Long-period oscillations in the lower solar atmosphere prior to flare events

Context. Multiple studies have identified a range of oscillation periods in active regions, from 3−5 min to long-period oscillations that last from tens of minutes to several hours. Recently, it was also suggested that these periods are connected with eruptive activity in the active regions. Thus, it is essential to understand the relation between oscillations in solar active regions and their solar eruption activity.

Aims. We investigate the long-period oscillations of NOAA 12353 prior to a series of C-class flares and correlate the findings with the 3- to 5-min oscillations that were previously studied in the same active region. The objective of this work is to elucidate the presence of various oscillations with long periods in the lower solar atmosphere both before and after the flare events.

Methods. To detect long-period oscillations, we assessed the emergence, shearing, and total magnetic helicity flux components from the photosphere to the top of the chromosphere. To analyze the magnetic helicity flux in the lower solar atmosphere, we used linear force-free field extrapolation to construct a model of the magnetic field structure of the active region. Subsequently, the location of long-period oscillations in the active region was probed by examining the spectral energy density of the measured intensity signal in the 1700 Å, 1600 Å, and 304 Å channels of the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO). Significant oscillation periods were determined by means of a wavelet analysis.

Results. Based on the evolution of the three magnetic helicity flux components, 3- to 8-h periods were found both before and after the flare events, spanning from the photosphere to the chromosphere. These 3- to 8-h periods were also evident throughout the active region in the photosphere in the 1700 Å channel. Observations of AIA 1600 Å and 304 Å channels, which cover the chromosphere to the transition region, revealed oscillations of 3−8 h near the region in which the flare occurred. The spatial distribution of the measured long-period oscillations mirror the previously reported distribution of 3- to 5-min oscillations in NOAA 12353 that were seen both before and after the flares.

Conclusions. This case study suggest that the varying oscillation properties in a solar active region could be indicative of future flaring activity.