Quasi-periodic Pulsations in a Solar Microflare

Abstract

Irregular time evolution of the radio emission generated in a B2-class microflare (SOL2017-01-25T10:15), occurring on 2017 January 25 in active region 12,628, is studied. The microflare was apparently initiated by an appearance of an s-shaped loop, observed in the EUV band. The radio emission is associated with the nonthermal electrons detected with Ramaty High Energy Solar Spectroscopic Imager, and originates simultaneously from two opposite footpoints of a magnetic fan structure beginning at a sunspot. According to the active region geometry, the footpoints are situated in the meridional direction, and hence are observed by RATAN-600 simultaneously. The radio emission intensity signal, as well as the left-hand and right-hand circular polarization signals in the lowfrequency band (3-4GHz) show good correlation with each other, with the average characteristic time of the variation 1.4±0.3s. The polarization signal shows a time variation with the characteristic time of about 0.7±0.2s. The irregular quasi-periodic pulsations of the radio emission are likely to be caused by the superposition of the signals generated at the local electron plasma frequencies by the interaction of nonthermal electrons with the plasma at the footpoints. In this scenario, the precipitation rate of the nonthermal electrons at the opposite footpoints could be modulated by the superposition of fundamental and second harmonic modes of sausage oscillations, resulting in the observed different characteristic times of the intensity and polarization signals. However, other mechanisms, e.g., the oscillatory regime of loop coalescence or magnetic null point oscillation could not be rigorously excluded.

Synthetic Emissions of the Fe xxi 1354 Å Line from Flare Loops Experiencing Fundamental Fast Sausage Oscillations

“…Likewise, signatures of FSMs were also found in the fine structures in a number of flare-associated radio bursts as measured by, say, the Chinese Solar Broadband Radio Spectrometer (Yu et al 2013(Yu et al , 2016, the Assembly of Metric-band Aperture Telescope and Real-time Analysis System (Kaneda et al 2018), and the LOw Frequency ARray (Kolotkov et al 2018). Furthermore, a recent multi-wavelength study by Nakariakov et al (2018) suggested the existence of FSMs in loops associated with microflares. In extreme ultraviolet (EUV), Su et al (2012) identified FSMs using imaging observations in the 171Å channel of the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO).…”

Fine structure events in microwave emission during solar minimum

“…The black arrow indicates the time of corresponding radio fine structure spectrum observed by SBRS at 1100-1900 MHz band, shown on the second panel. The four bottom panels are SDO AIA 171 Å images after running differentiation [Nakajima, 1994]. We can expect that the weak radio bursts observed with the high-sensitivity low-interference telescope will help us to understand physical characteristics of small-scale eruptions and nano-flares, and will further expand our understanding of solar radio emission and solar magnetic field.…”

Fast magnetoacoustic wave trains: from tadpoles to boomerangs

“…The high temporal resolution traditionally available in the radio band seems to be most suitable for the detection of these boomerangshaped fast magnetoacoustic wave trains in observations. In particular, signatures of quasi-periodic fast propagating wave trains in solar radio bursts were detected by Mészárosová et al (2009) with the characteristic oscillation period about 71 s; Mészárosová et al (2011) with multiple periods about 0.5 s, 2 s, and 81 s; Goddard et al (2016) with a period about 1.8 min; and Kumar et al (2017) with periods ranging from 70 s to 140 s. More recently, Nakariakov et al (2018) observed quasi-periodic pulsations in a solar microflare with periods about 1.4 s in the radio emission intensity and 0.7 s in the polarisation signal, which were interpreted in terms of fast sausage oscillations of the flaring loop. Kolotkov et al (2018) associated a 3-s quasi-periodic striation observed in the dynamic spectrum of a type III radio burst with the modulation of the local plasma density by a propagating fast wave train.…”