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 Chromospheric Condensation and Quasi-periodic Pulsations in a Circular-ribbon Flare Qingmin Zhang   Submitted: 2016-09-13 02:44 In this paper, we report our multiwavelength observations of the C3.1 circular-ribbon flare SOL2015-10-16T10:20 in active region (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). The PFRs located to the north of IFR were most striking in the Interface Region Imaging Spectrograph (IRIS) 1400 Å and 2796 Å images. For the first time, we observed the circular-ribbon flare in the Ca II H line of the Solar Optical Telescope (SOT) aboard Hinode, which has similar shape as observed in the Atmospheric Imaging Assembly (AIA) 1600 Å aboard the Solar Dynamic Observatory (SDO). Photospheric line-of-sight magnetograms from the Helioseismic and Magnetic Imager (HMI) aboard SDO show that the flare was associated with positive polarities and a negative polarity inside. The IFR and CFR were cospatial with the negative polarity and positive polarities, implying the existence of a magnetic null point (B=0) and the dome-like spine-fan topology. 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 \lambda1402.77 line (log T≈4.8), 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 Å. The radio dynamic spectra are characterized by a type III 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., 0.65x1010 erg cm-2 s-1. The Si IV line intensity and SXR derivative show quasi-periodic pulsations with periods of 32-42 s, which are likely caused by intermittent null-point magnetic reconnections modulated by the fast wave propagating along the fan surface loops at a phase speed of 950-1250 km s-1. Periodic accelerations and precipitations of the electrons result in periodic heating observed in the Si IV line and SXR. Authors: Q. M. Zhang, D. Li, and Z. J. Ning Projects: IRIS