arXiv subjects

Valery M. Nakariakov

Publications and source records attributed to Valery M. Nakariakov.

At least 19 recordsLinked to original sources

arXiv2026

Catalogue of Solar Radio Bursts Detected by the LOFAR LV614 Station at the Irbene Observatory during Solar Cycle 25

A catalogue (https://doi.org/10.5281/zenodo.20157853) of solar radio bursts detected with the LV614 LOFAR station at the Irbene Observatory, Latvia, in the stand-alone mode from June 2022 to August 2025 in the 29-60 MHz frequency range is presented. Observations were conducted with the weekly cadence, one-second time resolution, and 0.195 MHz spectral resolution. The cumulative duration of the observation was 1125.2 h. For all analysed observations, bandpass calibration was applied using Cassiopeia A as a reference source. In total, the catalogue includes 335 solar radio bursts. Radio bursts of all five main types, I-V, were detected. Specifically, the catalogue contains parameters of 23, 8, 293, 6, and 5 bursts of Type I, II, III, IV, and V, respectively. The detected burst peak flux ranges from 0.3 SFU to 234 SFU. The catalogue provides information about the peak flux and frequency, and the estimated height of the source above the solar surface. For Type II and III bursts, we also give the estimations of the frequency drifts, with mean values $ 0.043 ~\mathrm{MHz\,s^{-1}}$ and $4.383 ~\mathrm{MHz\,s^{-1}}$, respectively. We also estimated the fit $df/dt = Af^a$, with the best fit parameters $A=0.78$ and $a=0.43$. The numbers of Type I and III events are found to decrease with the peak flux, with the power law indices about $-0.9$ and $-1.25$, respectively. For Type III, we estimated electron beam speed ranging from 0.005 to 0.477 of the speed of light for the fundamental frequency, and 0.008 to 0.794 of the speed of light for its second harmonic.

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arXiv2026

Oblique kink waves in solar coronal streamers

Coronal streamer kink waves excited by an impact of coronal mass ejections (CMEs) are routinely used to estimate the coronal Alfvén speed. These estimates generally assume wave propagation parallel to the equilibrium magnetic field directed radially outward to the Sun, in the plane of the sky. However, the geometry of CME impacts naturally allows for oblique propagation. We investigate the properties of obliquely propagating kink waves guided by a plasma slab with a central current sheet representing a coronal streamer stalk. We consider both cold and warm streamer plasmas and analyse influence of propagation obliquity on wave dispersion, eigenfunctions, and seismological diagnostics. We find that increasing the propagation obliquity reduces the phase speed owing to both projection effects and the intrinsic dependence of the phase speed on the propagation angle. The wave structure also undergoes a gradual transition from a body mode to a surface mode, leading to stronger perturbations at the slab boundaries and enhanced wave localization. Consequently, for a given energy deposited by a CME impact, oblique kink waves are expected to produce larger observable amplitudes than field-aligned waves. Applying the developed theory to streamer waves reported previously, we infer Alfvén speeds in the range of 50-500 km/s. We show that neglecting wave propagation obliquity may underestimate the Alfvén speed by up to a factor of 1.5. These results demonstrate that wave propagation obliquity should be taken into account in coronal seismology of streamer waves and that stereoscopic observations will be required to constrain the true propagation direction.

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arXiv2026

Hard X-Ray Quasi-Periodic Pulsations in X-Class Solar Flares Observed by Aditya-L1/HEL1OS

We present the catalogue of quasi-periodic pulsations (QPPs) detected in hard X-ray (HXR) observations of X-class solar flares obtained with the HEL1OS instrument onboard Aditya-L1 during July 2024 - March 2026. The catalogue comprises 34 flares spanning GOES classes X1.1 - X7.1. QPPs are detected and studied using a uniform procedure based on Ensemble Empirical Mode Decomposition (EEMD) and wavelet analyses. Statistically significant QPPs are identified in 25 events (~74%), demonstrating that QPPs are a common property of X-class flares. The detected periods range from approximately 1 to 3 min, with a pronounced concentration between 1.3 and 1.7 min, suggesting the existence of a preferential timescale that may be associated with kink or slow magnetohydrodynamic oscillations in the flaring region. The dominant periods remain nearly independent of photon energy across the 8 - 20 keV range, while the modulation depth increases systematically with photon energy following a power-law relation with an exponent of about 1.5. QPPs detected in different HXR energy channels exhibit strong phase coherence and positive cross-correlation coefficients, indicating modulation by a common physical mechanism. Several intense flares display HXR modulation depths of up to 25%, providing evidence for nonlinear effects. The observed statistical properties support models in which QPPs are generated by repetitive magnetic reconnection, either spontaneous or periodically modulated by magnetohydrodynamic oscillations. This study establishes HEL1OS as a powerful instrument for quantitative studies of flare QPPs during Solar Cycle 25.

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arXiv2026

The Study of Quasi-Periodic Pulsations in Solar and Stellar Flares with SKA

An intensively studied phenomenon which is not described by the standard flare model are quasi-periodic pulsations (QPP) of the flaring emission. As analysis of the QPP phenomenon intrinsically requires a combination of high time and spatial resolutions, especially in the radio band, the unprecedented capabilities of SKA offer us a unique opportunity to reach a breakthrough progress in the observational study of QPP. The SKA-Mid-frequency band falls in a unique window where both coherent emissions from particle acceleration sites and incoherent gyrosynchrotron emissions from non-thermal particles in coronal loops can be studied. With an additional polarisation dimension and the capability to perform wideband spectroscopic imaging, the QPPs in gyrosynchrotron emission ($\ge 1$~GHz) and plasma emission will help understand the local magnetic field modulation due to active phenomena and the response seen in the particle acceleration observable below ~600~MHz. An incomplete list of specific science questions to be addressed with SKA includes (a) the role of QPP in the energy partition in flares, (b) seismology of flaring sites by QPP of different classes, (c ) differences and similarities between QPP in solar and stellar flares, (d) advancing the standard flare model, (e) the physics of repetitive magnetic reconnection: spontaneous vs induced, (f) ML techniques in the detection, classification and analysis of QPP, (g) QPP in weak flares. The latter topic could be especially advanced with SKA which will allow for high-cadence high fidelity radio imaging of weak energy release events.

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arXiv2026

A universal scaling between damping time and period of quasi-periodic pulsations from solar EUV brightenings to X-ray stellar flares

Recent high spatial and temporal resolution extreme-ultraviolet (EUV) imaging observations have revealed that quasi-periodic pulsations (QPPs), a ubiquitous signature of impulsive energy release in solar and stellar flares, are also present in much smaller-scale coronal events known as EUV brightenings. Whether QPPs observed across such disparate spatial and energetic scales share a common physical origin remains an open question. Here we analyse 2,146 EUV brightenings observed with Solar Orbiter/EUI and 300 EUV solar flares observed with SDO/AIA, identifying 185 brightenings and 89 flares exhibiting statistically significant damped QPPs. We show that the relationship between damping time and oscillation period follows a common power-law scaling for EUV brightenings and EUV solar flares, consistent with previously reported X-ray QPPs spanning both solar and stellar flares. The persistence of this scaling over a wide range of energies and scales suggests that QPPs are governed by a common underlying physical mechanism.

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arXiv2026

Phase-drifting with emitting plasma temperature in the quasi-periodic pulsations of an X-class solar flare

Recent multi-wavelength observations of solar flares have provided new constraints on the physical origin of quasi-periodic pulsations (QPPs). In an X-class flare, we detect a short-lived $\sim$5-minute QPP simultaneously in hard X-rays, extreme-ultraviolet (EUV), and soft X-ray emissions, exhibiting a clear phase-drifting behavior with emitting plasma temperature. Based on phase-resolved timing analysis, it is found that (i) the QPPs in all diagnostics share nearly identical oscillation periods, (ii) a systematic temperature-dependent phase drifting is present, with the phase delay relative to the hard X-ray emission increases systematically from the hottest to cooler EUV channels, and (iii) the QPP persists for only a few cycles during the impulsive phase. These properties imply that periodic magnetic reconnection, possibly triggered by the leakage of 5-minute oscillations from the lower atmosphere, modulates the non-thermal electrons responsible for the leading Hard X-ray QPPs. Subsequently, plasma heating and cooling processes manifest sequentially across passbands with different temperature responses, resulting in the observed temperature-dependent phase drifting. These results provide novel observational evidence supporting the use of multi-temperature, multi-wavelength phase relationships to constrain the temporal evolution of flare energy release and the origins of QPPs.

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arXiv2026

Effect of Solar Flares on Decayless Kink Oscillations in Nearby Coronal Loops

We present a statistical study of 130 solar flares (B to X class) that lack soft X-ray quasi-periodic pulsations and show no kink oscillations of nearby coronal loops visible in SDO/AIA 171~Å~images. The aim is to investigate whether decayless kink oscillations of coronal loops respond to nearby flaring activity. Using the Fractional Anisotropy-based Video Motion Magnification technique, we detected low-amplitude decayless oscillations in all 130 loops before, during, and after each flare, confirming their ubiquitous nature. Oscillation periods are found to range from 122~s to 268~s, and the projected displacement amplitudes are 0.023--0.111~Mm. No amplitude--period correlation is found. For each event, we estimated the amplitude before, during, and after the flare. Across all flare classes, the average amplitude remains unchanged. However, in some specific cases, the oscillation amplitude may exhibit minor changes. For B-, C-, and M-class flares, the fraction of events with an amplitude change exceeding 10% is approximately 23%, 41%, and 36%, respectively. In M-class flares, such minor amplitude increases occur four times more often than decreases; in X-class flares (only six events), decreases dominate by a factor of three. The fraction of events that exhibit an increase in the amplitude of more than 20% appears to be highest when the loop centre is located at a distance of 100--120~Mm from the flare site, reaching 33% (6 out of 18 events). Overall, the amplitude of decayless kink oscillations does not undergo a major change in response to nearby flares, especially for less powerful classes, suggesting that flare-related processes such as blast waves and reconnection inflows have little effect on the energy supply to oscillating loops.

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arXiv2026

Nonlinear steepening of a fast magnetoacoustic wave in the vicinity of a coronal magnetic null point

The interaction of a fast magnetoacoustic wave with a magnetic null point is studied in the context of the sympathetic flare phenomenon. Attention is paid to steepening the wave caused by the finite-amplitude effects in a non-uniform plasma environment. The null point is modelled by a potential magnetic configuration without a guiding field. The equilibrium plasma density and temperature are taken to be constant. The fast wave is excited by an impulsive point source outside the distance at which the local Alfvén and sound speeds are equal to each other. The incoming fast wave approaches the null point along the bisector of the magnetic configuration, i.e., across the local field. The fast-speed non-uniformity around the null point causes the refraction of the incident fast wave. However, the segment of the incoming wave, which approaches the null point is locally plane. The decrease in the fast speed towards the null point amplifies the nonlinear deformation of the incoming wave. Hence, the fast wave can become subject to nonlinear dissipation at a distance from the null point and not reach it.

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arXiv2026

UK White Paper on Magnetohydrodynamic (MHD) seismology of solar and heliospheric plasmas

Magnetohydrodynamic (MHD) seismology uses naturally occurring MHD waves to infer plasma properties that are otherwise hard to measure, especially magnetic field strength and topology, electric currents, fine structuring, transport coefficients, and energy release. Across the solar atmosphere, heliosphere, and planetary magnetospheres, multi-wavelength remote sensing and in-situ observations of waves provide powerful diagnostics that can address major open problems including chromospheric and coronal heating, flare and eruption physics, solar wind acceleration, and space weather impacts. This White Paper sets out the case for a coordinated UK programme that couples high precision observations with advanced theory and numerical modelling, modern time-frequency methods for non-stationary signals, and machine learning approaches for detection, classification, and parameter inference from rapidly growing multi-instrument datasets. It outlines priority needs such as robust mode identification, reliable density and temperature constraints, multi line-of-sight capability, and models that include partial ionisation and non-adiabatic/collisionless effects, alongside enabling instrumentation such as next-generation spectropolarimetry, integral field units, and radio facilities including the Square Kilometre Array. The paper highlights the UK's strong track record and infrastructure, and argues that sustained investment will amplify UK scientific return through international partnerships and mission involvement, delivering transformative plasma diagnostics and downstream benefits for space weather forecasting and related applications.

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arXiv2026

Power-law Indices of EUV Intensity Power Spectrum in Flaring Coronal Active Regions

Solar intensity power spectra are usually characterised by coloured noise, with the spectral energy following a segmented power-law function of frequency, $S(f)\propto f^{-α}$, over different frequency ranges. Typically, the power-law index exceeds 1 in the low-frequency part ($α_\mathrm{lf}$) and is around 0 at high frequencies ($α_\mathrm{hf}$). This work investigates the spatial and temporal evolution of the power-law indices of coronal EUV intensity power spectra in flare-hosting active regions. The spatial distribution of the power-law index in the low-frequency domain ($α_\mathrm{lf}$) closely mirrors EUV intensity images, indicating that $α_\mathrm{lf}$ can reveal the dynamics of coronal plasma structures. Temporally, $α_\mathrm{lf}$ remains stable in quiescent active regions, but it exhibits significant variability before the flare onset. Motivated by this behaviour, we analysed 14 flare events, quantifying the temporal variation of the indices $α_\mathrm{lf}$ and $α_\mathrm{hf}$ as potential flare precursors. In all flare events considered, notable deviations of $α_\mathrm{lf}$ beyond a defined threshold consistently occurred at the flare site within a few minutes before the flare. In some cases, the change in the value of $α_\mathrm{lf} - α_\mathrm{hf}$ was detected within 30--90\, minutes before the flare. This proof-of-concept study suggests that the temporal variation of the power-law indices in coronal EUV intensity power spectra could potentially serve as short-term precursors of solar flares, which needs to be validated on a larger flare sample.

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arXiv2025

A 50-min coronal kink oscillation and its possible photospheric counterpart

A coronal loop of 290~Mm length, observed at 171~Å with SDO/AIA on February 6th 2024 near AR 13571, is found to oscillate with two significantly different oscillation periods, $48.8 \pm 6.1$~min and $4.8\pm 0.3$~min. The oscillations occur in the time intervals without detected flares or eruptions. Simultaneously, near the Northern footpoint of the oscillating loop, we detect a $49.6 \pm 5.0$-min periodic variation of the average projected photospheric magnetic field observed with SDO/HMI. The shorter-period decayless oscillation is attributed to the eigen-mode, 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, \lq\lq violin\rq\rq\, 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.

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arXiv2025

Detection of kink oscillations in solar coronal loops by a CNN-LSTM neural network

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$\times$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.

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arXiv2024

Undersampling effects on observed periods of coronal oscillations

Context. Recent observations of decayless transverse oscillations have shown two branches in the relationship between periods and loop lengths. One is a linear relationship, interpreted as a standing mode. 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 modeled a signal that represents decayless oscillations where the period is proportional to the loop length and the amplitude and phase are randomly drawn. A downsampled signal was generated from the original signal by considering different sample rates that mimic temporal cadences of telescopes, and periods for sampled signals were analysed using the fast Fourier transform. Results. When the sampling cadence is getting closer to the actual oscillation period, a tendency for overestimating periods in short loops is enhanced. The relationship between loop lengths and periods of the sampled signals shows the two branches as in the observation. Conclusions. We find that long periods of decayless oscillations occurring in short loops could be the result of undersampling.

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arXiv2024

Detecting quasi-periodic pulsations in solar and stellar flares with a neural network

Quasi-periodic 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 non-stationary nature, contamination by noise, and the continuously increasing amount 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 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 QPPs 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.

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arXiv2024

Transition from decaying to decayless kink oscillations of solar coronal loops

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.

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arXiv2024

Diagnostics of the solar coronal plasmas by magnetohydrodynamic waves: Magnetohydrodynamic seismology

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 the 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.

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arXiv2024

Direct Imaging of Magnetohydrodynamic Wave Mode Conversion Near a 3D Null Point on the Sun

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 first 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.

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arXiv2023

The centroid speed as a characteristic of the group speed of solar coronal fast magnetoacoustic wave trains

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.

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