arXiv subjects

Valery M. Nakariakov

Publications and source records attributed to Valery M. Nakariakov.

At least 55 records · Page 3Linked to original sources

arXiv2019

A New Type of Jets in a Polar Limb of Solar Coronal Hole

A new type of chromospheric jets in a polar limb of a coronal hole is discovered in the Ca II filtergram of the Solar Optical Telescope on board the \textit{Hinode}. We identify 30 jets in the Ca II movie of duration of 53 min. The average speed at their maximum heights is found to be 132$\pm$44 km s$^{-1}$ ranging from 57 km s$^{-1}$ to 264 km s$^{-1}$ along the propagation direction. The average lifetime is 20$\pm$6 ranging from 11 seconds to 36 seconds. The speed and lifetime of the jets are located at end-tails of those parameters determined for type II spicules, hence implying a new type of jets. To confirm whether these jets are different from conventional spicules, we construct a time-height image averaged over horizontal region of 1$\arcsec$, and calculate lagged cross-correlations of intensity profiles at each height with the intensity at 2 Mm. From this, we obtain a cross-correlation map as a function of lag and height. We find that the correlation curve as a function of lag time is well fitted into three different Gaussian functions whose standard deviations of the lag time are 193 seconds, 42 seconds, and 17 seconds. The corresponding propagation speeds are calculated to be 9 km s$^{-1}$, 67 km s$^{-1}$, and 121 km s$^{-1}$, respectively. The kinematic properties of the former two components seem to correspond to the 3 minutes oscillations and type II spicules, while the latter component to the jets addressed in this study.

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arXiv2019

A blueprint of state-of-the-art techniques for detecting quasi-periodic pulsations in solar and stellar flares

Quasi-periodic pulsations (QPPs) appear to be a common feature observed in the light curves of both solar and stellar flares. However, their quasi-periodic nature, along with the fact that they can be small in amplitude and short-lived, makes QPPs difficult to unequivocally detect. In this paper, we test the strengths and limitations of state-of-the-art methods for detecting QPPs using a series of hare-and-hounds exercises. The hare simulated a set of flares, both with and without QPPs of a variety of forms, while the hounds attempted to detect QPPs in blind tests. We use the results of these exercises to create a blueprint for anyone who wishes to detect QPPs in real solar and stellar data. We present eight clear recommendations to be kept in mind for future QPP detections, with the plethora of solar and stellar flare data from new and future satellites. These recommendations address the key pitfalls in QPP detection, including detrending, trimming data, accounting for colored noise, detecting stationary-period QPPs, detecting QPPs with nonstationary periods, and ensuring that detections are robust and false detections are minimized. We find that QPPs can be detected reliably and robustly by a variety of methods, which are clearly identified and described, if the appropriate care and due diligence are taken.

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arXiv2019

Evidence for Vortex Shedding in the Sun's Hot Corona

Vortex shedding is an oscillating flow that is commonly observed in fluids due to the presence of a blunt body in a flowing medium. Numerical simulations have shown that the phenomenon of vortex shedding could also develop in the magnetohydrodynamic (MHD) domain. The dimensionless Strouhal number, the ratio of the blunt body diameter to the product of the period of vortex shedding and the speed of a flowing medium, is a robust indicator for vortex shedding, and, generally of the order of 0.2 for a wide range of Reynolds number. Using an observation from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we report a wavelike or oscillating plasma flow propagating upward against the Sun's gravitational force. A newly formed shrinking loop in the post-flare region possibly generates the oscillation of the upflow in the wake of the hot and dense loop through vortex shedding. The computed Strouhal number is consistent with the prediction from previous MHD simulations. Our observation suggests the possibility of vortex shedding in the solar corona.

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arXiv2019

The Physical Nature of Spiral Wave Patterns in Sunspots

Recently, spiral wave patterns (SWPs) have been detected in 3 minute oscillations of sunspot umbrae, but the nature of this phenomenon has remained elusive. We present a theoretical model that interprets the observed SWPs as the superposition of two different azimuthal modes of slow magnetoacoustic waves driven below the surface in an untwisted and non-rotating magnetic cylinder. We apply this model to SWPs of the line-of-sight (LOS) velocity in a pore observed by the Fast Imaging Solar Spectrograph installed at the 1.6 m Goode Solar Telescope. One- and two-armed SWPs were identified in instantaneous amplitudes of LOS Doppler velocity maps of 3 minute oscillations. The associated oscillation periods are about 160 s, and the durations are about 5 minutes. In our theoretical model, the observed spiral structures are explained by the superposition of non-zero azimuthal modes driven 1600 km below the photosphere in the pore. The one-armed SWP is produced by the slow-body sausage (m = 0) and kink (m = 1) modes, and the two-armed SWP is formed by the slow-body sausage (m = 0) and fluting (m = 2) modes of the magnetic flux tube forming the pore.

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arXiv2019

Seismological determination of the Alfvén speed and plasma-beta in solar photospheric bright points

The Alfvén speed and plasma beta in photospheric bright points observed by the Broadband Filter Imager (BFI) of the Solar Optical Telescope (SOT) on board the \textit{Hinode} satellite, are estimated seismologically. The diagnostics is based on the theory of slow magnetoacoustic waves in a non-isothermally stratified photosphere with a uniform vertical magnetic field. We identify and track bright points in a G-band movie by using the 3D region growing method, and align them with blue continuum images to derive their brightness temperatures. From the Fourier power spectra of 118 continuum light curves made in the bright points, we find that light curves of 91 bright points have oscillations with properties which are significantly different from oscillation in quiet regions, with the periods ranging 2.2--16.2~min. We find that the model gives a moderate value of the plasma beta when $γ$ lies at around 5/3. The calculated Alfvén speed is 9.68$\pm$2.02~km~s$^{-1}$, ranging in 6.3--17.4~km~s$^{-1}$. The plasma beta is estimated to be of 0.93$\pm$0.36, ranging in 0.2--1.9.

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arXiv2018

In-situ generation of transverse MHD waves from colliding flows in the solar corona

Transverse MHD waves permeate the solar atmosphere and are a candidate for coronal heating. However, the origin of these waves is still unclear. In this work, we analyse coordinated observations from \textit{Hinode}/SOT and \textit{IRIS} of a prominence/coronal rain loop-like structure at the limb of the Sun. Cool and dense downflows and upflows are observed along the structure. A collision between a downward and an upward flow with an estimated energy flux of $10^{7}-10^{8}$~erg~cm$^{-2}$~s$^{-1}$ is observed to generate oscillatory transverse perturbations of the strands with an estimated $\approx40~$km~s$^{-1}$ total amplitude, and a short-lived brightening event with the plasma temperature increasing to at least $10^{5}~$K. We interpret this response as sausage and kink transverse MHD waves based on 2D MHD simulations of plasma flow collision. The lengths, density and velocity differences between the colliding clumps and the strength of the magnetic field are major parameters defining the response to the collision. The presence of asymmetry between the clumps (angle of impact surface and/or offset of flowing axis) is crucial to generate a kink mode. Using the observed values we successfully reproduce the observed transverse perturbations and brightening, and show adiabatic heating to coronal temperatures. The numerical modelling indicates that the plasma $β$ in this loop-like structure is confined between $0.09$ and $0.36$. These results suggest that such collisions from counter-streaming flows can be a source of in-situ transverse MHD waves, and that for cool and dense prominence conditions such waves could have significant amplitudes.

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arXiv2018

2D solar wind speeds from 6 to 26 solar radii in solar cycle 24 by using Fourier filtering

Measurement of the solar wind speed near the Sun is important for understanding the acceleration mechanism of the solar wind. In this study, we determine 2D solar wind speeds from 6 to 26 solar radii by applying Fourier motion filters to \textit{SOHO}/LASCO C3 movies observed from 1999 to 2010. Our method successfully reproduces the original flow speeds in the artificially generated data as well as streamer blobs. We measure 2D solar wind speeds from 1-day to 1-year timescales and their variation in solar cycle 24. We find that the solar wind speeds at timescales longer than a month in the solar maximum period are relatively uniform in the azimuthal direction, while they are clearly bimodal in the minimum period, as expected from the \textit{Ulysses} observations and IPS reconstruction. The bimodal structure appears at around 2006, becomes most distinctive in 2009, and abruptly disappears in 2010. The radial evolution of the solar wind speeds resembles the Parker's solar wind solution.

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arXiv2018

The origin of the modulation of the radio emission from the solar corona by a fast magnetoacoustic wave

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 $\sim2$ Mm, phase speed of $\sim657$ km s$^{-1}$, which gives the oscillation period of $\sim3$ s, and the relative amplitude of $\sim0.35$%. The longer component has the wavelength of $\sim12$ Mm, and relative amplitude of $\sim5.1$%. 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 $\sim1.1$ G which is consistent with the radial magnetic field model.

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arXiv2018

Quasi-periodic pulsations in the most powerful solar flare of Cycle 24

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.

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arXiv2018

Finite amplitude transverse oscillations of a magnetic rope

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.

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arXiv2017

Quasi-Periodic Radio Bursts Associated with Fast-mode Waves near a Magnetic Null Point

This paper presents an observation of quasi-periodic rapidly-propagating waves observed in the AIA 171/193 Å~ channels during the impulsive phase of an M1.9 flare occurred on 7 May 2012. The instant period was found to decrease from 240~s to 120~s, the speed of the wave fronts was in the range of ~664-1416 km/s. Almost simultaneously, quasi-periodic bursts with similar instant periods, ~70~s and ~140~s, occur in the microwave emission and in decimetric type IV, and type III radio bursts, and in the soft X-ray emission. The magnetic field configuration of the flare site was consistent with a breakout topology, i.e., a quadrupolar field along with a magnetic null point. The quasi-periodic rapidly-propagating wavefronts of the EUV emission are interpreted as a fast magnetoacoustic wave train. The observations suggest that the fast-mode waves are generated during the quasi-periodic magnetic reconnection in the cusp-region above the flare arcade loops. For the first time, we provide the evidence of a tadpole wavelet signature at about 70--140~s in decimetric (245/610~MHz) radio bursts, along with the direct observation of a coronal fast-mode wave train in EUV. In addition, at AIA 131/193 Å we observed quasi-periodic EUV disturbances with the periods of 95~s and 240~s propagating downward at the apparent speed of 172-273 km/s. The nature of these downward propagating disturbances is not revealed, but they could be connected with magnetoacoustic waves or periodically shrinking loops.

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arXiv2017

Observation of a Short Period Quasi-Periodic Pulsation in Solar X-ray, Microwave and EUV Emissions

This paper presents the multi-wavelength analysis of a 13 s quasi-periodic pulsation (QPP) observed in hard X-ray (12-300 keV) and microwave (4.9-34 GHz) emissions during a C-class flare occurred on 21 September 2015. AIA 304 and 171 Å~ images show an emerging loop/flux tube (L1) moving radially outward, which interacts with preexisting structures within the active region. The QPP was observed during the expansion and rising motion of L1. The Nobeyama Radioheliograph (NoRH) microwave images in 17/34 GHz channels reveal a single radio source, which was co-spatial with a neighboring loop (L2). In addition, using AIA 304 Å~ images, we detected intensity oscillations in the legs of loop L2 with a period of about 26 s. A similar oscillation period was observed in the GOES soft X-ray flux derivative. This oscillation period seems to increase with time. We suggest that the observed QPP is most likely generated by the interaction between loops L2 and L3 observed in the AIA hot channels (131 and 94 Å). The merging speed of loops L2 and L3 was ~35 km/s. Loop L1 destroyed possibly by its interaction with preexisting structures in the active region and produced a cool jet with the speed ~106-118 km/s associated with a narrow CME (~770 km/s). Another mechanism of the QPP in terms of a sausage oscillation of the loop (L2) is also possible.

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arXiv2016

Multi-instrument observations of a failed flare eruption associated with MHD waves in a loop bundle

We present observations of a B7.9-class flare that occurred on the 24th January, 2015, using the Atmopsheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO), the EUV Imaging Spectrometer (EIS) and the X-Ray Telescope of Hinode. The flare triggers the eruption of a dense cool plasma blob as seen in AIA 171Å,\, which is unable to completely break out and remains confined within a local bundle of active region loops. During this process, transverse oscillations of the threads are observed. The cool plasma is then observed to descend back to the chromosphere along each loop strand. At the same time, a larger diffuse co-spatial loop observed in the hot wavebands of SDO/AIA and Hinode/XRT is formed, exhibiting periodic intensity variations along its length. The formation and evolution of magnetohydrodynamic (MHD) waves depend upon the values of the local plasma parameters (e.g. density, temperature and magnetic field), which can hence be inferred by coronal seismology. In this study we aim to assess how the observed MHD modes are affected by the variation of density and temperature. We combined analysis of EUV/X-ray imaging and spectroscopy using SDO/AIA, Hinode/EIS and XRT. We show that the evolution of the detected waves is determined by the temporal variations of the local plasma parameters, caused by the flare heating and the consequent cooling. We apply coronal seismology to both waves obtaining estimations of the background plasma parameters.

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arXiv2016

Motion magnification in coronal seismology

We introduce a new method for the investigation of low-amplitude transverse oscillations of solar plasma non-uniformities, such as coronal loops, individual strands in coronal arcades, jets, prominence fibrils, polar plumes, and other contrast features, observed with imaging instruments. The method is based on the two-dimensional dual tree complex wavelet transform (DT$\mathbb{C}$WT). It allows us to magnify transverse, in the plane-of-the-sky, quasi-periodic motions of contrast features in image sequences. The tests performed on the artificial data cubes imitating exponentially decaying, multi-periodic and frequency-modulated kink oscillations of coronal loops showed the effectiveness, reliability and robustness of this technique. The algorithm was found to give linear scaling of the magnified amplitudes with the original amplitudes provided they are sufficiently small. Also, the magnification is independent of the oscillation period in a broad range of the periods. The application of this technique to SDO/AIA EUV data cubes of a non-flaring active region allowed for the improved detection of low-amplitude decay-less oscillations in the majority of loops.

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arXiv2016

Observation of a Quasi-periodic Pulsation in Hard X-ray, Radio and Extreme-ultraviolet Wavelengths

We present multi-wavelength analysis of a quasi-periodic pulsation (QPP) observed in the hard X-ray, radio, and extreme-ultraviolet (EUV) channels during an M1.9 flare occurred on 23-24 September 2011. The non-thermal hard X-ray emission in 25-50 keV observed by RHESSI shows five distinct impulsive peaks of decaying amplitude with a period of about three minutes. Similar QPP was observed in the microwave emission recorded by the Nobeyama Radioheliograph and Polarimeter in the 8.8, 15, 17 GHz channels. Interestingly, the 3-min QPP was also observed in the metric and decimetric radio frequencies (25-180, 245, 610 MHz) as repetitive type III bursts. Multi-wavelength observations from the SDO/AIA, Hinode/SOT, and STEREO/SECCHI suggest a fan-spine topology at the eruption site, associated with the formation of a quasi-circular ribbon during the flare. A small filament was observed below the fan-loops before the flare onset. The filament rose slowly and interacted with the ambient field. This behaviour was followed by an untwisting motion of the filament. Two different structures of the filament showed $\sim$3-min periodic alternate rotation in the clockwise and counterclockwise directions. The 3-min QPP was found to highly correlate with 3-min oscillations in a nearby sunspot. We suggest that the periodic reconnection (modulated either by sunspot slow-mode wave or by untwisting filament) at a magnetic null-point most likely causes the repetitive particle acceleration, generating the QPP observed in hard X-ray, microwave and type III radio bursts.

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arXiv2015

A Multi-Period Oscillation in a Stellar Superflare

Flares that are orders of magnitude larger than the most energetic solar flares are routinely observed on Sun-like stars, raising the question of whether the same physical processes are responsible for both solar and stellar flares. In this letter we present a white-light stellar superflare on the star KIC9655129, observed by NASA's Kepler mission, with a rare multi-period quasi-periodic pulsation (QPP) pattern. Two significant periodic processes were detected using the wavelet and autocorrelation techniques, with periods of 78 +/- 12 min and 32 +/- 2 min. By comparing the phases and decay times of the two periodicities, the QPP signal was found to most likely be linear, suggesting that the two periodicities are independent, possibly corresponding either to different magnetohydrodynamic modes of the flaring region, or different spatial harmonics of the same mode. The presence of multiple periodicities is a good indication that the QPPs were caused by magnetohydrodynamic oscillations, and suggests that the physical processes in operation during stellar flares could be the same as those in solar flares.

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arXiv2015

North-South asymmetry in the magnetic deflection of polar coronal hole jets

Measurements of the magnetic field in the interplanetary medium, of the sunspots area, and of the heliospheric current sheet position, reveal a possible North-South asymmetry in the magnetic field of the Sun. We study the North-South asymmetry as inferred from measurements of the deflection of polar coronal hole jets when they propagate throughout the corona. Since the corona is an environment where the magnetic pressure is greater than the kinetic pressure, we can assume that magnetic field controls the dynamics of plasma. On average, jets during their propagation follow the magnetic field lines, highlighting its local direction. The average jet deflection is studied both in the plane perpendicular to the line of sight, and, for a reduced number of jets, in three dimensional space. The observed jet deflection is studied in terms of an axisymmetric magnetic field model comprising dipole We measured the position angles at 1 rs and at 2 rs of the 79 jets from the catalogue of Nistico et al 2009., based on the STEREO ultraviolet and white-light coronagraph observations during the solar minimum period March 2007-April 2008. We found that the propagation is not radial, in agreement with the deflection due to magnetic field lines. Moreover, the amount of the deflection is different between jets over the north and those from the south pole. Comparison of jet deflections and field line tracing shows that a ratio g2/g1 ~ -0.5 for the quadrupole and a ratio g3/g1 ~ 1.6-2.0 for the esapole can describe the field. The presence of a non-negligible quadrupole moment. We find that the magnetic deflection of jets is larger in the North than in the South of the order of 25-40%, with an asymmetry which is consistent with a southward deflection of the heliospheric current sheet of the order of 10 deg, consistent with that inferred from other, independent, datasets and instruments.

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arXiv2015

Solar and Heliospheric Physics with the Square Kilometre Array

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.

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