عنوان مقاله [English]
Solar flares are sudden bursts in the solar atmosphere, which have emissions, from radio wavelengths up to gamma rays, and according to their energy are classified into different classes (A, B, C, M, and X, respectively). The process of releasing magnetic energy in flares is done by magnetic reconnection, which is often created by a complex magnetic field. Flares accelerate many electrons and ions, raising their energy to the limit of relative energy. These accelerating particles play a very important role in the release of large solar flare energies. Considering the fact that flares emit radiation when they explode, most of them create light spectrum and sometimes X-rays and ultraviolet rays, which are emitted mainly by the photosphere and chromosphere into concentrated sources called footpoints and ribbons. These radiations and emissions occur when the lower layers of the sun's atmosphere heat up during a flare, and this heating due to the collision of particles probably plays an important role in the occurrence of the flare. In addition, they emit high-energy radiation such as hard X-rays (HXR) from electrons and gamma rays from ions. The main part of these emissions is in the form of electromagnetic emission (soft X-rays) and energetic particles. Emissions radiated from a large flare or a solar mass eruption (with an energy more than J), when reaching the earth, can have destructive effects on the Earth's atmosphere, as well as the orbits of satellites or magnetic and electrical facilities of devices like ships and airplanes. Therefore, predicting the time of the flare occurrence and determining its class type can help reduce these destructive effects.
One of the observable structures that can be seen before a flare occurs, are oscillations with very long period pulsations (VLPs) of the order of 8-30 minutes, which occur about one to two hours before the flare onset, and were first reported by Tan et al. (2016) in the pre-flare phase. MHD oscillations and longitudinal electric current in flare loops can be appropriate candidates to explain the formation of VLPs. Investigating pre-flare VLPs can also help us in understanding the origin of flares. With the help of observational data of X-ray radiation (SXR), onboard the GOES satellite, during the pre-flare phase, these pulses can be observed at similar time scales during flare processes.
In this paper, using the abovementioned data, we selected eighteen flares for the study of which 6 flares are in class C and 12 flares are in class M. Of these, twelve had typical VLPs before flare-onset, which were all in the M class, with the exception of one. The periodicity that we calculated for the VLPs of these flares, with the help of the Fast Fourier Transform is 14 to 28.9 minutes, which is in agreement with the results of Tan et al. (2016). The number of pulses observed in each pre-flare is between 3 and 7. For the other six remaining flares of our selection, no typical pre-flare VLP was observed, which all but one of them, were in class C.