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

TL;DR

This study investigates bright grains observed in IRIS transition region images, revealing they are caused by chromospheric shocks linked to dynamic fibrils, contributing to understanding solar atmospheric heating.

Contribution

It demonstrates that transition region bright grains are primarily driven by chromospheric shocks associated with dynamic fibrils, highlighting the chromosphere's role in transition region dynamics.

Findings

Bright grains are short-lived, moving limbward at up to 30 km/s.

Most grains result from chromospheric shocks impacting the transition region.

Shocks associated with dynamic fibrils influence transition region heating.

Abstract

The Interface Region Imaging Spectrograph (IRIS) provides spectroscopy and narrow band slit-jaw (SJI) imaging of the solar chromosphere and transition region at unprecedented spatial and temporal resolution. Combined with high-resolution context spectral imaging of the photosphere and chromosphere as provided by the Swedish 1-m Solar Telescope (SST), we can now effectively trace dynamic phenomena through large parts of the solar atmosphere in both space and time. IRIS SJI 1400 images from active regions, which primarily sample the transition region with the Si IV 1394 and 1403 {\AA} lines, reveal ubiquitous bright "grains" which are short-lived (2-5 min) bright roundish small patches of sizes 0.5-1.7" that generally move limbward with velocities up to about 30 km s$^{-1}$. In this paper we show that many bright grains are the result of chromospheric shocks impacting the transition region. These shocks are associated with dynamic fibrils (DFs), most commonly observed in H{\alpha}. We find 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 grains correspond to a blueshift and broadening of the Si IV lines. We note that the SJI 1400 grains can also be observed in the SJI 1330 channel which is dominated by C II lines. Our observations show that a significant part of the active region transition region dynamics is driven from the chromosphere below rather than from coronal activity above. We conclude that the shocks that drive DFs also play an important role in the heating of the upper chromosphere and lower transition region.