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The Interface Region Imaging Spectrograph (IRIS) provides spectroscopy andnarrow band slit-jaw (SJI) imaging of the solar chromosphere and transitionregion at unprecedented spatial and temporal resolution. Combined withhigh-resolution context spectral imaging of the photosphere and chromosphere asprovided by the Swedish 1-m Solar Telescope (SST), we can now effectively tracedynamic phenomena through large parts of the solar atmosphere in both space andtime. IRIS SJI 1400 images from active regions, which primarily sample thetransition region with the Si IV 1394 and 1403 {\AA} lines, reveal ubiquitousbright "grains" which are short-lived (2-5 min) bright roundish small patchesof sizes 0.5-1.7" that generally move limbward with velocities up to about 30km s$^{-1}$. In this paper we show that many bright grains are the result ofchromospheric shocks impacting the transition region. These shocks areassociated with dynamic fibrils (DFs), most commonly observed in H{\alpha}. Wefind that the grains show strongest emission in the ascending phase of the DF,that the emission is strongest towards the top of the DF and that the grainscorrespond to a blueshift and broadening of the Si IV lines. We note that theSJI 1400 grains can also be observed in the SJI 1330 channel which is dominatedby C II lines. Our observations show that a significant part of the activeregion transition region dynamics is driven from the chromosphere below ratherthan from coronal activity above. We conclude that the shocks that drive DFsalso play an important role in the heating of the upper chromosphere and lowertransition region.

On the Active Region Bright Grains Observed in the Transition Region Imaging Channels of IRIS