ejections (CMEs). EUV waves exhibit various features, including wave and non-wave components, stationary fronts, reflection, refraction, and mode conversion. Additionally, they can hit the nearby coronal loops and filaments/prominences during their propagation, triggering oscillations. These oscillations in loops and filaments/prominences enable us to diagnose coronal parameters such as the strength of the coronal magnetic field. In this article, we present the different observed features of the EUV waves along with existing models.

Detection of decayless oscillations in solar transition region loops

Context. Decayless kink oscillations have been frequently observed in coronal loops, serving as a valuable diagnostic tool for the coronal magnetic field. Such oscillations have never before been reported in low-lying loops of the transition region (TR). Aims. The aim of this study is to detect decayless kink oscillations in TR loops for the first time. Methods. We used the SI IV 1400 Å imaging data obtained from the Interface Region Imaging Spectrograph. We applied the Multiscale Gaussian Normalization method to highlight the TR loops, and generated time–distance maps to analyse the oscillation signals. Results. Seven oscillation events detected here exhibit a small but sustained displacement amplitude (0.04–0.10 Mm) for more than three cycles. Their periods range from 3 to 5 min. The phase speed is found to increase with loop length, which is consistent with the decrease in Alfvén speed with height. With these newly detected oscillations, we obtain a rough estimate of the magnetic field in the transition region, which is about 5–10 G. Conclusions. Our results further reveal the ubiquity of decayless kink oscillations in the solar atmosphere. These oscillations in TR loops have the potential to be a diagnostic tool for the TR magnetic field.

Decayless oscillations in 3D coronal loops excited by a power-law driver

Aims. We studied the manifestation of decayless oscillations in 3D simulations of coronal loops, driven by random motions. Methods. Using the PLUTO code, we ran magnetohydrodynamic (MHD) simulations of a straight gravitationally stratified flux tube, with its footpoints embedded in chromospheric plasma. We consider transverse waves drivers with a horizontally polarised red noise power-law spectrum. Results. Our broadband drivers lead to the excitation of standing waves with frequencies equal to the fundamental standing kink mode and its harmonics. These standing kink oscillations have non-decaying amplitudes, and spectra that depend on the characteristics of the loops, with the latter amplifying the resonant frequencies from the drivers. We thus report for the first time in 3D simulations the manifestation of decayless oscillations from broadband drivers. The spatial and temporal evolution of our oscillation spectra reveals the manifestation of a half harmonic, which exhibits half the frequency of the identified fundamental mode with a similar spatial profile. Our results suggest that this mode is related to the presence of the transition region in our model and could be interpreted as being the equivalent to the fundamental mode of standing sound waves driven on pipes closed at one end. Conclusions. The potential existence of this half harmonic has important implications for coronal seismology, since misinterpreting it for the fundamental mode of the system can lead to false estimations of the average kink speed profile along oscillating loops. Finally, its detection could potentially give us a tool for distinguishing between different excitation and driving mechanisms of decayless oscillations in observations.

Temperature inversion in a gravitationally bound plasma: Case of the solar corona

The temperature of the solar atmosphere increases from thousands to millions of degrees moving from the lower layer (chromosphere) to the outermost one (corona), while the density drops accordingly. The mechanism behind this phenomenon, known as a temperature inversion, is still unknown. In this work, we model a coronal loop as a collisionless plasma confined in a semicircular tube that is subject to the Sun’s gravity and in thermal contact with a fully collisional chromosphere behaving as a thermostat at the loop’s feet. By using kinetic N-particle simulations and analytical calculations, we show that rapid, intermittent, and short-lived heating events in the chromosphere drive the coronal plasma towards a non-equilibrium stationary state. The latter is characterized by suprathermal tails in the particles’ velocity distribution functions, exhibiting temperature and density profiles strikingly similar to those observed in the atmosphere of the Sun. These results suggest that a million-Kelvin solar corona can be produced without the local deposition of heat in the upper layer of the atmosphere that is typically assumed by standard approaches. We find that suprathermal distribution functions in the corona are self-consistently produced instead of postulated a priori, in contrast to classical kinetic models based on a velocity filtration mechanism.

Plantar Thermography in High-Risk Patients With Diabetes Mellitus Compared to Nondiabetic Individuals.

Diabetic foot (DF) is one of the most devastating complications of diabetes mellitus (DM). Infrared thermography has been studied for its potential in early diagnosis and preventive measures against DF ulcers, although its role in the management and prevention of DF complications remains uncertain. The objective of this study was to determine the average temperatures of different points of the plantar foot using infrared thermography in patients with DM and history of DF (DFa group, at the highest risk of developing foot ulcers) and compare them to people without DM (NoDM group). One hundred and twenty-three feet were included, 63 of them belonged to DFa Group and the other 60 to NoDM Group. The average temperature in the NoDM Group was 27.4 (26.3-28.5) versus 28.6 (26.8-30.3) in the DFa Group (p = .002). There were differences between both groups in temperatures at the metatarsal heads and heels, but not in the arch. Average foot temperatures did not relate to sex, ankle-brachial index, and age, and had a mild correlation with daily temperature (Spearman 0.51, p < .001). Data provided in our study could be useful in establishing a parameter of normal temperatures for high-risk patients. This could serve as a foundational framework for future research and provide reference values, not only for preventative purposes, as commonly addressed in most studies, but also to assess the applicability of thermography in clinical scenarios particularly when one foot cannot serve as a reference, suspected osteomyelitis of the remaining bone, or instances of increased temperature in specific areas which may necessitate adjustments to the insoles in secondary prevention.

Coronal Loop Detection Using Multiscale Convolutional Neural Networks

Solar magnetic fields play an important role in many solar activities, such as the solar wind, coronal mass ejections, and coronal oscillation. Coronal loops are curvilinear structures in the solar atmosphere and are closely related to coronal magnetic fields, so the study of their structure is very important. However, it is difficult to identify coronal loops accurately because of the complexity of their features. Therefore, we propose a two-stage detection method, using multiscale convolutional neural networks, to identify coronal loops. The regions including initial coronal loops are first marked by a improved Res-UNet model. The loop structures in the region are then detected using a improved dense extreme inception network for edge detection model. We selected the coronal images observed by the Transition and Coronal Explorer and the Atmospheric Imaging Assembly of the Solar Dynamics Observatory in the 171 Å channel to illustrate the detection processing. Meanwhile, we also compared the accuracy of our method to others. The results demonstrate that our proposed method has a high recognition rate and good robustness over previous identification methods and can be used to study the physical characteristics of coronal loops.

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.

Observational signature of continuously operating drivers of decayless kink oscillation

Context. Decayless kink oscillations, which are nearly omnipresent in the solar corona, are believed to be driven by continuously operating energy supply. Aims. In this Letter, we investigate an external continuous excitation of an apparent decayless oscillation during an X1.1 flare on June 20, 2023 (SOL2023-06-20T16:42). Methods. The decayless kink oscillation was identified in the coronal loop at extreme ultraviolet (EUV) wavelengths and the associated flare quasi-periodic pulsations (QPPs) were simultaneously observed in passbands of hard X-ray (HXR), microwave, and ultraviolet (UV) emissions. Results. The kink oscillation is detected as a transverse oscillation of the coronal loop, which reveals five apparent cycles with an average period of about 130 ± 10 s. The oscillation amplitude does not show any significantly decay, suggesting a decayless oscillation. At the same time, the solar flare occurs in the vicinity of the oscillating loop and exhibits five main pulses in HXR, microwave, and UV emissions, which could be regarded as flare QPPs. They have similar periods of about 100–130 s, which may indicate successive and repetitive energy releases during the flare impulsive phase. The peak of each loop oscillation cycle appears to follow the pulse of the QPPs, suggesting that the transverse oscillation is closely associated with flare QPPs. Conclusions. Our observations support the scenario where the repetitive energy released following flare QPPs could be invoked as external, continuously operating drivers of the apparent decayless kink oscillation.