Eigenspectra of solar active region long-period oscillations

Context.The low-frequency.0.5 hour−1(long-period&2 hours) oscillations of active regions(ARs) are studied. The investigation is based on an analysis of time series built from SDO/HMIphotospheric 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 oscillationsas 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 un-signed radial magnetic flux, and tilt angle, have been measured and recorded by using the image moment method. The power spectra have been built out of Gaussian apodized and zero paddeddatasets.Results.There are long period oscillations with periods ranging from 2 to 20 hours, similar tothe characteristic lifetimes of super-granulation, determined from the datasets of the AR total areaand radial magnetic flux, respectively. However, no periodicity in tilt angle data was found.Conclusions.Whatever nature these oscillations have, they must be energetically supported byconvective motions beneath the solar surface. The possible interpretations can be related to dif-ferent types of MHD oscillations of the multi-scale structure of the AR magnetic field, proba-bly linked with the characteristic turnover timescales of the super-granulation cells. The pres-ence of oscillations in the radial magnetic flux data may be connected to periodic flux emer-gence/cancellation processes.