In this paper , we report our multiwavelength observations of the C4.2 circular-ribbon flare in active region ( AR ) 12434 on 2015 October 16 .
The short-lived flare was associated with positive magnetic polarities and a negative polarity inside , as revealed by the photospheric line-of-sight magnetograms .
Such magnetic pattern is strongly indicative of a magnetic null point and spine-fan configuration in the corona .
The flare was triggered by the eruption of a mini-filament residing in the AR , which produced the inner flare ribbon ( IFR ) and the southern part of a closed circular flare ribbon ( CFR ) .
When the eruptive filament reached the null point , it triggered null point magnetic reconnection with the ambient open field and generated the bright CFR and a blowout jet .
Raster observations of the Interface Region Imaging Spectrograph ( IRIS ) show plasma upflow at speed of 35 - 120 km s ^ { -1 } in the Fe xxi 1354.09 Å line ( \log T \approx 7.05 ) and downflow at speed of 10 - 60 km s ^ { -1 } in the Si iv 1393.77 Å line ( \log T \approx 4.8 ) at certain locations of the CFR and IFR during the impulsive phase of flare , indicating explosive chromospheric evaporation .
Coincidence of the single HXR source at 12 - 25 keV with the IFR and calculation based on the thick-target model suggest that the explosive evaporation was most probably driven by nonthermal electrons .