Empirical mode decomposition analysis of random processes in the solar atmosphere 

Dmitrii Kolotkov Submitted: 20160610 08:12
Solar signals of various types often show appearance of coloured noisy components with a power law spectral energy distribution. Such a frequencydependent 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 quasiperiodic 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 nonstationary and amplitudemodulated 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, 3min oscillations in the analysed sunspot were detected to have energies significantly above the corresponding noise level. The correct accounting for the background frequencydependent 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: 20160610 15:23



Kinetic model of forcefree current sheets with nonuniform temperature 

Dmitrii Kolotkov Submitted: 20151028 07:27
The kinetic model of a onedimensional forcefree 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 forcefree 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: 20151028 15:23




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