We report the intriguing large-scale dynamic phenomena associated with the M6.5 flare ( SOL2015-06-22T18:23 ) in NOAA active region 12371 , observed by RHESSI , Fermi , and the Atmospheric Image Assembly ( AIA ) and Magnetic Imager ( HMI ) on the Solar Dynamic Observatory ( SDO ) .
The most interesting feature of this event is a third ribbon ( R3 ) arising in the decay phase , propagating along a dimming channel ( seen in EUV passbands ) towards a neighboring sunspot .
The propagation of R3 occurs in the presence of hard X-ray footpoint emission , and is broadly visible at temperatures from 0.6 MK to over 10 MK through the Differential Emission Measure ( DEM ) analysis .
The coronal loops then undergo an apparent slipping motion following the same path of R3 , after a \sim 80 min delay .
To understand the underlying physics , we investigate the magnetic configuration and the thermal structure of the flaring region .
Our results are in favor of a slipping-type reconnection followed by the thermodynamic evolution of coronal loops .
In comparison with those previously reported slipping reconnection events , this one proceeds across a particularly long distance ( \sim 60 Mm ) over a long period of time ( \sim 50 min ) , and shows two clearly distinguished phases : the propagation of the footpoint brightening driven by nonthermal particle injection and the apparent slippage of loops governed by plasma heating and subsequent cooling .