Eigenspectra of active region long-period oscillations obtained using the image processing moment method
Gulsun Dumbadze
Associate researcher - E. Kharadze Abastumani Astrophysical Observatory
PhD student - Ilia State University, Georgia
Title: Eigenspectra of active region long-period oscillations obtained using the image processing moment method
Authors: G.Dumbadze, B.M.Shergelashvili, S.Poedts, T.V.Zaqarashvili, M.Khodachenko, and P.De Causmaecker
Abstract
The low-frequency < 0.5 1/hour (long-period > 2 hour) oscillations of active regions (ARs) have been studied. The investigation is based on an analysis of time series built from SDO/HMI photospheric magnetograms. The investigation represents the case study of several typically structured ARs. In particular, we consider compact, dispersed, mixed and unipolar types of ARs. The main goal was to investigate whether ARs can be engaged in long-period oscillation as unified oscillatory units and to answer the question what the spectral pattern of such oscillations is, if they exist at all. Besides, the aim was to reveal the similarities and differences between the spectral properties of this oscillations between different types of ARs. The time series of AR characteristic parameters, such as, major and minor axes, tilt angle, total, positive and negative magnetic field polarity areas, have been measured and recorded by using the image moment calculation method. Fourier and wavelet spectra have been built out of Gaussian apodized and zero padded data sets and then Welch's window spectral averaging technique has been applied. Both methods of a priory apodization of data and a posteriori spectral averaging are used to maximally diminish spurious (artificial) peaks in the spectra and reveal the real justified peaks and their sequences. The data processing and analysis showed that the oscillations yield spectra of standing oscillation period harmonics of different types. In all cases they have been compared with two different standard reference spectra (Type (i) and Type (ii)) predicted by the theory of standing oscillations in magnetic loops. The sequences of frequency peaks found resemble either Type (i) or Type (ii) discrete reference spectrum but there are some noticeable deviations from standard spectra. These deviations may demonstrate the fact that there must be a leakage of oscillations through the foot points of magnetic loops that might be described in terms of different types of MHD oscillations depending on the structure of the ARs. In some cases, sequences of peak pairs are discovered instead of single peaks. This finding can be due to some physical conditions (plasma flow along the loop, non-uniform magnetic field, density or temperature, etc.) that break the symmetry between forward and backward propagating wave modes of the loop (scenario 1); or they can be the results of the complex internal structure of ARs, which can consist of two mutually independent oscillatory systems that resonate at different frequencies (scenario 2).
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