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

Abstract

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 $\approx$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 AIA and 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 HMI magnetogram shows evidence of flux emergence of both polarities. Using the IRIS \ion{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 \ion{O}{IV} and \ion{Si}{IV} show a redshift of 10-15 km s$^{-1}$, whereas neutral lines such as \ion{C}{I} and \ion{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 \ion{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.

Figures (9)

• Figure 1: Image of active region AR 12759 as recorded by AIA 171 Å passband on 4 April 2020 chosen for our analysis. The white box shows the region selected for detailed investigation.
• Figure 2: Images obtained from different AIA, HMI, and IRIS-SJI passbands as labelled, showing the region of detailed investigation. The HMI magnetic field image is scaled between $\pm$100 G. Different boxes on the AIA, HMI, and IRIS images represent the location of the transient, which is one of the foot-points (FP) of the transient loop under study and associated loop-top (LT). The vertical white line on IRIS 1400 Å image represents the position of the IRIS slit at the time as labelled. The image from AIA 1700 Å is a difference image with respect to an image obtained around 3:07 UT.
• Figure 3: The intensity evolution obtained at the transient loop foot-point in the boxed region from various AIA and IRIS passbands, as labelled, sensitive to coronal emissions (left panel) and lower atmospheric emissions (right panel). Asterisk (*) symbols represent observed data points, whereas overplotted continuous lines are spline interpolated light curves.
• Figure 4: Upper panels: Images of the photospheric LOS magnetic field ($\pm$60 G) underneath the transient loop foot-point as recorded by HMI magnetogram. Lower panels: Variation of the magnetic field underneath the transient loop foot-point in the boxed regions marked on the HMI magnetogram images above. The left panel is for the smaller yellow box region, whereas the middle and right panels are for the bigger black box region.
• Figure 5: Differential emission measure (DEM) curve and images obtained at different plasma temperatures as labelled. DEM distribution curves are shown for different locations marked with FP and LT in DEM images at different times, as labelled.