On the origin of non-stationary properties of QPP in radio emission of solar flares

Elena Kupriyanova, Pulkovo Observatory of RAS

Radio emission of solar flare radio is a sensitive indicator of various processes showing a significant part of the information in time profiles. The interesting feature of time profiles is a quasi-periodic modulation superposed over a slowly varying trend, or quasi-periodic pulsations (QPP) frequently observing in solar flare emission. The main aim of techniques based on QPP analysis is to extract hidden parameters of the QPP (period, amplitude, phase). Usually, these parameters are considered as stationary or slowly varying. However, special attention should be given to the non-stationary (or time-dependent) parameters of QPP signals. The non-stationarity appears as a strong variation of the oscillation amplitude, period, or phase with time making the signal looking anharmonic. Such features could be caused by the time variations of the physical parameters in the flaring site or as a superposition of several physical processes ongoing simultaneously, including power-law distributed background noise. In this study, we reveal and analyze the non-stationary properties of QPP observed in the microwave radio emission of three solar flares (SOL2017-07-14T01:07, SOL2017-09-05T01:30, SOL2017-09-05T07:06). The characteristic time scales of the QPP are from 15 s to 400 s. Initially, we revealed the non-stationarity in the correlation plots of Siberian Radioheliograph (SRH-48). We carried out a study based on the available microwave data from different instruments to exclude instrumental effects and looked for the same properties in X-ray data. The periodic properties are analyzed using a unique combination of the analytical methods, such as the autocorrelation, Fourier periodogram, wavelet, and EMD. Impacts of the flare trend and colored noise are considered. We distinguish between the real time-dependent signals and signals whose non-stationary characteristics are due to multi-mode composition. This study was supported by RFBR according to the research project No. 17-52-10001.