Eigenspectra of solar active region long-period oscillations

¹ Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
e-mail: stefaan.poedts@wis.kuleuven.be
² Centre for Computational Helio Studies, Ilia State University, G. Tsereteli Street 3, 0162 Tbilisi, Georgia
³ Evgeni Kharadze Georgian National Astrophysical Observatory, M. Kostava Street 47/57, 0179 Tbilisi, Georgia
⁴ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
⁵ Combinatorial Optimization and Decision Support, KU Leuven Campus Kortrijk, E. Sabbelaan 53, 8500 Kortrijk, Belgium
⁶ Institute of Physics, IGAM, University of Graz, Universitätsplatz 5, 8010 Graz, Austria
⁷ Ilia State University, Cholokashvili Ave 3/5, 0162 Tbilisi, Georgia
⁸ Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow 119992, Russia
⁹ Institute of Physics, University of Maria Curie-Skłodowska, Pl. M. Curie-Skłodowska 5, 20-031 Lublin, Poland
¹⁰ Institute of Astronomy, Russian Academy of Sciences, Moscow 119017, Russia

Received: 8 April 2020
Accepted: 12 June 2021

Abstract

Context. We studied the low-frequency ≲0.5 h⁻¹ (long-period ≳2 h) oscillations of active regions (ARs). The investigation is based on an analysis of a time series built from Solar Dynamics Observatory/Helioseismic and Magnetic Imager photospheric magnetograms and comprises case studies of several types of AR structures.

Aims. The main goals are to investigate whether ARs can be engaged in long-period oscillations as unified oscillatory entities and, if so, to determine the spectral pattern of such oscillations.

Methods. Time series of characteristic parameters of the ARs, such as, the total area, total unsigned radial magnetic flux, and tilt angle, were measured and recorded using the image moment method. The power spectra were built out of Gaussian-apodised and zero-padded datasets.

Results. There are long-period oscillations ranging from 2 to 20 h, similarly to the characteristic lifetimes of super-granulation, determined from the datasets of the AR total area and radial magnetic flux, respectively. However, no periodicity in tilt angle data was found.

Conclusions. Whatever nature these oscillations have, they must be energetically supported by convective motions beneath the solar surface. The possible interpretations can be related to different types of magnetohydrodynamic oscillations of the multi-scale structure of the AR magnetic field, which is probably linked with the characteristic turnover timescales of the super-granulation cells. The presence of oscillations in the radial magnetic flux data may be connected to periodic flux emergence or cancellation processes.