Propagating slow magnetoacoustic waves have long been observed in the corona and their use for magnetohydrodynamic seismology is growing. While these waves are thought to be generated by lower atmospheric oscillations, the exact mechanisms behind their generation and propagation remain unclear. This study aims to investigate the fine structure and drivers of slow waves through high-resolution solar observations. We observed slow magnetoacoustic waves with three distinct periodicities along sunspot-anchored coronal fan feathers using data from the Atmospheric Imaging Assembly. The waves were tracked for 3 h in active region 13100 on 2022 September 19. Time-distance analysis was used to determine wave periods and projected phase speeds, and Fourier analysis for period intensity maps. We detect distinct periods of 3.06 +/- 0.04, 2.47 +/- 0.02, and 2.82 +/- 0.02 min in three feathers of a coronal fan. Increased intensity of chromospheric oscillations with the same distinct periods was observed in the 304 & Aring; channel in the umbral region where the feathers are anchored, which suggests that those locations could be footpoints of the specific feathers. These results indicate that propagating slow waves exhibit fine structuring in their oscillation periods. The specific period is a unique signature of each feather, which can be utilized for seismological diagnostics of the local coronal magnetic geometry, in stereoscopic observations.

A hybrid machine learning model, which combines a shallow convolutional neural network and a long short-term memory (CNN-LSTM) network, has been developed to automate the detection of kink oscillations in coronal plasma loops within large volumes of high-cadence sequences of imaging data. The network was trained on a set of 10,000 synthetic data cubes designed to mimic sequences of coronal images, achieving an accuracy greater than 98% on this synthetic data set. The model was then applied to detect kink oscillations in real data cubes of coronal active regions observed with Solar Dynamics Observatory/Atmospheric Imaging Assembly in the 171 Å channel. This data set consisted of 50 samples with visually detected kink oscillations and 128 samples without. Each sample covered an area of 260 × 260 pixels in the spatial domain and a duration of 30 minutes with a 12 s cadence in the time domain. Both off-limb and on-disk regions of interest were used. The data were preprocessed by median filtering in the time domain, and Gaussian smoothing and contrast-limited adaptive histogram equalization in the spatial domain. In the real data set, the performance of the model was 83.7%. The model is fully available in open access. We regard the CNN-LSTM model developed as a first step toward creating robust tools for routine solar coronal data mining in the context of coronal oscillation studies.

Societal dependence on space-based services demands major advances in predicting the impacts of eruptive solar events. Millimeter-wavelength observations offer uniquely direct access to the time-dependent physical conditions in the atmospheric layers of the Sun where these events originate. A facility capable of full-disk, high-cadence, multi-frequency imaging would provide a transformative view of the Sun and its influence on the heliosphere. AtLAST is ideally suited to deliver this capability, and to establish a European leadership role in advancing the scientific foundations that will enable reliable, operational space-weather forecasting for the first time.

A hybrid machine learning model which combines a shallow convolutional neural network and a long short-term memory network (CNN–LSTM), has been developed to automate the detection of kink oscillations in coronal plasma loops within large volumes of high-cadence sequences of imaging data. The network was trained on a set of 10,000 synthetic data cubes designed to mimic sequences of coronal images, achieving an accuracy greater than 98% on this synthetic dataset. The model was then applied to detect kink oscillations in real data cubes of coronal active regions observed with SDO/AIA in the 171 Å channel. This dataset consisted of 50 samples with visually detected kink oscillations and 128 samples without. Each sample covered an area of 260×260 pixels in the spatial domain and a duration of 30 min with a 12 s cadence in the time domain. Both off-limb and on-disk regions of interest were used. The data were pre-processed by median filtering in the time domain, and Gaussian smoothing and Contrast Limited Adaptive Histogram Equalization in the spatial domain. In the real dataset, the performance of the model was 83.7%.The model is fully available in open access. We regard the CNN–LSTM model developed as a first step toward creating robust tools for routine solar coronal data mining in the context of coronal oscillation study.

The response of a solar coronal plasma loop to an impulsive perturbation, demonstrating the simultaneous formation of guided and leaky fast magnetoacoustic wave trains, is numerically studied. The equilibrium magnetic configuration corresponds to a potential arcade. A loop is modeled by filling a bent magnetic flux bundle with a plasma denser than in the surrounding. The plasma parameter β is much lower than unity everywhere in the computational domain. The initial perturbation is a pulse of the plasma velocity of sausage symmetry. The extent of the velocity pulse along the field is about the minor diameter of the loop. The perturbation generates both a guided fast-mode disturbance along the loop and leaky fast waves that propagate outward into the ambient corona. Both guided and leaky fast waves are quasiperiodic. The external component experiences refraction, which, in the chosen equilibrium, turns the wave toward the vertical. The guided component consists of a leading dispersive precursor, composed predominantly of higher harmonics, followed by a trailing wave train, dominated by the fundamental harmonic. In the external medium, the higher harmonic precursor generates leaky fast perturbations, while perturbations in the trailing wave train are evanescent. Our results confirm that both narrow and broad quasiperiodic fast-propagating waves detected in the corona can be excited by the same spatially localised impulsive energy deposition. The narrow counterpart is associated with the guided component, while the broad component is caused by the leaky component.

A coronal loop of 290 Mm length, observed at 171 Å with Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly on 2024 February 6 near active region 13571, is found to oscillate with two significantly different oscillation periods, 48.8 ± 6.1 and 4.8 ± 0.3 minutes. The oscillations occur in the time intervals without detected flares or eruptions. Simultaneously, near the northern footpoint of the oscillating loop, we detect a 49.6 ± 5.0 minute periodic variation of the average projected photospheric magnetic field observed with SDO/Helioseismic and Magnetic Imager. The shorter-period decayless oscillation is attributed to the eigenmode, standing kink oscillation of the loop, while the longer-period oscillation may be the oscillatory motion caused by the periodic footpoint driver. The photospheric long-period process can also drive the short-period, eigen oscillation of the loop via the self-oscillatory “violin” mechanism, in which a transverse oscillation is excited by an external quasi-steady flow. This finding indicates that the most powerful, lower-frequency spectral components of photospheric motions, which are well below the Alfvénic/kink cutoff, can reach the corona.

The transition of an impulsively excited kink oscillation of a solar coronal loop to an oscillation with a stationary amplitude, i.e., the damping pattern, is determined using the low-dimensional self-oscillation model. In the model, the decayless kink oscillations are sustained by the interaction of the oscillating loop with an external quasi-steady flow. The analytical solution is based on the assumption that the combined effect of the effective dissipation, for example, by resonant absorption, and interaction with an external flow, is weak. The effect is characterised by a dimensionless coupling parameter. The damping pattern is found to depend upon the initial amplitude and the coupling parameter. The approximate expression shows a good agreement with a numerical solution of the self-oscillation equation. The plausibility of the established damping pattern is demonstrated by an observational example. Notably, the damping pattern is not exponential, and the characteristic decay time is different from the time determined by the traditionally used exponential damping fit. Implications of this finding for seismology of the solar coronal plasmas are discussed. In particular, it is suggested that a very rapid, in less than the oscillation period, decay of the oscillation to the stationary level, achieved for larger values of the coupling parameter, can explain the relative rareness of the kink oscillation events.

Numerical simulations of a propagating slow magnetoacoustic wave guided by a field-aligned low- β plasma nonuniformity are performed in terms of ideal magnetohydrodynamics, aiming at modeling propagating extreme ultraviolet (EUV) emission disturbances observed in the solar corona. The perpendicular profiles of the equilibrium density and temperature are smoothly nonuniform, resulting in smoothly nonuniform profiles of the sound and tube speeds. It is found that an initially plane wavefront perpendicular to the magnetic field experiences a growing deformation with the distance from the driver. The segments of the wavefront located at higher sound speed regions propagate along the field faster. This results in progressively increasing phase mixing. At some distance from the wave driver, at a certain perpendicular cross-section of the nonuniformity, there are opposite phases of the wave. As local perpendicular phase and group speeds are opposite to each other, the slow wave energy tends towards regions of the higher local sound speed. This effect increases with the increase in the plasma- β . Thus, plasma nonuniformities with temperature decreases are slow magnetoacoustic anti-waveguides, while those with temperature increases are waveguides. In the optically thin radiation regime, typical for the EUV emission from the solar corona, phase mixing of slow waves leads to apparent damping of the waves. This damping is not connected with any dissipative process, and is caused by the destructive interference of slow perturbations with different phases, integrated along the line of sight. The apparent damping depends on the combination of magnetic-field strengths, plasma- β , and viewing angles. This effect could be responsible for nonsystematic dependencies of the damping length upon the oscillation periods and the plasma temperature, appearing in observations.

Analysis of more than 300 M-class solar flares observed with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory in the 131 Å channel, revealed 16 events of sloshing oscillations in hot solar coronal loops. Time–distance maps made along the loops demonstrated EUV emission intensity blobs bouncing between the footpoints, i.e., showing characteristic zigzagging patterns, of the size shorter than 25

Quasiperiodic pulsations (QPP) are often detected in solar and stellar flare lightcurves. These events may contain valuable information about the underlying fundamental plasma dynamics as they are not described by the standard flare model. The detection of QPP signals in flare lightcurves is hindered by their intrinsically nonstationary nature, contamination by noise, and the continuously increasing number of flare observations. Hence, the creation of automated techniques for QPP detection is imperative. We implemented the fully convolution network (FCN) architecture to classify the flare lightcurves depending on whether they have exponentially decaying harmonic QPP or not. To train the FCN, 90,000 synthetic flare lightcurves with and without QPP were generated. After training, it showed an accuracy of 87.2% on the synthetic test data and did not experience overfitting. To test the FCN performance on real data, we used the subset of stellar flare lightcurves observed by Kepler, with strong evidence of decaying QPP identified hitherto with other methods. Then, the FCN was applied to find QPP in a larger-scale Kepler flare catalogue comprised of 2274 events, resulting in a 7% QPP detection rate with a probability above 95%. The FCN, implemented in Python, is accessible through a browser application with a user-friendly graphical interface and detailed installation and usage guide. The obtained results demonstrate that the developed FCN performs well and successfully detects exponentially decaying harmonic QPP in real flare data, and can be used as a tool for preliminary sifting of the QPP events of this type in future large-scale observational surveys.

Sharp boundaries are a key feature of space plasma environments universally, with their wave-like motion (driven by pressure variations or flow shears) playing a key role in mass, momentum, and energy transfer. This review summarises magnetohydrodynamic surface wave theory with particular reference to Earth’s magnetopause, due to its mediation of the solar-terrestrial interaction. Basic analytic theory of propagating and standing surface waves within simple models are presented, highlighting many of the typically-used assumptions. We raise several conceptual challenges to understanding the nature of surface waves within a complex environment such as a magnetosphere, including the effects of magnetic topology and curvilinear geometry, plasma inhomogeneity, finite boundary width, the presence of multiple boundaries, turbulent driving, and wave nonlinearity. Approaches to gain physical insight into these challenges are suggested. We also discuss how global simulations have proven a fruitful tool in studying surface waves in more representative environments than analytic theory allows. Finally, we highlight strong interdisciplinary links with solar physics which might help the magnetospheric community. Ultimately several upcoming missions provide motivation for advancing magnetopause surface wave theory towards understanding their global role in filtering, accumulating, and guiding turbulent solar wind driving.

Context. Recent observations of decayless transverse oscillations have revealed two branches in the relationship between period and loop length. One is a linear relationship, interpreted as a standing mode, while the other shows almost no correlation and has not yet been interpreted conclusively. Aims. We investigated the undersampling effect on observed periods of decayless oscillations. Methods. We considered oscillating coronal loops that closely follow the observed loop length distribution. Assuming that all oscillations are standing waves, we modelled a signal that represents decayless oscillations where the period is proportional to the loop length and the amplitude and phase are randomly drawn. We generated a downsampled signal from the original signal by considering different sample rates that mimic temporal cadences of telescopes, and analysed the periods for sampled signals using the fast Fourier transform. Results. When the sampling cadence approaches the actual oscillation period, there is a greater tendency to overestimate the periods in short loops. We find the same two branches in the relationship between loop length and period of the sampled signals as those seen in the observations. Conclusions. We find that long periods of decayless oscillations occurring in short loops could be the result of undersampling.

The highly-filamented nature of the coronal plasma significantly influences dynamic processes in the corona such as magnetohydrodynamic waves and oscillations. Fast magnetoacoustic waves, guided by coronal plasma non-uniformities, exhibit strong geometric dispersion, forming quasi-periodic fast-propagating (QFP) wave trains. QFP wave trains are observed in extreme-ultraviolet imaging data and indirectly in microwaves and low-frequency radio, aiding in understanding the magnetic connectivity, energy, and mass transport in the corona. However, measuring the field-aligned group speed of QFP wave trains, as a key parameter for seismological analysis, is challenging due to strong dispersion and associated rapid evolution of the wave train envelope. We demonstrate that the group speed of QFP wave trains formed in plane low-β coronal plasma non-uniformities can be assessed through the propagation of the wave train's effective centre of mass, referred to as the wave train's centroid speed. This centroid speed, as a potential observable, is shown empirically to correspond to the group speed of the most energetic Fourier harmonic in the wave train. The centroid speed is found to be almost insensitive to the waveguide density contrast with the ambient corona, and to vary with the steepness of the transverse density profile. The discrepancy between the centroid speed as the group speed measure and the phase speed at the corresponding wavelength is shown to reach 70%, which is crucial for the energy flux estimation and interpretation of observations.

ABSTRACT Simultaneous observations of a propagating disturbance of EUV intensity, with SDO/AIA at 171 Å and SolO/EUI-HRIEUV at 174 Å, are investigated. The disturbance moves outwards along a plasma fan structure in active region AR 12941 on February 7th 2022, at 12:45–14:15 UT. The spacecraft line-of-sight separation is 19°. The variation of the EUV intensity resembles an almost harmonic wave with an oscillation period of $2.7^{+0.1}_{-0.2}$ and $2.6^{+0.1}_{-0.1}$ min for AIA and HRIEUV, respectively. Over 30 oscillation cycles are detected. The wave originated at the footpoint of the fan, anchored in a sunspot. The projected phase speeds are 60.5 ± 5.2 and 74.4 ± 6.2 km s−1 in the AIA and HRIEUV data, respectively, determined by the cross-correlation technique. The observed parameters of the propagating EUV disturbance suggest its interpretation as a slow magnetoacoustic wave. Observations with AIA show that the wave decays with height, with a calculated e-folding length of $6.9^{+1.3}_{-0.8}$ Mm. In contrast, in the HRIEUV data, the propagating EUV disturbance is also seen much higher, with the e-folding length of $12.8^{+1.1}_{-1.7}$ Mm. This observation demonstrates, for the first time, that the apparent spatial damping of propagating slow waves depends on the observational instrument. Moreover, our work shows that the study of slow waves can be advanced with the use of HRIEUV, and multiple instruments with non-parallel lines of sight.

Mutual conversion of various kinds of magnetohydrodynamic (MHD) waves can have profound impacts on wave propagation, energy transfer, and heating of the solar chromosphere and corona. Mode conversion occurs when an MHD wave travels through a region where the Alfvén and sound speeds are equal (e.g., a 3D magnetic null point). Here we report the direct extreme ultraviolet (EUV) imaging of mode conversion from a fast-mode to a slow-mode MHD wave near a 3D null point using Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) observations. An incident fast EUV wavefront associated with an adjacent eruptive flare propagates laterally through a neighboring pseudostreamer. Shortly after the passage of the fast EUV wave through the null point, a slow-mode wave appears near the null that propagates upward along the open structures and simultaneously downward along the separatrix encompassing the fan loops of the pseudostreamer base. These observations suggest the existence of mode conversion near 3D nulls in the solar corona, as predicted by theory and MHD simulations. Moreover, we observe decaying transverse oscillations in both the open and closed structures of the pseudostreamer, along with quasiperiodic type III radio bursts indicative of repetitive episodes of electron acceleration.

Macroscopic wave and oscillatory phenomena ubiquitously detected in the plasma of the corona of the Sun are interpreted in terms of magnetohydrodynamic theory. Fast and slow magnetoacoustic waves are clearly distinguished in observations. Properties of coronal magnetohydrodynamic waves are determined by local parameters of the plasma, including the field-aligned filamentation typical for the corona. It makes coronal magnetohydrodynamic waves reliable probes of the coronal plasma structures by the method of magnetohydrodynamic seismology. For example, propagating slow waves indicate the local direction of the guiding magnetic field. Standing, sloshing and propagating slow waves can be used for probing the coronal heating function and the polytropic index. Kink oscillations of coronal plasma loops provide us with estimations of the absolute value of the magnetic field in oscillating plasma loops. This tutorial introduces several techniques of magnetohydrodynamic seismology of solar coronal plasmas. It includes the description of practical steps in the data acquisition, pre-processing, and processing using the open-access data of the Atmospheric Imaging Assembly on the Solar Dynamics Observatory spacecraft, and elaborated data analysis techniques of motion magnification and Bayesian statistics.

The energy balance in the corona of the Sun is the key to the long-standing coronal heating dilemma, which could be potentially revealed by observational studies of decayless kink oscillations of coronal plasma loops. A bundle of very long off-limb coronal loops with the length of [Formula: see text] Mm and a lifetime of about 2 days are found to exhibit decayless kink oscillations. The oscillations are observed for several hours. The oscillation amplitude is measured at 0.3-0.5 Mm, and the period at 28-33 min. The existence of 30-min periodicity of decayless kink oscillations indicates that the mechanism compensating the wave damping is still valid in such a massive plasma structure. It provides important evidence for the non-resonant origin of decayless kink oscillations with 2-6 min periods, i.e., the lack of their link with the leakage of photospheric and chromospheric oscillations into the corona and the likely role of the broadband energy sources. Magnetohydrodynamic seismology based on the reported detection of the kink oscillation, with the assistance of the differential emission measure analysis and a background coronal model provides us with a comprehensive set of plasma and magnetic field diagnostics, which is of interest as input parameters of space weather models.

Solar flares are among the most powerful and disruptive events in our solar system, however the physical mechanisms driving and transporting this energetic release are not fully understood. An important signature associated with flare energy release is highly variable emission on timescales of sub-seconds to minutes which often exhibit oscillatory behaviour, features collectively known as quasi-periodic pulsations (QPPs). To fully identify the driving mechanism of QPPs, exploit their potential as a diagnostic tool, and incorporate them into our understanding of solar and stellar flares, new observational capabilities and initiatives are required. There is a clear community need for flare-focused, rapid cadence, high resolution, multi-wavelength imaging of the Sun, with high enough sensitivity and dynamic range to observe small fluctuations in intensity in the presence of a large overall intensity. Furthermore, multidisciplinary funding and initiatives are required to narrow the gap between numerical models and observations. QPPs are direct signatures of the physics occurring in flare magnetic reconnection and energy release sites and hence are critical to include in a unified flare model. Despite significant modelling and theoretical work, no single mechanism or model can fully explain the presence of QPPs in flares. Moreover, it is also likely that QPPs fall into different categories that are produced by different mechanisms. At present we have insufficient information to observationally distinguish between mechanisms. The motivation to understand QPPs is strengthened by the geo-effectiveness of flares on the Earth's ionosphere, and by the fact that stellar flares exhibit similar QPP signatures. QPPs present a golden opportunity to better understand flare physics and exploit the solar-stellary analogy, benefiting both astrophysics, heliophysics, and the solar-terrestrial connection.

Decayless kink oscillations of plasma loops in the solar corona may contain an answer to the enigmatic problem of solar and stellar coronal heating. The polarisation of the oscillations gives us a unique information about their excitation mechanisms and energy supply. However, unambiguous determination of the polarisation has remained elusive. Here, we show simultaneous detection of a 4-min decayless kink oscillation from two non-parallel lines-of-sights, separated by about 104∘, provided by unique combination of the High Resolution Imager on Solar Orbiter and the Atmospheric Imaging Assembly on Solar Dynamics Observatory. The observations reveal a horizontal or weakly oblique linear polarisation of the oscillation. This conclusion is based on the comparison of observational results with forward modelling of the observational manifestation of various kinds of polarisation of kink oscillations. The revealed polarisation favours the sustainability of these oscillations by quasi-steady flows which may hence supply the energy for coronal heating.

Plumes are bright structures in coronal holes extending from the solar surface into the corona and are considered as a possible source of the solar wind. Plumes are thought to be rooted in strong unipolar photospheric flux patches (network/plage region). The magnetic activities at the base of plumes may play a crucial role in producing outflows and propagating disturbances (PDs). However, the role of photospheric/chromospheric activities (e.g., jets/spicules) at the base of plumes and their connection to PDs is poorly understood. Using high-resolution observations of a plume taken on 2020 July 23 with the 1.6 m Goode Solar Telescope (GST), Interface Region Imaging Spectrograph (IRIS), and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, we analyzed chromospheric/transition region activities at the base of the plume and their connection to outflows/PDs in the plume. The GST Visible Imaging Spectrometer images reveal repetitive spicules with blueshifted emission (pseudo-Doppler maps) at the plume’s footpoint. In addition, the photospheric magnetograms provide evidence of mixed polarities at the base of the plume. The IRIS Mg ii k Dopplergrams show strong blueshifted emission (∼50 km s−1) and a high brightness temperature (Mg ii k2 line) at the footpoint of the plume. The long-period PDs (P ≈ 20–25 minutes) along the plume (AIA 171 Å) match the periodicity of spicules in the chromospheric images, suggesting a close connection between the spicules and the PDs. We suggest that the interchange reconnection between the closed and open flux of the coronal bright point at the plume’s footpoint is the most likely candidate to produce upflows and associated PDs along the plume.