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A new look at the frequency-dependent damping of slow-mode waves in the solar corona  

Dmitrii Kolotkov   Submitted: 2022-05-12 08:48

Being directly observed in the Doppler shift and imaging data and indirectly as quasi-periodic pulsations in solar and stellar flares, slow magnetoacoustic waves offer an important seismological tool for probing many vital parameters of the coronal plasma. A recently understood active nature of the solar corona for magnetoacoustic waves, manifested through the phenomenon of wave-induced thermal misbalance, led to the identification of new natural mechanisms for the interpretation of observed properties of waves. A frequency-dependent damping of slow waves in various coronal plasma structures remains an open question, as traditional wave damping theories fail to match observations. We demonstrate that accounting for the back-reaction caused by thermal misbalance on the wave dynamics leads to a modification of the relationship between the damping time and oscillation period of standing slow waves, prescribed by the linear theory. The modified relationship is not of a power-law form and has the equilibrium plasma conditions and properties of the coronal heating/cooling processes as free parameters. It is shown to readily explain the observed scaling of the damping time with period of standing slow waves in hot coronal loops. Functional forms of the unknown coronal heating process, consistent with the observed frequency-dependent damping, are seismologically revealed.

Authors: Dmitrii Y. Kolotkov, Valery M. Nakariakov
Projects: SoHO-SUMER

Publication Status: In press in MNRAS Letters
Last Modified: 2022-05-16 08:37
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Multi-wavelength quasi-periodic pulsations in a stellar superflare  

Dmitrii Kolotkov   Submitted: 2021-12-16 01:56

We present the first multi-wavelength simultaneous detection of QPP in a superflare (more than a thousand times stronger than known solar flares) on a cool star, in soft X-rays (SXR, with XMM-Newton) and white light (WL, with Kepler). It allowed for the first-ever analysis of oscillatory processes in a stellar flare simultaneously in thermal and non-thermal emissions, conventionally considered to come from the corona and chromosphere of the star, respectively. The observed QPP have periods 1.5 ± 0.15 hours (SXR) and 3 ± 0.6 hours (WL), and correlate well with each other. The unique relationship between the observed parameters of QPP in SXR and WL allowed us to link them with oscillations of the electric current in the flare loop, which directly affect the dynamics of non-thermal electrons and indirectly (via Ohmic heating) the thermal plasma. These findings could be considered in favour of the equivalent LCR-contour model of a flare loop, at least in the extreme conditions of a stellar superflare.

Authors: Dmitrii Y. Kolotkov, Valery M. Nakariakov, Robin Holt, Alexey A. Kuznetsov
Projects: Other

Publication Status: Accepted for publication in ApJ Letters
Last Modified: 2021-12-22 13:14
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The solar corona as an active medium for magnetoacoustic waves  

Dmitrii Kolotkov   Submitted: 2021-11-05 09:00

The presence and interplay of continuous cooling and heating processes maintaining the corona of the Sun at the observed one million K temperature were recently understood to have crucial effects on the dynamics and stability of magnetoacoustic waves. These essentially compressive waves perturb the coronal thermal equilibrium, leading to the phenomenon of a wave-induced thermal misbalance. Representing an additional natural mechanism for the exchange of energy between the plasma and the wave, thermal misbalance makes the corona an active medium for magnetoacoustic waves, so that the wave can not only lose but also gain energy from the coronal heating source (similarly to burning gases, lasers and masers). We review recent achievements in this newly emerging research field, focussing on the effects that slow-mode magnetoacoustic waves experience as a back-reaction of this perturbed coronal thermal equilibrium. The new effects include enhanced frequency-dependent damping or amplification of slow waves, and effective, not associated with the coronal plasma non-uniformity, dispersion. We also discuss the possibility to probe the unknown coronal heating function by observations of slow waves and linear theory of thermal instabilities. The manifold of the new properties that slow waves acquire from a thermodynamically active nature of the solar corona indicate a clear need for accounting for the effects of combined coronal heating/cooling processes not only for traditional problems of the formation and evolution of prominences and coronal rain, but also for an adequate modelling and interpretation of magnetohydrodynamic waves.

Authors: D. Y. Kolotkov, D. I. Zavershinskii, V. M. Nakariakov
Projects: None

Publication Status: Accepted for publication in Plasma Phys. Control. Fusion
Last Modified: 2021-11-05 17:48
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Mixed properties of slow magnetoacoustic and entropy waves in a plasma with heating/cooling misbalance  

Dmitrii Kolotkov   Submitted: 2021-04-27 02:46

The processes of the coronal plasma heating and cooling were previously shown to significantly affect the dynamics of slow magnetoacoustic (MA) waves, causing amplification or attenuation, and also dispersion. However, the entropy mode is also excited in such a thermodynamically active plasma and is affected by the heating/cooling misbalance too. This mode is usually associated with the phenomenon of coronal rain and formation of prominences. Unlike the adiabatic plasmas, the properties and evolution of slow MA and entropy waves in continuously heated and cooling plasmas get mixed. Different regimes of the misbalance lead to a variety of scenarios for the initial perturbation to evolve. In order to describe properties and evolution of slow MA and entropy waves in various regimes of the misbalance, we obtained an exact analytical solution of the linear evolutionary equation. Using the characteristic timescales and the obtained exact solution, we identified regimes with qualitatively different behaviour of slow MA and entropy modes. For some of those regimes, the spatio-temporal evolution of the initial Gaussian pulse is shown. In particular, it is shown that slow MA modes may have a range of non-propagating harmonics. In this regime, perturbations caused by slow MA and entropy modes in a low-beta plasma would look identically in observations, as non-propagating disturbances of the plasma density (and temperature) either growing or decaying with time. We also showed that the partition of the initial energy between slow MA and entropy modes depends on the properties of the heating and cooling processes involved. The obtained exact analytical solution could be further applied to the interpretation of observations and results of numerical modelling of slow MA waves in the corona and the formation and evolution of coronal rain.

Authors: D.I. Zavershinskii, D.Y. Kolotkov, D.S. Riashchikov, N.E. Molevich
Projects: None

Publication Status: Accepted for publication in Solar Physics
Last Modified: 2021-04-27 17:08
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Seismological constraints on the solar coronal heating function  

Dmitrii Kolotkov   Submitted: 2020-10-07 02:38

The hot solar corona exists because of the balance between radiative and conductive cooling and some counteracting heating mechanism which remains one of the major puzzles in solar physics. The coronal thermal equilibrium is perturbed by magnetoacoustic waves which are abundantly present in the corona, causing a misbalance between the heating and cooling rates. Due to this misbalance, the wave experiences a back-reaction, either losing or gaining energy from the energy supply that heats the plasma, at the time scales comparable to the wave period. In particular, the plasma can be subject to wave-induced instability or over-stability, depending on the specific choice of the coronal heating function. In the unstable case, the coronal thermal equilibrium would be violently destroyed, which does not allow for the existence of long-lived plasma structures typical for the corona. Based on this, we constrained the coronal heating function using observations of slow magnetoacoustic waves in various coronal plasma structures.

Authors: D. Y. Kolotkov, T. J. Duckenfield, V. M. Nakariakov
Projects: None

Publication Status: Accepted for publication in A&A
Last Modified: 2020-10-07 10:17
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The origin of the modulation of the radio emission from the solar corona by a fast magnetoacoustic wave  

Dmitrii Kolotkov   Submitted: 2018-05-21 07:20

Observational detection of quasi-periodic drifting fine structures in a type III radio burst associated with a solar flare SOL2015-04-16T11:22, with Low Frequency Array, is presented. Although similar modulations of the type III emission have been observed before and were associated with the plasma density fluctuations, the origin of those fluctuations was unknown. Analysis of the striae of the intensity variation in the dynamic spectrum allowed us to reveal two quasi-oscillatory components. The shorter component has the apparent wavelength of ~2 Mm, phase speed of ~657 km s-1, which gives the oscillation period of ~3 s, and the relative amplitude of ~0.35%. The longer component has the wavelength of ~12 Mm, and relative amplitude of ~5.1%. The short frequency range of the detection does not allow us to estimate its phase speed. However, the properties of the shorter oscillatory component allowed us to interpret it as a fast magnetoacoustic wave guided by a plasma non-uniformity along the magnetic field outwards from the Sun. The assumption that the intensity of the radio emission is proportional to the amount of plasma in the emitting volume allowed us to show that the superposition of the plasma density modulation by a fast wave and a longer-wavelength oscillation of an unspecified nature could readily reproduce the fine structure of the observed dynamic spectrum. The observed parameters of the fast wave give the absolute value of the magnetic field in the emitting plasma of ~1.1 G which is consistent with the radial magnetic field model.

Authors: Dmitrii Y. Kolotkov, Valery M. Nakariakov, Eduard P. Kontar
Projects: LOw Frequency ARray (LOFAR)

Publication Status: accepted for publication in ApJ
Last Modified: 2018-05-21 10:03
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Quasi-periodic pulsations in the most powerful solar flare of Cycle 24  

Dmitrii Kolotkov   Submitted: 2018-04-13 07:42

Quasi-periodic pulsations (QPP) are common in solar flares and are now regularly observed in stellar flares. We present the detection of two different types of QPP signals in the thermal emission light curves of the X9.3 class solar flare SOL2017-09-06T12:02, which is the most powerful flare of Cycle 24. The period of the shorter-period QPP drifts from about 12 to 25 seconds during the flare. The observed properties of this QPP are consistent with a sausage oscillation of a plasma loop in the flaring active region. The period of the longer-period QPP is about 4 to 5 minutes. Its properties are compatible with standing slow magnetoacoustic oscillations, which are often detected in coronal loops. For both QPP signals, other mechanisms such as repetitive reconnection cannot be ruled out, however. The studied solar flare has an energy in the realm of observed stellar flares, and the fact that there is evidence of a short-period QPP signal typical of solar flares along with a long-period QPP signal more typical of stellar flares suggests that the different ranges of QPP periods typically observed in solar and stellar flares is likely due to observational constraints, and that similar physical processes may be occurring in solar and stellar flares.

Authors: Dmitrii Kolotkov, Chloe Pugh, Anne-Marie Broomhall, and Valery Nakariakov
Projects: GOES X-rays,SDO-EVE

Publication Status: accepted for publication in ApJL
Last Modified: 2018-04-13 18:14
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Finite amplitude transverse oscillations of a magnetic rope  

Dmitrii Kolotkov   Submitted: 2018-03-13 18:45

The effects of finite amplitudes on the transverse oscillations of a quiescent prominence represented by a magnetic rope are investigated in terms of the model proposed by Kolotkov et al. 2016. We consider a weakly nonlinear case governed by a quadratic nonlinearity, and also analyse the fully nonlinear equations of motion. We treat the prominence as a massive line current located above the photosphere and interacting with the magnetised dipped environment via the Lorentz force. In this concept the magnetic dip is produced by two external current sources located at the photosphere. Finite amplitude horizontal and vertical oscillations are found to be strongly coupled between each other. The coupling is more efficient for larger amplitudes and smaller attack angles between the direction of the driver and the horizontal axis. Spatial structure of oscillations is represented by Lissajous-like curves with the limit cycle of a hourglass shape, appearing in the resonant case, when the frequency of the vertical mode is twice the horizontal mode frequency. A metastable equilibrium of the prominence is revealed, which is stable for small amplitude displacements, and becomes horizontally unstable, when the amplitude exceeds a threshold value. The maximum oscillation amplitudes are also analytically derived and analysed. Typical oscillation periods are determined by the oscillation amplitude, prominence current, its mass and position above the photosphere, and the parameters of the magnetic dip. The main new effects of the finite amplitude are the coupling of the horizontally and vertically polarised transverse oscillations (i.e. the lack of a simple, elliptically polarised regime) and the presence of metastable equilibria of prominences.

Authors: Dmitrii Y. Kolotkov, Giuseppe Nistico, George Rowlands, Valery M. Nakariakov
Projects: None

Publication Status: accepted for publication in JASTP
Last Modified: 2018-03-14 13:30
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Long-period quasi-periodic oscillations of a small-scale magnetic structure on the Sun  

Dmitrii Kolotkov   Submitted: 2016-12-22 14:43

Long-period quasi-periodic variations of the average magnetic field in a small-scale magnetic structure on the Sun are analysed. The structure is situated at the photospheric level and is involved in a facula formation in the chromosphere. The observational signal obtained from the SDO/HMI line-of-sight magnetograms of the target structure has a non-stationary behaviour, and is therefore processed with the Hilbert-Huang Transform spectral technique. The empirical decomposition of the original signal and subsequent testing of the statistical significance of its intrinsic modes reveal the presence of the white and pink noisy components for the periods shorter and longer than 10 min, respectively, and a significant oscillatory mode. The oscillation is found to have a non-stationary period growing from about 80 to 230 min and an increasing relative amplitude, while the mean magnetic field in the oscillating structure is seen to decrease. The observed behaviour could be interpreted either by the dynamical interaction of the structure with the boundaries of supergranula cells in the region of interest or in terms of the vortex shedding appearing during the magnetic flux emergence.

Authors: D. Y. Kolotkov, V. V. Smirnova, P. V. Strekalova, A. Riehokainen, and V. M. Nakariakov
Projects: None

Publication Status: accepted for publication in A&A letters
Last Modified: 2016-12-28 11:41
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Nonlinear oscillations of coalescing magnetic flux ropes  

Dmitrii Kolotkov   Submitted: 2016-05-12 16:35

An analytical model of highly nonlinear oscillations occurring during a coalescence of two magnetic flux ropes, based upon two-fluid hydrodynamics, is developed. The model accounts for the effect of electric charge separation, and describes perpendicular oscillations of the current sheet formed by the coalescence. The oscillation period is determined by the current sheet thickness, the plasma parameter beta, and the oscillation amplitude. The oscillation periods are typically greater than or about the ion plasma oscillation period. In the nonlinear regime, the oscillations of the ion and electron concentrations have a shape of a narrow symmetric spikes.

Authors: Dmitrii Y. Kolotkov, Valery M. Nakariakov, and George Rowlands
Projects: None

Publication Status: in press in Phys. Rev. E
Last Modified: 2016-05-14 07:55
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Transverse oscillations and stability of prominences in a magnetic field dip  

Dmitrii Kolotkov   Submitted: 2016-05-03 04:51

Aims. We developed an analytical model of the global transverse oscillations and mechanical stability of a quiescent prominence in the magnetised environment with a magnetic field dip that accounts for the mirror current effect. Methods. The model is based on the interaction of line currents through the Lorentz force. Within this concept the prominence is treated as a straight current-carrying wire, and the magnetic dip is provided by two photospheric current sources. Results. Properties of both vertical and horizontal oscillations are determined by the value of the prominence current, its density and height above the photosphere, and the parameters of the magnetic dip. The prominence can be stable in both horizontal and vertical directions simultaneously when the prominence current dominates in the system and its height is less than the half-distance between the photospheric sources.

Authors: D.Y. Kolotkov, G. Nistico and V.M. Nakariakov
Projects: None

Publication Status: in press in A&A
Last Modified: 2016-05-04 09:22
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Abstracts by Author
A new look at the frequency-dependent damping of slow-mode waves in the solar corona
Multi-wavelength quasi-periodic pulsations in a stellar superflare
The solar corona as an active medium for magnetoacoustic waves
Mixed properties of slow magnetoacoustic and entropy waves in a plasma with heating/cooling misbalance
Seismological constraints on the solar coronal heating function
The origin of the modulation of the radio emission from the solar corona by a fast magnetoacoustic wave
Quasi-periodic pulsations in the most powerful solar flare of Cycle 24
Finite amplitude transverse oscillations of a magnetic rope
Long-period quasi-periodic oscillations of a small-scale magnetic structure on the Sun
Nonlinear oscillations of coalescing magnetic flux ropes
Transverse oscillations and stability of prominences in a magnetic field dip

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