Valery Nakariakov - Academia.edu (original) (raw)

Papers by Valery Nakariakov

Decayless kink oscillations of plasma loops in the solar corona may contain an answer to the enig... more 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. 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, reveals, for the first time, 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 f...

Solar-Terrestrial Physics

The paper discusses the diagnostics of plasma jets in the solar corona with the use of data from ... more The paper discusses the diagnostics of plasma jets in the solar corona with the use of data from modern space- and ground-based telescopes observing the Sun in the extreme ultraviolet (EUV) and micro- wave bands. We examine observational parameters of EUV and radio emission in events associated with plasma jets, depending on the mechanism of formation, initiation conditions, and evolution of the jets. The opportunities provided by the study of plasma jets, which relies on simultaneous observations in different bands, are highlighted. For a number of jets, we have measured their primary parameters; and in this paper we present preliminary results of statistical processing of the data obtained. Microwave observations of several specific events, made by ground-based instruments RATAN-600, SRH, and Nobeyama Radioheliograph, are considered in detail. The diagnostic capabilities of these instruments for studying coronal jets are shown. To analyze the three-dimensional structure of the cor...

Proceedings of Advancing Astrophysics with the Square Kilometre Array — PoS(AASKA14), 2015

The fields of solar radiophysics and solar system radio physics, or radio heliophysics, will bene... more 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.

Physics of Plasmas, 2014

Quasilinear theory has long been used to treat the problem of a weak electron beam interacting wi... more Quasilinear theory has long been used to treat the problem of a weak electron beam interacting with plasma and generating Langmuir waves. Its extension to weak-turbulence theory treats resonant interactions of these Langmuir waves with other plasma wave modes, in particular, ion-sound waves. These are strongly damped in plasma of equal ion and electron temperatures, as sometimes seen in, for example, the solar corona and wind. Weak turbulence theory is derived in the weak damping limit, with a term describing ion-sound wave damping then added. In this paper, we use the EPOCH particle-in-cell code to numerically test weak turbulence theory for a range of electron-ion temperature ratios. We find that in the cold ion limit, the results agree well, but for increasing ion temperature the three-wave resonance becomes broadened in proportion to the ion-sound wave damping rate. Additionally, we establish lower limits on the number of simulation particles needed to accurately reproduce the e...

Space Science Reviews, 2009

ABSTRACT We present the first identification of the global (or fundamental) fast magnetoacoustic ... more ABSTRACT We present the first identification of the global (or fundamental) fast magnetoacoustic sausage mode of a coronal loop in the X-ray band, based upon the detailed analysis of multi-wavelength spatially-resolving observations of the M9.3 solar flare on 6 November 2004 by RHESSI. High quality harmonic oscillations with the time period of about 78 s and Q=240 have been detected in the thermal X-ray (with energies lower than 25 keV) emission flux in the decay phase of the flare. Similar quasi-periodic pulsations were also observed in the decimetric-centimetric radio emission flux (as pulsations of type IV radio burst). In the non-thermal hard X-ray emission (higher than 25 keV) the oscillations were less pronounced. It is found that the area of the quasi-stationary and cooling soft X-ray source (lower than 15 keV), which was situated near the top of the flaring loop, varied in anti-phase with the oscillating flux of its radiation. The oscillation period remained constant during all the oscillations. The observed properties of the oscillations coincide with the theoretically predicted properties of standing sausage modes. Cooling of heated plasma during the oscillations is explained mainly by the conductive losses.

The phase mixing of Alfvén waves in planar two-dimensional open magnetic configurations is consid... more The phase mixing of Alfvén waves in planar two-dimensional open magnetic configurations is considered. It is assumed that the characteristic vertical spatial scale of the configuration is much larger than the horizontal scale, and that the latter is of the order of a wavelength. The WKB method is used to derive the governing equation for the wave amplitude, which in appropriate coordinates is the diffusion equation with the diffusion coefficient being spatially dependent. The dependency of the energy flux on the vertical coordinate is obtained for monochromatic waves, and illustrated for three particular cases. The theory is applied to Alfvén wave damping in coronal holes.

We develop a new method for the determination of the absolute value of the magnetic field strengt... more We develop a new method for the determination of the absolute value of the magnetic field strength in coronal closed magnetic structures, based on the analysis of flare-generated oscillations of coronal loops. Interpretation of the quasi-harmonic oscillations observed in terms of global standing kink waves allows to connect the period of the oscillations and the loops length with the magnetic field strength in the loops. Improved diagnostic of the loop length, the oscillation period, and the plasma density in the loop will significantly improve the method's precision.

The Astrophysical Journal, 2000

The evolution of quasi-isentropic magnetohydrodynamic waves of small but finite amplitude in an o... more The evolution of quasi-isentropic magnetohydrodynamic waves of small but finite amplitude in an optically thin plasma is analyzed. The plasma is assumed to be initially homogeneous, in thermal equilibrium and with a straight and homogeneous magnetic field frozen in. Depending on the particular form of the heating/cooling function, the plasma may act as a dissipative or active medium for magnetoacoustic waves, while Alfvén waves are not directly affected. An evolutionary equation for fast and slow magnetoacoustic waves in the single wave limit, has been derived and solved, allowing us to analyse the wave modification by competition of weakly nonlinear and quasiisentropic effects. It was shown that the sign of the quasi-isentropic term determines the scenario of the evolution, either dissipative or active. In the dissipative case, when the plasma is first order isentropically stable the magnetoacoustic waves are damped and the time for shock wave formation is delayed. However, in the active case when the plasma is isentropically overstable, the wave amplitude grows, the strength of the shock increases and the breaking time decreases. The magnitude of the above effects depends upon the angle between the wave vector and the magnetic field. For hot (T > 10 4 K) atomic plasmas with solar abundances either in the interstellar medium or in the solar atmosphere, as well as for the cold (T < 10 3 K) ISM molecular gas, the range of temperature where the plasma is isentropically unstable and the corresponding time and length-scale for wave breaking have been found.

The Astrophysical Journal, 2009

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2012

Recent observations have revealed that magnetohydrodynamic (MHD) waves and oscillations are ubiqu... more Recent observations have revealed that magnetohydrodynamic (MHD) waves and oscillations are ubiquitous in the solar atmosphere, with a wide range of periods. We give a brief review of some aspects of MHD waves and coronal seismology that have recently been the focus of intense debate or are newly emerging. In particular, we focus on four topics: (i) the current controversy surrounding propagating intensity perturbations along coronal loops, (ii) the interpretation of propagating transverse loop oscillations, (iii) the ongoing search for coronal (torsional) Alfvén waves, and (iv) the rapidly developing topic of quasi-periodic pulsations in solar flares.

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2006

Polar plumes are cool, dense, linear, magnetically open structures that arise from predominantly ... more Polar plumes are cool, dense, linear, magnetically open structures that arise from predominantly unipolar magnetic footpoints in the solar polar coronal holes. As the Alfvén speed is decreased in plumes in comparison with the surrounding medium, these structures are natural waveguides for fast and slow magnetoacoustic waves. The simplicity of the geometry of polar plumes makes them an ideal test ground for the study of magnetohydrodynamic (MHD) wave interaction with solar coronal structures. The review covers recent observational findings of compressible and incompressible waves in polar plumes with imaging and spectral instruments, and interpretation of the waves in terms of MHD theory.

A new mechanism for the enhanced generation of compressible fluctuations by Alfven waves is prese... more A new mechanism for the enhanced generation of compressible fluctuations by Alfven waves is presented. A strongly nonlinear regime of Alfven wave phase-mixing is numerically simulated in a one-dimensionally inhomogeneous plasma of finite temperature. It is found that the inhomogeneity of the medium determines the efficiency of nonlinear excitation of magnetoacoustic waves. The level of the compressible fluctuations is found to be higher (up to the factor of two) in inhomogeneous regions. The amplitude of the generated magnetoacoustic wave can reach up to 30% of the source Alfven wave amplitude, and this value is practically independent of the Alfven wave amplitude and the steepness of Alfven speed profile. The highest amplitudes of compressible disturbances are reached in plasmas with beta of about 0.5. The further growth of the amplitude of compressible fluctuations is depressed by saturation.

The Astrophysical Journal, 2012

The Astrophysical Journal, 2011

Flows and instabilities play a major role in the dynamics of magnetized plasmas including the sol... more Flows and instabilities play a major role in the dynamics of magnetized plasmas including the solar corona, magnetospheric and heliospheric boundaries, cometary tails, and astrophysical jets. The nonlinear effects, multiscale and microphysical interactions inherent to the flow-driven instabilities, are believed to play a role, e.g., in plasma entry across a discontinuity, generation of turbulence, and enhanced drag. However, in order to clarify the efficiency of macroscopic instabilities in these processes, we lack proper knowledge of their overall morphological features. Here we show the first observations of the temporally and spatially resolved evolution of the magnetic Kelvin-Helmholtz instability in the solar corona. Unprecedented high-resolution imaging observations of vortices developing at the surface of a fast coronal mass ejecta are taken by the new Solar Dynamics Observatory, validating theories of the nonlinear dynamics involved. The new findings are a cornerstone for developing a unifying theory on flow-driven instabilities in rarefied magnetized plasmas, which is important for understanding the fundamental processes at work in key regions of the Sun-Earth system.

The Astrophysical Journal, 2010