arXiv subjects

Dmitrii Kolotkov

Publications and source records attributed to Dmitrii Kolotkov.

8 recordsLinked to original sources

arXiv2026

Entropy-mode imprints in the solar corona: non-exponential damping and phase shifts of compressive oscillations

Magnetohydrodynamic (MHD) waves in coronal loops provide key seismological diagnostics through their characteristic time signatures. While fast and slow magnetoacoustic modes are routinely exploited, the entropy mode, despite being another eigenmode of the system, remains largely inaccessible due to its non-propagating and non-oscillatory nature. We identify possible observable time-domain signatures of the entropy mode and its indirect effects. Our approach exploits the intrinsically non-adiabatic conditions of the solar corona, under which the entropy mode is closely linked to the compressive slow mode. We consider a one-dimensional coronal loop model with field-aligned thermal conduction, where standing slow and entropy modes are simultaneously excited. We show that the entropy mode leaves distinct imprints on the total loop temperature and density perturbations. Specifically, its rapid decay relative to the slow mode produces a non-exponential damping profile during the initial oscillation cycles and introduces a pronounced asymmetry between the upper and lower temperature and density envelopes. These effects arise naturally from the superposition of two exponentially decaying components with different damping timescales. Furthermore, deviations from the canonical quarter-period phase shift between temperature/density and velocity perturbations in the standing slow mode are explained by the entropy-mode effect. We conclude that the entropy mode may be detected through its impact on compressive oscillations. Revealing its role in non-exponential damping, envelope asymmetry, and phase shifts of compressive oscillations makes the entropy mode potentially accessible to observations and lays the foundation for solar and stellar seismological applications.

astro-ph.SR↗

arXiv2026

Spatio-temporal analysis of helioseismic quasi-biennial oscillations

Quasi-biennial oscillations (QBOs) are shorter-term periodic signals that occur alongside the dominant 11-year solar cycle. In this study, we examine the spatial and temporal evolution of QBOs using helioseismic p-mode frequency shifts from the Global Oscillation Network Group (GONG) across solar Cycles 23 and 24 and the ascending phase of Cycle 25. By applying wavelet analysis to frequency shifts, we studied the changes in QBO periodicities to determine whether the QBO period and amplitude vary with latitude. Our results show that QBO periods exhibit a weak latitudinal dependence, with shorter and less persistent signals at low latitudes, while at higher latitudes the periods are nearly constant at $\sim$3 years. Cycle 24 tends to display slightly longer periods than Cycle 23, though within uncertainties. At all latitudes, QBO amplitudes increase with mode frequency, which is consistent with previous studies. Higher amplitude QBOs are found at low latitudes, reflecting the distribution of surface magnetic activity. The ratio of QBO to cycle amplitude is systematically higher in Cycle 24 than in Cycle 23, and above $20^\circ$ latitude the amplitude ratio is nearly uniform in Cycle 23 but shows modest variations in Cycle 24. A linear relation between QBO amplitude and cycle amplitude is found in both cycles, but with significantly different slopes, indicating that QBO amplitudes are not wholly governed by the solar cycle strength and are at least partially decoupled from it. Finally, we find no evidence that QBO period depends on QBO amplitude, consistent with a linear oscillation regime.

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arXiv2026

Cycle dependence of helioseismic oscillations above the acoustic cut-off frequency

Helioseismic and recent asteroseismic observations reveal fine structure in the power spectrum with alternating peaks and troughs above the acoustic cut-off frequency. This structure is interpreted as the interference patterns of high-frequency acoustic waves excited in the solar interior and propagating into the atmosphere, known as pseudomodes. Pseudomodes exhibit clear solar-cycle variability, with frequency shifts that occur predominantly in anti-phase with the activity cycle, although the underlying mechanism remains uncertain. This work investigates how the subsurface excitation source location and the photospheric acoustic cut-off frequency influence the formation, frequency distribution, and solar-cycle variability of pseudomodes. We employ an analytical Klein-Gordon subsurface cavity model, which is shown to act as an effective Fabry-Pérot interferometer for high-frequency waves that experience constructive and destructive interference between the source location and the lower turning point. We derive an effective dispersion relation isolating the effects of the source location and photospheric cut-off on the pseudomode frequency. The model reproduces the observed peak-trough pseudomode spectrum for reasonable parameter values constrained by Bayesian MCMC best-fitting to GONG observations. We also find that solar-cycle-associated 11-year modulations of the source location result in anti-phase pseudomode frequency shifts, whereas similar cyclic variations in the cut-off frequency produce harmonic-dependent behaviour, yielding both in-phase and anti-phase shifts. As the acoustic cut-off and mode excitation relate to stratification and flows in the solar interior, the results highlight pseudomodes as a powerful diagnostic tool for changes in subsurface solar and stellar structure through the solar cycle.

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arXiv2025

Investigating magnetic activity cycles in solar-like oscillators using asteroseismic data from the K2 mission

We present the results of an investigation into the possible presence of magnetic activity cycles in stars observed in two observational campaigns by the K2 mission. This study was based on the KEYSTONE asteroseismic sample of solar-like oscillators, which contained 20 stars for which we were able to determine whether the asteroseismic p-mode frequencies varied in time. These frequency shifts ($δν$) were determined using a cross-correlation method and using the individual mode frequencies, obtained by fitting power spectra. Three stars were found to exhibit $δν$ larger than their associated errors ($σ_{δν}$) using both methods, while two more stars exhibited $δν>σ_{δν}$ when the cross correlation was used and a further two stars exhibited $δν>σ_{δν}$ when the fitted frequencies were used. When considering the whole sample of 20 stars, the amplitude of $δν$ showed no dependence on the large frequency separation and metallicity. However, $δν$ was observed to increase with rotation rate and effective temperature. Our sample contained a number of evolved subgiant stars, allowing us to expand the parameter space usually considered when comparing $δν$ with stellar parameters. While $δν$ was small for all of the evolved stars, one was found to have $δν>σ_{δν}$, raising the possibility that these evolved stars may still exhibit variable magnetic activity.

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arXiv2023

Quasi-periodic pulsations in solar flares: a key diagnostic of energy release on the Sun

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.

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arXiv2021

Novel data analysis techniques in coronal seismology

We review novel data analysis techniques developed or adapted for the field of coronal seismology. We focus on methods from the last ten years that were developed for extreme ultraviolet (EUV) imaging observations of the solar corona, as well as for light curves from radio and X-ray. The review covers methods for the analysis of transverse and longitudinal waves; spectral analysis of oscillatory signals in time series; automated detection and processing of large data sets; empirical mode decomposition; motion magnification; and reliable detection, including the most common pitfalls causing artefacts and false detections. We also consider techniques for the detailed investigation of MHD waves and seismological inference of physical parameters of the coronal plasma, including restoration of the three-dimensional geometry of oscillating coronal loops, forward modelling and Bayesian parameter inference.

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arXiv2021

Common origin of quasi-periodic pulsations in microwave and decimetric solar radio bursts

We analyse quasi-periodic pulsations (QPP) detected in the microwave and decimeter radio emission of the SOL2017-09-05T07:04 solar flare, using simultaneous observations by the Siberian Radioheliograph 48 (SRH-48, 4-8 GHz) and Mingantu Spectral Radioheliograph (MUSER-I, 0.4-2 GHz). The microwave emission was broadband with a typical gyrosynchrotron spectrum, while a quasi-periodic enhancement of the decimetric emission appeared in a narrow spectral band (500-700 MHz), consistent with the coherent plasma emission mechanism. The periodicity that we found in microwaves is about 30 s, coming from a compact loop-like source with a typical height of about 31 Mm. The decimetric emission demonstrated a periodicity about 6 s. We suggested a qualitative scenario linking the QPPs observed in both incoherent and coherent spectral bands and their generation mechanisms. The properties of the QPPs found in the microwave signal are typical for perturbations of the flare loop by the standing sausage mode of a fast magnetohydrodynamic (MHD) wave. Our analysis indicated that this sausage-oscillating flare loop was the primary source of oscillations in the discussed event. The suggested scenario is that a fundamental sausage harmonic is the dominant cause for the observed QPPs in the microwave emission. The initiation of oscillations in the decimetric emission is caused by the third sausage harmonic via periodic and nonlinear triggering of the acceleration processes in the current sheets, formed at the interface between the sausage-oscillating flare loop and the external coronal loop that extended to higher altitudes. Our results demonstrate the possible role of MHD wave processes in the release and transport of energy during solar flares, linking coherent and incoherent radio emission mechanisms.

astro-ph.SR↗

arXiv2021

TESS observations of flares and quasi-periodic pulsations from low mass stars and potential impact on exoplanets

We have performed a search for flares and Quasi-Periodic Pulsations (QPPs) from low mass M dwarf stars using TESS 2 min cadence data. We find seven stars which show evidence of QPPs. Using Fourier and Empirical Mode Decomposition techniques, we confirm the presence of 11 QPPs in these seven stars with a period between 10.2 and 71.9 min, including an oscillation with strong drift in the period and a double-mode oscillation. The fraction of flares we examined which showed QPPs (7 percent) is higher than other studies of stellar flares, but is very similar to the fraction of Solar C-class flares. Based on the stellar parameters taken from the TESS Input Catalog, we determine the lengths and magnetic field strengths of the flare coronal loops using the period of the QPPs and various assumptions about the origin of the QPPs. We also use a scaling relationship based on flares from Solar and Solar-type stars and the observed energy, plus the duration of the flares, finding that the different approaches predict loop lengths which are consistent to a factor of $\sim$2. We also discuss the flare frequency of the seven stars determining whether this could result in ozone depletion or abiogenesis. Three of our stars have a sufficiently high rate of energetic flares which are likely to cause abiogenesis. However, two of them are also in the range where ozone depletion is likely to occur. We speculate on the implications for surface life on these stars and the effects of the loop lengths and QPPs on potential exoplanets in the habitable zone.

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