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 that exhibit QPPs, and in particular look for any indication of the QPP periods relating to active region properties (namely photospheric area, bipole separation distance, and average magnetic field strength at the photosphere), 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 active region 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 (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 active region 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, which 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 active region properties implies that the small-scale structure of the active region is important, and/or that different QPP mechanisms act in different cases.

Non-stationary quasi-periodic pulsations in solar and stellar flares

“…Obviously, one should count only those oscillation cycles whose instantaneous amplitudes exceed some threshold value, for example, of one third of the highest amplitude in the QPP. Figure 3 shows the statistics of this effective quality estimated for the QPP events that occurred in the same longlived solar active region and described in [11]. In addition, the right panel of figure 3 shows the lack of the dependence of the number of oscillation cycles on the mean period.…”

Quasi-Periodic Pulsations in Solar and Stellar Flares. Review

“…Statistical studies of the oscillation quality (Qfactor) of OPPs, i.e. the number of oscillation cycles (periods), and its relationship with the QPP period based on the events occurring in the same flaring active region [Pugh et al, 2017b] have shown that the typical duration of QPPs is only several (2-10) oscillation cycles [Nakariakov et al, 2019]. Also shown is the complete absence of the dependence of the mean QPP period on the number of QPP cycles.…”

“…The Fourier spectrum of the total light curve of a flare is a power-law function, where the low-frequency trend and the high-frequency noise have the maximum and minimum spectral power respectively. As a special case the broken power-law spectrum with different slopes (different power-law indices) in the low-frequency and highfrequency ranges are considered [Pugh et al, 2017b]. The noise level, or the significance level of spectral peaks, is determined from the criterion  2 [Vaughan, 2005;Pugh et al, 2017a].…”

“…As a special case the broken power-law spectrum with different slopes (different power-law indices) in the low-frequency and highfrequency ranges are considered [Pugh et al, 2017b]. The noise level, or the significance level of spectral peaks, is determined from the criterion  2 [Vaughan, 2005;Pugh et al, 2017a]. In this case, the noise color is determined empiricallyas a slope of the Fourier spectrum.…”

Quasi-periodic Pulsations in a Solar Microflare

“…It has been demonstrated that QPP are a statistically significant, common feature of both thermal and nonthermal emission of flaring energy releases (Kupriyanova et al 2010 andSimões et al 2015, respectively, see also Inglis et al 2016;Pugh et al 2016Pugh et al , 2017 for recent statistical studies). The mechanisms for QPP are intensively studied, and it is commonly accepted that they can be connected with repetitive regimes of charged particle acceleration in magnetic reconnection, and/or the periodic modulation of the accelerated particle kinematics.…”