Chromospheric Condensation and Quasi-periodic Pulsations in a Circular-ribbon Flare
Abstract
In this paper, we report our multiwavelength observations of the C3.1 circular-ribbon flare SOL2015-10-16T10:20 in AR 12434. The flare consisted of a circular flare ribbon (CFR), an inner flare ribbon (IFR) inside, and a pair of short parallel flare ribbons (PFRs). During the impulsive phase of the flare, "two-step" raster observations of IRIS with a cadence of 6 s and an exposure time of 2 s show plasma downflow at the CFR in the Si iv λ1402.77 line, suggesting chromospheric condensation. The downflow speeds first increased rapidly from a few km s-1 to the peak values of 45-52 km s-1, before decreasing gradually to the initial levels. The decay timescales of condensation were 3-4 minutes, indicating ongoing magnetic reconnection. Interestingly, the downflow speeds are positively correlated with logarithm of the Si iv line intensity and time derivative of the GOES soft X-ray (SXR) flux in 1-8 . The radio dynamic spectra are characterized by a type 3 radio burst associated with the flare, which implies that the chromospheric condensation was most probably driven by nonthermal electrons. Using an analytical expression and the peak Doppler velocity, we derived the lower limit of energy flux of the precipitating electrons, i.e., 0.65×1010 erg cm-2 s-1. The Si iv line intensity and SXR derivative show quasi-periodic pulsations with periods of 32-42 s.
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