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Long-period quasi-periodic oscillations of a small-scale magnetic structure on the Sun  

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

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

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

Publication Status: accepted for publication in A&A letters
Last Modified: 2016-12-28 11:41
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Empirical mode decomposition analysis of random processes in the solar atmosphere  

Dmitrii Kolotkov   Submitted: 2016-06-10 08:12

Solar signals of various types often show appearance of coloured noisy components with a power law spectral energy distribution. Such a frequency-dependent noise may indicate the operation of various randomly distributed dynamical processes in the solar atmosphere. We develop a recipe for the correct usage of the empirical mode decomposition (EMD) technique in the presence of coloured noise, allowing for the clear distinguishing between quasi-periodic oscillatory phenomena in the solar atmosphere and superimposed random background processes. For illustration, we statistically investigate EUV emission intensity variations observed with SDO/AIA in the coronal (171 Å), chromospheric (304 Å), and upper photospheric (1600 Å) layers of the solar atmosphere, from a quiet sun and a sunspot umbrae regions. EMD has been used for analysis due to its adaptive nature and essential applicability to the processing non-stationary and amplitude-modulated time series. For the comparison of the results obtained with EMD, we use the Fourier transform technique as an etalon. We empirically revealed statistical properties of synthetic coloured noises in EMD, and suggested a scheme allowing for the detection of noisy components among the intrinsic modes obtained with EMD in real signals. Application of the method to the solar EUV signals showed that they indeed behave randomly and could be represented as a combination of different coloured noises characterised by a specific value of the power law indices in their spectral energy distributions. On the other hand, 3-min oscillations in the analysed sunspot were detected to have energies significantly above the corresponding noise level. The correct accounting for the background frequency-dependent random processes is essential when using EMD for analysis of oscillations in the solar atmosphere. For the quiet sun region the power law index was found to increase with height above the photosphere, indicating that the higher frequency processes are trapped deeper in the quiet sun atmosphere. In contrast, lower levels of the sunspot umbrae were found to be characterised by higher values of the power law index, meaning the domination of lower frequencies deep inside the sunspot atmosphere. Comparison of the EMD results with those obtained with the Fourier transform showed good consistency, justifying the applicability of EMD.

Authors: D. Y. Kolotkov, S. A. Anfinogentov, and V. M. Nakariakov
Projects: None

Publication Status: accepted, A&A
Last Modified: 2016-06-10 15:23
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Nonlinear oscillations of coalescing magnetic flux ropes  

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

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

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

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

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

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

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

Publication Status: in press in A&A
Last Modified: 2016-05-04 09:22
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Kinetic model of force-free current sheets with non-uniform temperature  

Dmitrii Kolotkov   Submitted: 2015-10-28 07:27

The kinetic model of a one-dimensional force-free current sheet (CS) developed recently by M. Harrison and T. Neukirch (Phys. Rev. Lett., vol. 6, 2009) predicts uniform distributions of the plasma temperature and density across the CS. However, in realistic physical systems inhomogeneities of these plasma parameters may arise quite naturally due to the boundary conditions or local plasma heating. Moreover, as the CS spatial scale becomes larger than the characteristic kinetic scales (the regime often referred to as the MHD limit) it should be possible to set arbitrary density and temperature profiles. Thus, an advanced model has to allow for inhomogeneities of the macroscopic plasma parameters across the CS, to be consistent with the MHD limit. In this paper we generalise the kinetic model of a force-free current sheet, taking into account the inhomogeneity of the density and temperature across the CS. In the developed model the density may either be enhanced or depleted in the CS central region. The temperature profile is prescribed by the density profile, keeping the plasma pressure uniform across the CS. All macroscopic parameters, as well as the distribution functions for the protons and electrons, are determined analytically. Applications of the developed model to current sheets observed in space plasmas are discussed.

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

Publication Status: accepted for publication in Physics of Plasmas
Last Modified: 2015-10-28 15:23
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Abstracts by Author
Long-period quasi-periodic oscillations of a small-scale magnetic structure on the Sun
Empirical mode decomposition analysis of random processes in the solar atmosphere
Nonlinear oscillations of coalescing magnetic flux ropes
Transverse oscillations and stability of prominences in a magnetic field dip
Kinetic model of force-free current sheets with non-uniform temperature

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