Investigating the propagation of small-scale flare energy in the lower and upper atmosphere of solar active region

During solar flares, a considerable portion of the flare atmosphere becomes heated; however, the energy deposition process is still unclear, especially in the lower solar atmosphere. Here, we present spectroscopic and imaging observations of a small-scale transient of lifetime <1-min and further formation of a hot loop of lifetime ≈2-min in a solar active region. The observed transient shows the appearance of hot plasma >10 MK at the loop foot-point and the subsequent formation of a small-scale transient loop with a loop-top temperature >8 MK. The transient shows an enhancement in intensities in several Atmospheric Imaging Assembly and Interface Region Imaging Spectrograph (IRIS) passbands. Light curves obtained from several lower atmospheric passbands show consistent time lags in several peak intensities, which, to our knowledge, has never been reported before. Beneath the transient, associated Helioseismic and Magnetic Imager magnetogram shows evidence of flux emergence of both polarities. Using the IRIS O iv line pair, we obtained the average electron number density of 10 11.22 cm-3 at the transient. IRIS transition region lines such as O iv and Si iv show a redshift of 10–15 km s-1 ⁠, whereas neutral lines such as C i and S i show a redshift of about 5 km s-1⁠. These Doppler shifts suggest a down-flowing warmer plasma in the lower atmosphere. The appearance of Mg ii triplets in emission is also observed. We interpret these enhancements in intensities in the lower atmosphere as a result of heating due to both non-thermal electrons and thermal conduction operating simultaneously.