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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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A Combined Analysis of the Observational Aspects of the Quasi-biennial Oscillation in Solar Magnetic Activity  

Valery Nakariakov   Submitted: 2014-11-02 06:34

Solar quasi-biennial oscillations (QBOs) with the time scale of 0.6?4 yrs appear to be a basic feature of the Sun's activity. Observational aspects of QBOs are reviewed on the basis of recent publications. Solar QBOs are shown to be ubiquitous and very variable. We demonstrate that many features of QBOs are common to different observations. These features include variable periodicity and intermittence with signs of stochastisity, a presence at all levels of the solar atmosphere and even in the convective zone, independent development in the northern and southern solar hemispheres, most pronounced amplitudes during the maximum phase of the 11-yr cycle and the transition of QBOs into interplanetary space. Temporal weakening of solar activity around the maximum of the 11-yr cycle (Gnevyshev Gap) can be considered an integral part of QBOs. The exact mechanism by which the solar QBO is produced is poorly understood. We describe some of the most plausible theoretical mechanisms and discuss observational features that support/contradict the theory. QBOs have an important meaning as a benchmark of solar activity, not only for investigation of the solar dynamo but also in terms of space weather.

Authors: G. Bazilevskaya, A.-M. Broomhall, Y. Elsworth, V. M. Nakariakov
Projects: None

Publication Status: Space Science Reviews, published online
Last Modified: 2014-11-03 07:55
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Nonlinear slow magnetoacoustic waves in coronal plasma structures  

Valery Nakariakov   Submitted: 2014-11-02 06:27

Context. There is abundant observational evidence of longitudinal waves in the plasma structures of the solar corona. These essentially compressive waves are confidently interpreted as slow magnetoacoustic waves. The use of the slow waves in plasma diagnostics and estimating their possible contribution to plasma heating and acceleration require detailed theoretical modelling. Aims. We investigate the role of obliqueness and magnetic effects in the evolution of slow magnetoacoustic waves, also called tube waves, in field-aligned plasma structures. Special attention is paid to the wave damping caused by nonlinear steepening. Methods. We considered an untwisted straight axisymmetric field-aligned plasma cylinder and analysed the behaviour of the slow magnetoacoustic waves that are guided by this plasma structure. We adopted a thin flux tube approximation. We took into account dissipation caused by viscosity, resistivity and thermal conduction, and nonlinearity. Effects of stratification and dispersion caused by the finite radius of the flux tube were neglected. Results. We derive the Burgers-type evolutionary equation for tube waves in a uniform plasma cylinder. Compared with a plane acoustic wave, the formation of shock fronts in tube waves is found to occur at a larger distance from the source. In addition, tube waves experience stronger damping. These effects are most pronounced in plasmas with the parameter beta at about or greater than unity. In a low-beta plasma, the evolution of tube waves can satisfactorily be described with the Burgers equation for plane acoustic waves.

Authors: A. N. Afanasyev, V.M. Nakariakov
Projects: None

Publication Status: A&A, accepted
Last Modified: 2014-11-03 07:57
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Wave dynamics in a sunspot umbra  

Valery Nakariakov   Submitted: 2014-09-01 02:05

Context. Sunspot oscillations are one of the most frequently studied wave phenomena in the solar atmosphere. Understanding the basic physical processes responsible for sunspot oscillations requires detailed information about their fine structure. Aims. We aim to reveal the relationship between the fine horizontal and vertical structure, time evolution, and the fine spectral structure of oscillations in a sunspot umbra. Methods. The high spatial and time resolution data obtained with SDO/AIA for the sunspot in active region NOAA 11131 on 08 December 2010 were analysed with the time-distance plot technique and the pixelised wavelet filtering method. Different levels of the sunspot atmosphere were studied from the temperature minimum to the corona. Results. Oscillations in the 3 min band dominate in the umbra. The integrated spectrum of umbral oscillations contains distinct narrowband peaks at 1.9 min, 2.3 min, and 2.8 min. The power significantly varies in time, forming distinct 12?20 min oscillation trains. The oscillation power distribution over the sunspot in the horizontal plane reveals that the enhancements of the oscillation amplitude, or wave fronts, have a distinct structure consisting of an evolving two-armed spiral and a stationary circular patch at the spiral origin, situated near the umbra centre. This structure is seen from the temperature minimum at 1700? to the 1.6 MK corona at 193?. In time, the spiral rotates anti-clockwise. The wave front spirality is most pronounced during the maximum amplitude phases of the oscillations, and in the bandpasses where umbral oscillations have the highest power, 304? and 171?. In the low-amplitude phases the spiral breaks into arc-shaped patches. The 2D cross-correlation function shows that the oscillations at higher atmospheric levels occur later than at lower layers. The phase speed is estimated to be about 100 km s-1. The fine spectral analysis shows that the central patch corresponds to the high-frequency oscillations, while the spiral arms highlight the lower-frequency oscillations in the 3-min band. Conclusions. The vertical and horizontal radial structure of the oscillations is consistent with the model that interprets umbral oscillations as slow magnetoacoustic waves filtered by the atmospheric temperature non-uniformity in the presence of the magnetic field inclination from the vertical. The mechanism for the polar-angle structure of the oscillations, in particular the spirality of the wave fronts, needs to be revealed.

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

Publication Status: A&A, accepted
Last Modified: 2014-09-03 13:12
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Sausage oscillations of coronal plasma slabs  

Valery Nakariakov   Submitted: 2014-05-15 06:41

Context. Sausage oscillations are observed in plasma non-uniformities of the solar corona as axisymmetric perturbations of the nonuniformity. Often, these non-uniformities can be modelled as field-aligned slabs of the density enhancement. Aims. We perform parametric studies of sausage oscillations of plasma slabs, aiming to determine the dependence of the oscillation period on its parameters, and the onset of leaky and trapped regimes of the oscillations. Methods. Slabs with smooth transverse profiles of the density of a zero-beta plasma are perturbed by an impulsive localised perturbation of the sausage symmetry. In particular, the slab can contain an infinitely thin current sheet in its centre. The initial value problem is then solved numerically. The numerical results are subject to spectral analysis. The results are compared with analytical solutions for a slab with a step-function profile and also with sausage oscillations of a plasma cylinder. Results. We established that sausage oscillations in slabs generally have the same properties as in plasma cylinders. In the trapped regime, the sausage oscillation period increases with the increase in the longitudinal wavelength. In the leaky regime, the dependence of the period on the wavelength experiences saturation, and the period becomes independent of the wavelength in the long-wavelength limit. In the leaky regime the period is always longer than in the trapped regime. The sausage oscillation period in a slab is always longer than in a cylinder with the same transverse profile. In slabs with steeper transverse profiles, sausage oscillations have longer periods. The leaky regime occurs at shorter wavelengths in slabs with smoother profiles.

Authors: Hornsey, C., Nakariakov, V.M. and Fludra, A.
Projects: None

Publication Status: A&A, accepted
Last Modified: 2014-05-15 15:19
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Quasi-periodic wiggles of microwave zebra structures in a solar flare  

Valery Nakariakov   Submitted: 2013-09-23 10:03

Quasi-periodic wiggles of microwave zebra pattern structures with period range from about 0.5 s to 1.5 s are found in a X-class solar flare on 2006 December 13 at the 2.6-3.8 GHz with the Chinese Solar Broadband Radio Spectrometer (SBRS/Huairou). Periodogram and correlation analysis show that the wiggles have two-three significant periodicities and almost in phase between stripes at different frequency. The Alfvén speed estimated from the zebra pattern structures is about 700 km s-1. We obtain the spatial size of the waveguiding plasma structure to be about 1 Mm with the detected period of about 1 s. It suggests the ZP wiggles can be associated with the fast magnetoacoustic oscillations in the flaring active region. The lack of a significant phase shift between wiggles of different stripes suggests that the ZP wiggles are caused by a standing sausage oscillation.

Authors: Sijie Yu, V. M. Nakariakov, L. A. Selzer, Baolin Tan and Yihua Yan
Projects: None

Publication Status: ApJ (accepted)
Last Modified: 2013-09-25 17:56
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Decay-less kink oscillations in coronal loops  

Valery Nakariakov   Submitted: 2013-08-22 14:49

Kink oscillations of coronal loops in an off-limb active region are detected with the Imaging Assembly Array (AIA) instruments of the Solar Dynamics Observatory (SDO) at 171~AA. We aim to measure periods and amplitudes of kink oscillations of different loops and to determinate the evolution of the oscillation phase along the oscillating loop. Oscillating coronal loops were visually identified in the field of view of SDO/AIA and STEREO/EUVI-A: the loop length was derived by three-dimensional analysis. Several slits were taken along the loops to assemble time-distance maps. We identified oscillatory patterns and retrieved periods and amplitudes of the oscillations. We applied the cross-correlation technique to estimate the phase shift between oscillations at different segments of oscillating loops. We found that all analysed loops show low-amplitude undamped transverse oscillations. Oscillations periods of loops in the same active region range from 2.5 to 11 min, and are different for different loops. The displacement amplitude is lower than 1~Mm. The oscillation phase is constant along each analysed loop. The spatial structure of the phase of the oscillations corresponds to the fundamental standing kink mode. We conclude that the observed behaviour is consistent with the empirical model in terms of a damped harmonic resonator affected by a non-resonant continuously operating external force.

Authors: S.A. Anfinogentov, G. Nistico, V. M. Nakariakov
Projects: SDO-AIA

Publication Status: A&A (accepted)
Last Modified: 2013-08-22 14:49
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The decaying long-period oscillation of a stellar megaflare  

Valery Nakariakov   Submitted: 2013-08-06 00:11

We analyze and interpret the oscillatory signal in the decay phase of the U-band light curve of a stellar megaflare observed on 2009 January 16 on the dM4.5e star YZ CMi. The oscillation is well approximated by an exponentially decaying harmonic function. The period of the oscillation is found to be 32 minutes, the decay time about 46 minutes, and the relative amplitude 15%. As this observational signature is typical of the longitudinal oscillations observed in solar flares at extreme ultraviolet and radio wavelengths, associated with standing slow magnetoacoustic waves, we suggest that this megaflare may be of a similar nature. In this scenario, macroscopic variations of the plasma parameters in the oscillations modulate the ejection of non-thermal electrons. The phase speed of the longitudinal (slow magnetoacoustic) waves in the flaring loop or arcade, the tube speed, of about 230 km s?1 would require a loop length of about 200 Mm. Other mechanisms, such as standing kink oscillations, are also considered.

Authors: S. Anfinogentov, V. M. Nakariakov, M. Mathioudakis, T. Van Doorsselaere and A. F. Kowalski
Projects: None

Publication Status: ApJ 773, 156, 2013; doi:10.1088/0004-637X/773/2/156
Last Modified: 2013-08-06 14:52
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Sausage oscillations of coronal plasma structures  

Valery Nakariakov   Submitted: 2012-10-30 16:08

Dependence of the period of sausage oscillations of coronal loops on its length and the radial profile depth and steepness is determined. We performed a parametric study of linear axisymmetric fast magnetoacoustic (sausage) oscillations of coronal loops modelled as a field-aligned low-beta plasma cylinder with a smooth inhomogeneity of the plasma density in the radial direction. The density decreases smoothly in the radial direction. Sausage oscillations are impulsively excited by a perturbation of the radial velocity, localised at the cylinder axis and having a harmonic dependence on the longitudinal coordinate. The initial perturbation results either in a leaky or trapped sausage oscillation, depending upon whether the longitudinal wavenumber is smaller or greater than a cut-off value, respectively. The period of the sausage oscillations was found to always grow with the increase in the longitudinal wavelength, with the saturation of this dependence in the long-wavelength limit. Deeper and steeper radial profiles of the Alfvén speed correspond to more efficient trapping of sausage modes: the cutoff value of the wavelength increases with the steepness and the density (or Alfvén speed) contrast ratio. In the leaky regime, the period is always longer than the period of a trapped mode of a shorter wavelength in the same cylinder. For shallow profiles of the density and shorter wavelengths, the period grows with the wavelength. In the long wavelength limit, the period becomes independent of the wavelength, and increases with the depth and steepness of the radial profile of the Alfvén speed.

Authors: Nakariakov, V.M., Hornsey, C., Melnikov, V.F.
Projects: None

Publication Status: accepted
Last Modified: 2012-10-30 19:45
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Slow Magnetoacoustic Waves in Two-Ribbon Flares  

Valery Nakariakov   Submitted: 2011-02-24 11:05

We demonstrate that disturbances observed to propagate along the axis of the arcade in two-ribbon solar flares at the speed of a few tens km s-1, well below the Alfvén and sound speeds, can be interpreted in terms of slow magnetoacoustic waves. The waves can propagate across the magnetic field, parallel to the magnetic neutral line, because of the wave-guiding effect due to the reflection from the footpoints. The perpendicular group speed of the perturbation is found to be a fraction of the sound speed, which is consistent with observations. The highest value of the group speed grows with the increase in the ratio of the sound and Alfvén speeds. For a broad range of parameters, the highest value of the group speed corresponds to the propagation angle of 25-28 degrees to the magnetic field. This effect can explain the temporal and spatial structure of quasi-periodic pulsations observed in two-ribbon flares.

Authors: Nakariakov, V.M., Zimovets, I.V.
Projects: None

Publication Status: ApJ L, accepted
Last Modified: 2011-02-24 19:33
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Oscillatory processes in solar flares  

Valery Nakariakov   Submitted: 2010-10-05 07:21

Electromagnetic (radio, visible-light, UV, EUV, X-ray and gamma-ray) emission generated by solar and stellar flares often contains pronounced quasi-periodic pulsations (QPP). Physical mechanisms responsible for the generation of long-period QPP (with the periods longer than one second) are likely to be associated with MHD processes. The observed modulation depths, periods and anharmonicity of QPP suggest that they can be linked with some kind of MHD auto-oscillations, e.g. an oscillatory regime of magnetic reconnection. Such regimes, of both spontaneous and induced nature, have been observed in resistive-MHD numerical simulations. The oscillations are essentially nonlinear and non-stationary. We demonstrate that a promising novel method for their analysis is the Empirical Mode Decomposition technique.

Authors: Authors: V.M. Nakariakov, A.R. Inglis, I.V. Zimovets, C. Foullon, E. Verwichte, R. Sych, I. N. Myagkova
Projects: RHESSI

Publication Status: Plasma Phys. Controlled Fusion (in press)
Last Modified: 2010-10-05 08:00
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Abstracts by Author
Non-stationary quasi-periodic pulsations in solar and stellar flares
Quasi-periodic Pulsations in a Solar Microflare
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Related Pages
MSU Solar Physics.
Max Millennium Science Mail Archive.
Max Millennium Message of the Day Mail Archive.
Max Millennium Flare Catalog

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