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Comparison of damping models for kink oscillations of coronal loops  

Valery Nakariakov   Submitted: 2023-08-24 02:05

Kink oscillations of solar coronal loops are of intense interest due to their potential for diagnosing plasma parameters in the corona. The accurate measurement of the kink oscillation damping time is crucial for precise seismological diagnostics, such as the transverse density profile, and for the determination of the damping mechanism. Previous studies of large-amplitude rapidly-decaying kink oscillations have shown that both an exponential damping model and a generalised model (consisting of Gaussian and exponential damping patterns) fit observed damping profiles sufficiently well. However, it has recently been shown theoretically that the transition from the decaying regime to the decayless regime could be characterised by a super-exponential damping model. In this work, we re-analyse a sample of decaying kink oscillation events, and utilise the Markov Chain Monte Carlo Bayesian approach to compare the exponential, Gaussian–exponential and super-exponential damping models. It is found that in seven out of ten analysed oscillations, the preferential damping model is the super-exponential one. In two events, the preferential damping is exponential, and in one it is Gaussian–exponential. This finding indicates the plausibility of the superexponential damping model. The possibility of a non-exponential damping pattern needs to be taken into account in the analysis of a larger number of events, especially in the estimation of the damping time and its associated empirical scalings with the oscillation period and amplitude, and in seismological inversions.

Authors: Zhong, Y., Kolotkov, D.Y., Zhong, S., Nakariakov, V.M.
Projects: SDO-AIA

Publication Status: MNRAS (accepted)
Last Modified: 2023-08-30 13:11
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Polarisation of decayless kink oscillations of solar coronal loops  

Valery Nakariakov   Submitted: 2023-08-21 00:30

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 degrees, 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.

Authors: Zhong, S., Nakariakov, V.M., Kolotkov, D.Y., Chitta, L.P., Antolin, P., Verbeeck, C., Berghmans, D.
Projects: SO/EUI

Publication Status: Nat. Comm. (in press)
Last Modified: 2023-08-23 13:11
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30-min Decayless Kink Oscillations in a Very Long Bundle of Solar Coronal Plasma Loops  

Valery Nakariakov   Submitted: 2023-08-09 20:51

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 736±80Mm and a lifetime of about 2 days are found to exhibit decayless kink oscillations. The oscillations were observed for several hours. The oscillation amplitude was 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.

Authors: Sihui Zhong, Valery M. Nakariakov, Yuhu Miao, Libo Fu, Ding Yuan
Projects: SDO-AIA

Publication Status: Nat. Sci. Rep. (in press)
Last Modified: 2023-08-10 04:59
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Do periods of decayless kink oscillations of solar coronal loops depend on noise?  

Valery Nakariakov   Submitted: 2022-09-13 13:03

Decayless kink oscillations of solar coronal loops are studied in terms of a low-dimensional model based on a randomly driven Rayleigh oscillator with coefficients experiencing random fluctuations. The model considers kink oscillations as natural modes of coronal loops, decaying by linear resonant absorption. The damping is counteracted by random motions of the loop footpoints and the interaction of the loop with external quasi-steady flows with random fluctuations. In other words, the model combines the self-oscillatory and randomly driven mechanisms for the decayless behaviour. The random signals are taken to be of the stationary red noise nature. In the noiseless case, the model has an asymptotically stationary oscillatory solution, i.e., a kink self-oscillation. It is established that the kink oscillation period is practically independent of noise. This finding justifies the seismological estimations of the kink and Alfvén speeds and the magnetic field in an oscillating loop by kink oscillations, based on the observed oscillation period. The oscillatory patterns are found to be almost harmonic. Noisy fluctuations of external flows modulate the amplitude of the almost monochromatic oscillatory pattern symmetrically, while random motions of the loop footpoints cause antisymmetric amplitude modulation. Such modulations are also consistent with the observed behaviour.

Authors: Nakariakov, V.M., Kolotkov, D.Y., Zhong, S.
Projects: None

Publication Status: MNRAS (accepted)
Last Modified: 2022-09-14 13:14
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Two-spacecraft detection of short-period decayless kink oscillations of solar coronal loops  

Valery Nakariakov   Submitted: 2022-09-08 04:12

Decayless kink oscillations of an ensemble of loops are captured simultaneously by the High Resolution Imager (HRI) of the Extreme Ultraviolet Imager (EUI) and the Atmospheric Imaging Assembly (AIA) from 22:58 UT on 5 November to 00:27 UT on 6 November 2021. Oscillations are analysed by processing image sequences taken by the two instruments with a motion magnification technique. The analysed loops are around 51 Mm in length, and oscillate with short periods of 1-3 min (1.6 min in average) and displacement amplitudes of 27-83 km. The signals recorded by AIA are delayed by 66 s as compared to HRI, which coincides with the light travel time difference from the Sun to each instrument. After correction of this time difference, the cross-correlation coefficient between the signals from the two data varies from 0.82 to 0.97, indicating that they are well consistent. This work confirms that HRI sees the same oscillations as AIA, which is the necessary first step before proceeding to the detection of shorter time scales by EUI. In addition, our results indicate the robustness of the de-jittering procedure in the study of kink oscillations with HRI.

Authors: Zhong, S., Nakariakov, V.M., Kolotkov, D.Y., Verbeeck, C., Berghmans, D.
Projects: SO/EUI

Publication Status: MNRAS (accepted)
Last Modified: 2022-09-11 17:48
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Slow magnetoacoustic oscillations in stellar coronal loops  

Valery Nakariakov   Submitted: 2022-05-11 03:44

Slow magnetoacoustic oscillations in stellar coronal loops with gravitational stratification are analyzed with a numerical solution of the boundary-value problem for eigenvalues and eigen functions. In this study, we only focus on the resonant periods. The effects of the gravitational stratification, star mass, loop temperature, and loop length on the properties of slow magnetoacoustic oscillations are investigated. It is shown that the discrepancy between stratified and non-stratified loops is higher in density perturbations than velocity perturbations. When the star has larger mass, higher coronal temperature, and longer loop, the density perturbations in stratified loop are significantly different from the harmonic functions. The periods in the stratified loop are slightly longer than in the non-stratified loop. The periods calculated in our model (14-644 min) are consistent with the periods of stellar quasi-periodic pulsations observed in both soft x-rays (2-70 min) and white light (8-390 min).

Authors: Lim, D., Nakariakov, V.M., Moon, Y.-J.
Projects: None

Publication Status: ApJ (accepted)
Last Modified: 2022-05-11 12:28
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Kink oscillations of coronal loops  

Valery Nakariakov   Submitted: 2021-08-23 12:29

Kink oscillations of coronal loops, i.e., standing kink waves, is one of the most studied dynamic phenomena in the solar corona. The oscillations are excited by impulsive energy releases, such as low coronal eruptions. Typical periods of the oscillations are from a few to several minutes, and are found to increase linearly with the increase in the major radius of the oscillating loops. It clearly demonstrates that kink oscillations are natural modes of the loops, and can be described as standing fast magnetoacoustic waves with the wavelength determined by the length of the loop. Kink oscillations are observed in two different regimes. In the rapidly decaying regime, the apparent displacement amplitude reaches several minor radii of the loop. The damping time which is about several oscillation periods decreases with the increase in the oscillation amplitude, suggesting a nonlinear nature of the damping. In the decayless regime, the amplitudes are smaller than a minor radius, and the driver is still debated. The review summarises major findings obtained during the last decade, and covers both observational and theoretical results. Observational results include creation and analysis of comprehensive catalogues of the oscillation events, and detection of kink oscillations with imaging and spectral instruments in the EUV and microwave bands. Theoretical results include various approaches to modelling in terms of the magnetohydrodynamic wave theory. Properties of kink oscillations are found to depend on parameters of the oscillating loop, such as the magnetic twist, stratification, steady flows, temperature variations and so on, which make kink oscillations a natural probe of these parameters by the method of magnetohydrodynamic seismology.

Authors: V. M. Nakariakov, S. A. Anfiinogentov, P. Antolin, R. Jain, D. Y. Kolotkov, E. G. Kupriyanova, D. Li, N. Magyar, G. Nistico, D. J. Pascoe, A. K. Srivastava, J. Terradas, S. Vasheghani Farahani, G. Verth, D. Yuan, I. V. Zimovets
Projects: None

Publication Status: Space Sci. Rev. (accepted)
Last Modified: 2021-08-23 13:59
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Excitation of negative energy surface magnetohydrodynamic waves in an incompressible cylindrical plasma  

Valery Nakariakov   Submitted: 2020-04-29 10:56

Negative energy wave phenomena may appear in shear flows in the presence of a wave decay mechanism and external energy supply. We study the appearance of negative energy surface waves in a plasma cylinder in the incompressible limit. The cylinder is surrounded by an axial magnetic field and by a plasma of different density. Considering flow inside and viscosity outside the flux tube, we derive dispersion relations, and obtain analytical solutions for the phase speed and growth rate (increment) of the waves. It is found that the critical speed shear for the occurrence of the dissipative instability associated with negative energy waves (NEWs) and the threshold of Kelvin-Helmholtz instability (KHI) depend on the axial wavelength. The critical shear for the appearance of sausage NEW is lowest for the longest axial wavelengths, while for kink waves the minimum value of the critical shear is reached for the axial wavelength comparable to the diameter of the cylinder. The range between the critical speed of the dissipative instability and the KHI threshold is shown to depend on the difference of the Alfvén speeds inside and outside of the cylinder. For all axial {wavenumbers}, NEW appears for the shear flow speeds lower than the KHI threshold. It is easier to excite NEW in an underdense cylinder than in an overdense one. The negative energy surface waves can be effectively generated for azimuthal number m=0 with a large axial wave number and for higher modes (m>0) with a small axial wave number.

Authors: Yu, D.J., Nakariakov, V.M.
Projects: None

Publication Status: ApJ, accepted
Last Modified: 2020-04-29 15:36
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Oscillation of a small Hα surge in a solar polar coronal hole  

Valery Nakariakov   Submitted: 2019-06-10 03:17

Hα surges(i.e., cool/dense collimated plasma ejections) may act as a guide for the propagation of magnetohydrodynamic waves. We report a high-resolution observation of a surge observed with 1.6 m Goode Solar Telescope (GST) on 2009 August 26, from 18:20 UT to 18:45 UT. Characteristics of plasma motions in the surge are determined with the normalizing radial gradient filter and the Fourier motion filter. The shape of the surgeis found to change from a"C" shape to an inverse "C" shape after a formation of a cusp, a signature ofreconnection. There are apparent upflows seen above the cusp top and downflows below it. The upflows showrising and rotational motions in the right-hand direction, with the rotational speed decreasing with height. Near thecusp top, wefind a transverse oscillation of the surge, with the period of ∼2 minutes. There is no change of theoscillation phase below the cusp top, but above the top a phase change is identified, giving a vertical phase speedabout 86 km s-1. As the height increases, the initial amplitude of the oscillation increases, and the oscillationdamping time decreases from 5.13 to 1.18 minutes. We conclude that the oscillation is a propagating kink wavethat is possibly excited by the repetitive spontaneous magnetic reconnection.

Authors: Cho, K.-S., Cho, I.-H., Nakariakov, V.M., Yurchyshyn, V.B., Yang, H., Kim, Y.-H., Kumar, P., Magara, T.
Projects: None

Publication Status: Astrophysical Journal Letters, 877, L1, 2019
Last Modified: 2019-06-12 12:21
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Properties of slow magnetoacoustic oscillations of solar coronal loops by multi-instrumental observations  

Valery Nakariakov   Submitted: 2019-03-05 08:52

Rapidly decaying oscillations of the thermal emission detected in the decay phase of solar and stellar flares are usually interpreted as standing or sloshing (reflecting) slow magnetoacoustic oscillations. We determine the scalings of the oscillation periods, damping times and amplitudes with the temperature, considering both standing and sloshing oscillations detected with different instruments. In addition, the time evolution of different spatial harmonics of a sloshing oscillation is considered. Parameters of slow oscillations observed in the EUV, X-ray, and microwave bands, and published in the literature, are used. The damping time of slow oscillations is found to scale almost linearly with the oscillation period, as the period to 0.87± 0.1, giving the average Q-factor determined as the ratio of the damping time to the period, of about 1. The Q-factor is found to scale with the relative amplitude to the power of 0.33+0.10-0.11 with 95 % confidence. The amplitudes of different spatial harmonics forming a sloshing pulse show similar time evolution, suggesting that the period-dependent dissipation is counteracted by another mechanism. The results obtained indicate that the damping of slow oscillations depends on the oscillation amplitude, and that the competition of nonlinear and dissipative effects could allow for the existence of wave pulses of a sustained shape.

Authors: Nakariakov, V. M., Kosak, M. K., Kolotkov, D. Y., Anfinogentov, S. A., Kumar, P., Moon, Y.-J.
Projects: SDO-AIA

Publication Status: ApJL, accepted
Last Modified: 2019-03-05 12:47
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Non-stationary quasi-periodic pulsations in solar and stellar flares  

Valery Nakariakov   Submitted: 2018-08-10 08:41

Often the enhanced electromagnetic radiation generated in solar and stellar flares shows a pronounced (quasi)-oscillatory pattern - quasi-periodic pulsations (QPP), with characteristic periods ranging from a fraction of a second to several tens of minutes. We review recent advances in the empirical study of QPP in solar and stellar flares, addressing the intrinsic non-stationarity of the signal, i.e. the variation of its amplitude, period or phase with time. This non-stationarity could form a basis for a classification of QPP, necessary for revealing specific physical mechanisms responsible for their appearance. We could identify two possible classes of QPP, decaying harmonic oscillations, and trains of symmetric triangular pulsations. Apparent similarities between QPP and irregular geomagnetic pulsations Pi offer a promising avenue for the knowledge transfer in both analytical techniques and theory. Attention is also paid to the effect of the flare trend on the detection and analysis of QPP.

Authors: Nakariakov, V.M., Kolotkov, D., Kupriyanova, E.G., Mehta, T., Pugh, C.E., Lee, D.-H., Broomhall, A.M.
Projects: None

Publication Status: PPCF, accepted
Last Modified: 2018-08-10 08:56
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Quasi-periodic Pulsations in a Solar Microflare  

Valery Nakariakov   Submitted: 2018-06-07 11:07

Irregular time evolution of the radio emission generated in a B2-class microflare (SOL2017-01-25T10:15), occurring on 2017 January 25 in active region 12,628, is studied. The microflare was apparently initiated by an appearance of an s-shaped loop, observed in the EUV band. The radio emission is associated with the nonthermal electrons detected with Ramaty High Energy Solar Spectroscopic Imager, and originates simultaneously from two opposite footpoints of a magnetic fan structure beginning at a sunspot. According to the active region geometry, the footpoints are situated in the meridional direction, and hence are observed by RATAN-600 simultaneously. The radio emission intensity signal, as well as the left-hand and right-hand circular polarization signals in the low-frequency band (3-4 GHz) show good correlation with each other, with the average characteristic time of the variation 1.4 ± 0.3 s. The polarization signal shows a time variation with the characteristic time of about 0.7 ± 0.2 s. The irregular quasi-periodic pulsations of the radio emission are likely to be caused by the superposition of the signals generated at the local electron plasma frequencies by the interaction of nonthermal electrons with the plasma at the footpoints. In this scenario, the precipitation rate of the nonthermal electrons at the opposite footpoints could be modulated by the superposition of fundamental and second harmonic modes of sausage oscillations, resulting in the observed different characteristic times of the intensity and polarization signals. However, other mechanisms, e.g., the oscillatory regime of loop coalescence or magnetic null point oscillation could not be rigorously excluded.

Authors: Nakariakov, V. M., Anfinogentov, S., Storozhenko, A. A., Kurochkin, E. A., Bogod, V. M., Sharykin, I. N., Kaltman, T. I.
Projects: RATAN-600

Publication Status: ApJ 859:154, 2018
Last Modified: 2018-06-07 12:36
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Effect of local thermal equilibrium misbalance on long-wavelength slow magnetoacoustic waves  

Valery Nakariakov   Submitted: 2017-11-03 05:51

Evolution of slow magnetoacoustic waves guided by a cylindrical magnetic flux tube that represents a coronal loop or plume, is modelled accounting for the effects of finite gas pressure, weak nonlinearity, dissipation by thermal conduction and viscosity, and the misbalance between the cooling by optically thin radiation and unspecified heating of the plasma. An evolutionary equation of the Burgers-Malthus type is derived. It is shown that the cooling/heating misbalance, determined by the derivatives of the combined radiative cooling and heating function, with respect to the density, temperature and magnetic field at the thermal equilibrium affect the wave rather strongly. This effect may either cause additional damping, or counteract it, or lead to the gradual amplification of the wave. In the latter case the coronal plasma acts as an active medium for the slow magnetoacoustic waves. The effect of the cooling/heating misbalance could be important for coronal slow waves, and could be responsible for certain discrepancies between theoretical results and observations, in particular the increased or decreased damping lengths and times, detection of the waves at certain heights only, and excitation of compressive oscillations. The results obtained open up a possibility for the diagnostics of the coronal heating function by slow magnetoacoustic waves.

Authors: Nakariakov, V.M., Afanasyev, A.N., Kumar, S., Moon, Y.-J.
Projects: None

Publication Status: ApJ 849:62, 2017
Last Modified: 2017-11-03 13:41
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Comparison of damped oscillations in solar and stellar X-ray flares  

Valery Nakariakov   Submitted: 2016-08-09 20:18

We explore the similarity and difference of the quasi-periodic pulsations (QPPs) observed in the decay phase of solar and stellar flares at X-rays. We identified 42 solar flares with pronounced QPPs, observed with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and 36 stellar flares with QPPs, observed with X-ray Multi Mirror Newton observatory (XMM-Newton). The Empirical Mode Decomposition (EMD) method and least-square fit by a damped sine function were applied to obtain the periods (P) and damping times (τ) of the QPPs. We found that (1) the periods and damping times of the stellar QPPs are 5.00+70.45-4.57 min and 17.29+60.80-17.02 min, which are comparable with those of the solar QPPs (0.57+3.01-0.30 and 1.20+2.77-0.76 min). (2) The ratio of the damping times to the periods (τ/P) observed in the stellar QPPs (1.46+2.54-0.48) are statistically identical to those of solar QPPs (1.49+2.49-0.66). (3) The scalings of the QPP damping time with the period are well described by the power law in both solar and stellar cases. The power indices of the solar and stellar QPPs are 0.96±{0.10} and 0.98±{0.05}, respectively. This scaling is consistent with the scalings found for standing slow magnetoacoustic and kink modes in solar coronal loops. Thus, we propose that the underlying mechanism responsible for the stellar QPPs is the natural magnetohydrodynamic oscillations in the flaring or adjacent coronal loops, as in the case of solar flares.

Authors: Cho, I.-H., Cho, K.-S., Nakariakov, V.M., Kim, S., Kumar, P.
Projects: RHESSI

Publication Status: Astrophysical Journal, 2016, accepted
Last Modified: 2016-08-10 16:05
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Undamped transverse oscillations of coronal loops as a self-oscillatory process  

Valery Nakariakov   Submitted: 2016-05-18 13:18

Context. Standing transverse oscillations of coronal loops are observed to operate in two regimes, the rapidly decaying large amplitude oscillations, and undamped small amplitude oscillations. In the latter regime the damping should be compensated by energy supply, which allows the loop to perform almost monochromatic oscillations with almost constant amplitude. Different loops oscillate with different periods. The oscillation amplitude does not show dependence on the loop length or the oscillation period. Aims. We aim to develop a low-dimensional model explaining the undamped kink oscillations as a self-oscillatory process caused by the effect of negative friction. The source of energy is an external quasi-steady flow, e.g. supergranulation motions near the loop footpoints or external flows in the corona. Methods. We demonstrate that the interaction of a quasi-steady flow with a loop can be described by a Rayleigh oscillator equation that is a nonlinear ordinary differential equation, with the damping and resonant terms determined empirically. Results. Low-amplitude self-oscillatory solutions to the Rayleigh oscillator equation are harmonic signals of constant amplitude, which is consistent with the observed properties of undamped kink oscillations. The period of self-oscillations is determined by the frequency of the kink mode. The damping by dissipation and mode conversion is compensated by the continuous energy deposition at the frequency of the natural oscillation. Conclusions. We propose that undamped kink oscillations of coronal loops may be caused by the interaction of the loops with quasi-steady flows, and hence are self-oscillations, in analogy with producing a tune by a stick moving across a violin string.

Authors: Nakariakov, V. M., Anfinogentov, S., Nistico, G., Lee, D.-H.
Projects: SDO-AIA

Publication Status: A&A, Letter, Accepted
Last Modified: 2016-05-18 15:32
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Dependence of kink oscillation damping on the amplitude  

Valery Nakariakov   Submitted: 2016-05-15 14:00

Context. Kink oscillations of coronal loop are one of the most intensively studied oscillatory phenomena in the solar corona. In the large-amplitude rapidly damped regime these oscillations are observed to have a low quality-factor, with only a few cycles of oscillation detected before they are damped. The specific mechanism responsible for the rapid damping is commonly accepted to be associated with the linear coupling between collective kink oscillations and localised torsional oscillations, the phenomenon ofresonant absorption of the kink mode. However, the role of finite amplitude effects is still not clear.
Aims. We investigated the empirical dependence of the kink oscillation damping time and its quality factor, defined as the ratio of the damping time to the oscillation period, on the oscillation amplitude.
Methods. We analysed decaying kink oscillation events detected previously with TRACE, SDO/AIA and and STEREO/EUVI in the EUV 171 Å band.
Results. We found that the ratio of the kink oscillation damping time to the oscillation period systematically decreases with the oscillation amplitude. The quality factor dependence on the oscillation displacement amplitude has been approximated by the powerlaw dependence with the exponent of -1/2, however we stress that this is a "by eye" estimate, and a more rigorous estimation of the scaling law requires more accurate measurements and increased statistics. We conclude that damping of kink oscillations of coronal loops depends on the oscillation amplitude, indicating the possible role of nonlinear mechanisms for damping.

Authors: Goddard, C.R., Nakariakov, V.M.
Projects: SDO-AIA

Publication Status: A&A, accepted, DOI:10.1051/0004-6361/201628718
Last Modified: 2016-05-16 09:09
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Magnetohydrodynamic oscillations in the solar corona and Earth's magnetosphere: Towards consolidated understanding  

Valery Nakariakov   Submitted: 2016-05-13 03:25

Magnetohydrodynamic (MHD) oscillatory processes in different plasma systems, such as the corona of the Sun and the Earth's magnetosphere, show interesting similarities and differences, which so far received little attention and remain under-exploited. The successful commissioning within the past ten years of THEMIS, Hinode, STEREO and SDO spacecraft, in combination with matured analysis of data from earlier spacecraft (Wind, SOHO, ACE, Cluster, TRACE and RHESSI) makes it very timely to survey the breadth of observations giving evidence for MHD oscillatory processes in solar and space plasmas, and state-of-the-art theoretical modelling. The paper reviews several important topics, such as Alfvénic resonances and mode conversion; MHD waveguides, such as the magnetotail, coronal loops, coronal streamers; mechanisms for periodicities produced in energy releases during substorms and solar flares, possibility of Alfvénic resonators along open field lines; possible drivers of MHD waves; diagnostics of plasmas with MHD waves; interaction of MHD waves with partly-ionised boundaries (ionosphere and chromosphere). The review is mainly oriented to specialists in magnetospheric physics and solar physics, but not familiar with specifics of the adjacent research fields.

Authors: V.M. Nakariakov, V. Pilipenko, B. Heilig, P. Jelínek, M. Karlický́, D.Y. Klimushkin, D.Y. Kolotkov, D.-H. Lee, G. Nisticò, T. Van Doorsselaere, G. Verth, I.V. Zimovets
Projects: None

Publication Status: Space Science Reviews, published
Last Modified: 2016-05-14 07:55
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Solar and Heliospheric Physics with the Square Kilometre Array  

Valery Nakariakov   Submitted: 2014-12-23 05:09

The fields of solar radiophysics and solar system radio physics, or radio heliophysics, will benefit immensely from an instrument with the capabilities projected for SKA. Potential applications include interplanetary scintillation (IPS), radio-burst tracking, and solar spectral radio imaging with a superior sensitivity. These will provide breakthrough new insights and results in topics of fundamental importance, such as the physics of impulsive energy releases, magnetohydrodynamic oscillations and turbulence, the dynamics of post-eruptive processes, energetic particle acceleration, the structure of the solar wind and the development and evolution of solar wind transients at distances up to and beyond the orbit of the Earth. The combination of the high spectral, time and spatial resolution and the unprecedented sensitivity of the SKA will radically advance our understanding of basic physical processes operating in solar and heliospheric plasmas and provide a solid foundation for the forecasting of space weather events.

Authors: Valery M. Nakariakov, Mario M. Bisi, Philippa K. Browning, Dalmiro Maia, Eduard P. Kontar, Divya Oberoi, Peter T. Gallagher, Iver H. Cairns, Heather Ratcliffe
Projects: None

Publication Status: Proc. of Science (in press), 2015
Last Modified: 2014-12-23 14:49
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Excitation of kink oscillations of coronal loops: statistical study  

Valery Nakariakov   Submitted: 2014-12-18 10:01

Context. Solar flares are often accompanied by kink (transverse) oscillations of coronal loops. Despite intensive study of these oscillations in recent years mechanisms for their excitation remain unrevealed. Aims. To clarify the excitation mechanisms for kink oscillations of coronal loops. Methods. We analyse 58 kink oscillation events observed by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) during its first four years (2010-2014) with the use of the JHelioviewer. Association of these oscillation events with flares, lower coronal eruptions and plasma ejections, coronal mass ejections (CMEs) and coronal type II radio bursts is studied. Results. It is found that 44 out of these 58 oscillation events (76%) were associated with CMEs observed in the white light emission. Moreover, 57 events (98%) were accompanied by lower coronal eruptions/ejections (LCEs) observed in the EUV band in the parental active regions. An LCE was not clearly seen only in one event but it was definitely associated with a CME. The main observational finding is that the kink oscillations were excited by the deviation of loops from their equilibria by a nearby LCEs in 55 events (95%). In 3 remaining events it was difficult to reliably determine the cause of the oscillations because of limitations of the observational data. We also found that 53 events (91%) were associated with flares. In five remaining events the parental active regions were behind the limb and we could not directly see flare sites. It indicates that there is a close relationship between these two kinds of the solar activity. However, the estimated speeds of a hypothetical driver of kink oscillations by flares were found to be low than 500 km s-1 in 80% of cases. Such low speeds are not in favour of the association of the oscillation excitation with a shock wave, as it has usually been assumed. The fact that only 23 (40%) of the oscillation events were found to be associated with coronal type II radio bursts is also against the shock wave mechanism for the excitation of kink oscillations. Conclusions. The performed statistical analysis shows that the most probable mechanism for the excitation of kink oscillations of coronal loops is the deviation of loops from their equilibrium by nearby eruptions or plasma ejections rather than a blast shock wave ignited by a flare.

Authors: Zimovets, I.V., Nakariakov, V.M.
Projects: SDO-AIA

Publication Status: A&A, accepted
Last Modified: 2014-12-21 19:18
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Multi-mode quasi-periodic pulsations in a solar flare  

Valery Nakariakov   Submitted: 2014-12-18 09:53

Context. Quasi-periodic pulsations (QPP) of the electromagnetic radiation emitted in solar and stellar flares are often detected in microwave, white light, X-ray and gamma-ray bands. Mechanisms for QPP are intensively debated in the literature. Previous studies revealed that QPP may manifest nonlinear, non-stationary and, perhaps, multi-modal processes operating in flares. Aims. We study QPP of the microwave emission generated in an X3.2-class solar flare on 14 May, 2013, observed with the Nobeyama Radioheliograph (NoRH), aiming to reveal signatures of the nonlinear, non-stationary, and multi-modal processes in the signal. Methods. The NoRH correlation signal obtained at the 17 GHz intensity has a clear QPP pattern. The signal was analysed with the Hilbert?Huang transform (HHT) that allows one to determine its instant amplitude and frequency, and their time variation. Results. It was established that the QPP consists of at least three well-defined intrinsic modes, with the mean periods of 15, 45 and 100 seconds. All the modes have quasi-harmonic behaviour with different modulation patterns. The 100-second intrinsic mode is a decaying oscillation, with the decay time of 250 seconds. The 15-second intrinsic mode shows a similar behaviour, with the decay time of 90 seconds. The 45-s mode has a wave-train behaviour. Conclusions. Dynamical properties of detected intrinsic modes indicate that the 100-s and 15-s modes are likely to be associated with fundamental kink and sausage modes of the flaring loop, respectively. The 100-s oscillation could also be caused by the fundamental longitudinal mode,while this interpretation requires the plasma temperature of about 30 million K and hence is not likely. The 45-s mode could be the second standing harmonics of the kink mode.

Authors: Kolotkov, D. Y., Nakariakov, V. M., Kupriyanova, E. G., Ratcliffe, H., Shibasaki, K.
Projects: Nobeyama Radioheliograph

Publication Status: A&A, accepted
Last Modified: 2014-12-21 19:19
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