Analyses of periodic variations in small-scale solar magnetic structures, of the sizes less than 10 arcsec, such as faculae and pores, require cutting edge instrumental sensitivity and resolution. So far the longest detected periodicities in such structures were limited by a few tens of minutes. Our study shows the presence of a much longer-period oscillation in a photospheric facula. This object is a photospheric crosscut of a magnetic flux tube, generated in the solar interior by dynamo, and emerging in the upper regions of the solar atmosphere.

We analysed time-series of the average line-of-sight magnetic field in the facula, obtained with the SDO/HMI instrument, spanning 13 hours, and processed with the Hilbert-Huang transform (HHT) spectral technique. It allowed for the statistically robust detection of the white and pink noisy components in the original signal, as well as a single intrinsic quasi-oscillatory mode. Its period is found to grow from about 80 to 230 minutes with the increase in the amplitude and decrease in the mean magnetic field.

The detected white and coloured noises could manifest different random processes of a natural or instrumental origin. The detected oscillation period is far too long to be linked with a magnetohydrodynamic mode of the observed magnetic structure. On the other hand, similar periodicities could be induced by the interaction of photospheric magnetic structures with the boundaries of supergranula cells. However, it is unclear how the detected variation of the period with the amplitude and mean magnetic field can be produced by this mechanism. Another option is connected with periodic motions appearing during the magnetic flux emergence. In this effect, the oscillation period is prescribed by the vortex shedding, and its scaling with the mean magnetic field in the structure coincides with the observations. However, more detailed modelling of this phenomenon is required.

• Caption to figures: Left: SDO/HMI line-of-sight magnetogram of the analysed magnetic structure, measured at 23:00:45 UT on 6 July 2013. Blue contours show the magnetic field intensity levels of 70 (dash), 100 (solid, analysed magnetic contour), and 600 (dash) G. Right: variations of the average magnetic field in the structure (black) with the long-term trend subtracted, and its intrinsic mode (red) detected with HHT. 
• Publication: D. Y. Kolotkov, V. V. Smirnova, P. V. Strekalova, A. Riehokainen and V. M. Nakariakov, Long-period quasi-periodic oscillations of a small-scale magnetic structure on the Sun, A&A, 598 (2017) L2
• DOI:  https://doi.org/10.1051/0004-6361/201629951