Statistics of transition-region loop brightenings and their heating implication

Abstract

Transition-region loops are a type of critical magnetic structure in the solar atmosphere, yet their physical properties and evolutionary characteristics remain statistically poorly constrained. We aim to statistically characterize the physical properties of propagating brightening events in transition-region loops and to explore the underlying heating mechanism responsible for these brightenings.Using coordinated observations from the Extreme Ultraviolet Imager onboard the Solar Orbiter and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory, we analyze 42 propagating brightening events in loops that are unambiguously detected in both instrument data. Each of these events evolve simultaneously in the AIA 94, 131, 171, 193, 211, 304, and 335 passband images, suggesting that they are in the transition-region or low-coronal temperature range. Our analyses show that these brightenings are impulsive, with an average brightening time of 118.4 s and a mean intensity decreasing time of 159.4 s. The propagating brightenings are predominantly subsonic, with velocities in the range of 0-90 km/s and an average of 51.3 km/s. The lengths of brightenings range from 3 to 11 Mm, with an average and standard deviation of 6.3 Mm, which are closely related to the propagation velocity and the lifetime. The initial brightening sites are predominantly located near the footpoints of these loops, and the number of brightening events decreases systematically with increasing of loop height. Our results are consistent with an energizing mechanism regulated by enthalpy flows and radiative cooling.

Figures (7)

• Figure 1: Overview of the study. Panel (a) shows the observation positions of SDO and SO. Panel (b) presents the full disk of the sun in EUI 174 Å passband, of which the purple box indicates the ROI location in the HRI field of view, situated near the equatorial region on the left side of the solar surface. Panel (c) is the ROI in a high-resolution image of HRI$_{\mathrm{EUV}}$ passband. Panel (d) shows the full disk of the Sun in AIA 171 Å passband, of which the blue box marks the ROI position in the AIA field of view, located near the equatorial region on the right side of the solar surface. Panels (e)–(j) display the ROI images of AIA passbands, where multiple distinct loops connect the magnetic polarity regions in the upper left and lower right. Panel (k) shows the HMI magnetic field image of the ROI scaled from $-$300 G (black) to 300 G (white), with the primary positive polarity region in the upper left field of view, the main negative polarity region in the lower right, and numerous small-scale polarity points distributed in the central region.
• Figure 2: Case-analysis methodology. Panel (a) shows the evolution of Case-13, the green dashed line marks the trajectory of the brightening loop. Panel (b) presents the T--S map, constructed along the green dashed trajectory in panel (a3); the white solid line indicates the propagation velocity of the brightening front. The T--S map is used to derive the characteristic timescale, $L_b$, propagation velocity, and other physical properties of the event. Panel (c) shows the schematic illustration of the timescale determination based on the light curve obtained by integrating the intensity along the selected loop path. The blue dashed line denotes the mean intensity of the lowest 50% of pixels, and the red dashed lines indicate the reference baseline used to identify the brightening and intensity decreasing phases. Panel (d) displays the light curves from multiple AIA passbands and HRI$_{\mathrm{EUV}}$ passband.
• Figure 3: Statistical properties of 42 brightening events. Panels (a)--(c) show the distributions of the brightening time, intensity decreasing time, and total duration, the red dashed lines mark the mean values, and blue dashed lines mark the medians. Panel (d) shows the distribution of the propagation velocities of the brightening fronts. The red and green dashed lines denote the mean and median. Panel (e) displays the distribution of the maximum $L_b$, defined as the length of the contiguous brightened segment along the loop at the time of peak brightening. The red and blue dashed lines indicate the mean and median. Panel (f) presents the distribution of the initial brightening location, defined as the distance between the first clearly visible bright structure and the footpoint, normalized by the loop length. The detailed parameters of each event are listed in Table \ref{['table1']}.
• Figure 4: Two-dimensional correlation analysis. Panel (a) shows the correlation of $L_b$ and the footpoint distance. Panel (b) displays the correlation of $L_b$ and the propagation velocity. The green solid line is obtained from grouped mean data by linear fitting. Panel (c) presents the correlation of $L_b$ and the brightening time. Panel (d) presents the correlation of the brightening and intensity decreasing time and footpoint distance. The pink dots represent the brightening time, and the blue dots represent the intensity decreasing time.
• Figure 5: Evolution detail of brightening events. Panels (a1)--(a8) present the brightening process of Case-3; the event brightens at a footpoint, and the brightening front propagates along the loop over a finite distance. Panels (b1)--(b8) show the brightening process of Case-6; the event also brightens from the footpoint. As indicated by the green arrows, small, moving, brightening points are observed near the footpoint, which may provide evidence of reconnection between shorter and longer loops.