Properties of quasi-periodic pulsations in solar flares from a single active region

Abstract

Context. Quasi-periodic pulsations (QPPs) are a common feature of solar and stellar flares, and so the nature of these pulsations should be understood in order to fully understand flares. Aims. We investigate the properties of a set of solar flares originating from a single active region (AR) that exhibit QPPs, and in particular look for any indication of QPP periods relating to AR properties, as might be expected if the characteristic timescale of the pulsations corresponds to a characteristic length scale of the structure from which the pulsations originate. The three AR properties used for this study are the photospheric area, bipole separation distance, and average magnetic field strength at the photosphere. The AR studied, known as NOAA 12172/12192/12209, was unusually long-lived and persisted for over three Carrington rotations between September and November 2014. During this time a total of 181 flares were observed by GOES. Methods. Data from the GOES/XRS, SDO/EVE/ESP, Fermi/GBM, Vernov/DRGE and Nobeyama Radioheliograph observatories were used to determine if QPPs were present in the flares. For the soft X-ray GOES/XRS and EVE/ESP data, the time derivative of the signal was used so that any variability in the impulsive phase of the flare was emphasised. Periodogram power spectra of the time series data, without any form of detrending, were inspected and flares with a peak above the 95% confidence level in the power spectrum were labelled as having candidate QPPs. The confidence levels were determined taking full account of data uncertainties and the possible presence of red noise. Active region properties were determined using SDO/HMI line of sight magnetogram data. Results. A total of 37 flares, i.e. 20% of the sample, show good evidence of having stationary or weakly non-stationary QPPs, and some of the pulsations can be seen in data from multiple instruments and in different wavebands. Because the detection method used was rather conservative, this may be a lower bound for the true number of flares with QPPs. The QPP periods were found to show a weak correlation with the flare amplitude and duration, but this is likely due to an observational bias. A stronger correlation was found between the QPP period and duration of the QPP signal, which can be partially but not entirely explained by observational constraints. No correlations were found with the AR area, bipole separation distance, or average magnetic field strength. Conclusions. The fact that a substantial fraction of the flare sample showed evidence of QPPs, using a strict detection method with minimal processing of the data, demonstrates that these QPPs are a real phenomenon that cannot be explained by the presence of red noise or the superposition of multiple unrelated flares. The lack of correlation between the QPP periods and AR properties implies that the small-scale structure of the AR is important and/or that different QPP mechanisms act in different cases.