The Interface Region Imaging Spectrometer ( IRIS ) is the first solar instrument to observe \sim 10 MK plasma at subarcsecond spatial resolution through imaging spectroscopy of the Fe xxi \lambda 1354.1 forbidden line .
IRIS observations of the X1 class flare that occurred on 2014 March 29 at 17:48 UT reveal Fe xxi emission from both the flare ribbons and the post-flare loop arcade .
Fe xxi appears at all of the chromospheric ribbon sites , although typically with a delay of one raster ( 75 seconds ) and sometimes offset by up to 1 { { } ^ { \prime } } { { } ^ { \prime } } .
100–200 km s ^ { -1 } blue-shifts are found at the brightest ribbons , suggesting hot plasma upflow into the corona .
The Fe xxi ribbon emission is compact with a spatial extent of < 2 { { } ^ { \prime } } { { } ^ { \prime } } , and can extend beyond the chromospheric ribbon locations .
Examples are found of both decreasing and increasing blue-shift in the direction away from the ribbon locations , and blue-shifts were present for at least 6 minutes after the flare peak .
The post-flare loop arcade , seen in Atmospheric Imaging Assembly ( AIA ) 131 Å filtergram images that are dominated by Fe xxi , exhibited bright loop-tops with an asymmetric intensity distribution .
The sizes of the loop-tops are resolved by IRIS at \geq 1 { { } ^ { \prime } } { { } ^ { \prime } } , and line widths in the loop-tops are not broader than in the loop-legs suggesting the loop-tops are not sites of enhanced turbulence .
Line-of-sight speeds in the loop arcade are typically < 10 km s ^ { -1 } , and mean non-thermal motions fall from 43 km s ^ { -1 } at the flare peak to 26 km s ^ { -1 } six minutes later .
If the average velocity in the loop arcade is assumed to be at rest , then it implies a new reference wavelength for the Fe xxi line of 1354.106 \pm 0.023 Å .