Kolotkov, Dmitrii, Anfinogentov, Sergey and Nakariakov, V. M. (Valery M.) (2016) Empirical mode decomposition analysis of random processes in the solar atmosphere. Astronomy & Astrophysics, 592 . A153. doi:10.1051/0004-6361/201628306 ISSN 0004-6361.

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Abstract

Context. Coloured noisy components with a power law spectral energy distribution are often shown to appear in solar signals of various types. Such a frequency-dependent noise may indicate the operation of various randomly distributed dynamical processes in the solar atmosphere.
Aims. We develop a recipe for the correct usage of the empirical mode decomposition (EMD) technique in the presence of coloured noise, allowing for clear distinguishing between quasi-periodic oscillatory phenomena in the solar atmosphere and superimposed random background processes. For illustration, we statistically investigate extreme ultraviolet (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 region.
Methods. EMD has been used for analysis because of 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.
Results. We empirically revealed statistical properties of synthetic coloured noises in EMD, and suggested a scheme that allows 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.
Conclusions. 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.

Item Type:	Journal Article
Divisions:	Faculty of Science, Engineering and Medicine > Science > Physics
Journal or Publication Title:	Astronomy & Astrophysics
Publisher:	EDP Sciences
ISSN:	0004-6361
Official Date:	22 August 2016
Dates:	Date Event 22 August 2016 Available 10 June 2016 Accepted 13 February 2016 Submitted
Volume:	592
Number of Pages:	9
Article Number:	A153
DOI:	10.1051/0004-6361/201628306
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access
Date of first compliant deposit:	13 June 2016
Date of first compliant Open Access:	22 August 2016
Is Part Of:	European Research Council under the SeismoSun Research Project No. 321141, the STFC consolidated grant ST/L000733/1.

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