Energetics of Hi-C EUV brightenings

Abstract

We study the thermal structure and energetics of the point-like EUV brightenings within a system of fan loops observed in the active region AR 11520. These brightenings were simultaneously observed on 2012 July 11 by the HIgh-resolution Coronal (Hi-C) imager and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). We identified 27 brightenings by automatically determining intensity enhancements in both Hi-C and AIA 193 Å light curves. The energetics of these brightenings were studied by using the Differential Emission Measure (DEM) diagnostics. The DEM weighted temperatures of these transients are in the range log T (K) = 6.2 − 6.6 with radiative energies ≈10 24−25 ergs and densities ≈ a few times 10 9 cm −3 . To the best of our knowledge, these are the smallest brightenings in EUV ever detected. We used these results to determine the mechanism of energy loss in these brightenings. Our analysis reveals that the dominant mechanism of energy loss for all the identified brightenings is conduction rather than radiation.A&A proofs: manuscript no. hic_final_2C_final spheric Imaging Assembly (AIA; Lemen et al. 2012) on board the Solar Dynamic Observatory (SDO). These tiny dots are the smallest EUV brightenings ever observed or reported in the literature and have energies a few orders of magnitude more than the nanoflares energy budget.Using a potential field extrapolation of line-of-sight magnetograms obtained with Helioseismic and Magnetic Imager (HMI; Scherrer et al. 2012) on board SDO, Régnier et al. (2014) found that these events are predominantly located at the footpoints of large scale trans-equatorial coronal loops with energy content of 10 26 ergs per brightening. Régnier et al. (2014) concluded that these bright dots are multi-thermal in nature with temperatures ranging between log T (K) = 5.3 − 6.5 using the EM loci analysis method (see e.g., Jordan et al. 1987;Tripathi et al. 2010).Since direct observations of the heating mechanisms responsible for these brightenings are still infeasible, a comprehensive understanding of such small scale brightenings is crucial. A quantitative analysis of the energetics involved in these transients is of paramount importance to establish their role in heating the fan loops and the solar corona in general. Here, we study the brightenings as a statistical ensemble, i.e., by analysing all the individual brightenings that occurred near the footpoint of the trans-equatorial fan loop system during the course of the Hi-C observations. Our objective is to identify such brightenings in Hi-C observations and study their energetics by employing Differential Emission Measure (DEM) diagnostics. For such diagnostics, it is mandatory to have multi-filter observations. Therefore, we searched for the same brightening in the AIA 193 Å observations. The DEMs are then derived using AIA filters. Even though the AIA 193 Å channel's effective area and thermal response are similar to that of Hi-C, these events are readily observed in the latter but not in the former...

Observation and Modeling of Chromospheric Evaporation in a Coronal Loop Related to Active Region Transient Brightening

“…The finding that the conductive losses are much more efficient suggest that there is more energy involved in the events than that being seen as was also suggested by Peter et al (2014). Similar results were obtained for Hi-C EUV bright dots studied by Subramanian et al (2018).…”

Transient brightenings in the quiet Sun detected by ALMA at 3 mm

“…Furthermore, the range of computed energies falls within the cluster of values that have been reported in the literature: they are consistent with or smaller than the values reported by 8 × 10 24 −1.6 × 10 26 erg), Berghmans et al 1998; 5 × 10 24 −3 × 10 27 erg), and Winebarger et al (2013; 2.0−6.3 × 10 24 erg). On the other hand, Aschwanden et al (2000), Parnell & Jupp (2000), and Subramanian et al (2018) reported energy ranges (5 × 10 23 −5 × 10 26 erg, 10 23 −10 26 erg, and 0.3−30.0 × 10 24 erg, respectively) whose low-end values are below the lowest energies we detected. Moreover, studies of active region weak transient brightenings report larger energies than ours (e.g., Shimizu 1995; 10 25 −10 29 erg and Hannah et al 2008; 10 26 −10 30 erg).…”

“…Nanoflares, events with energies less than 10 24 erg, are expected to occur as a result of magnetic reconnection in elemental, tangled magnetic flux tubes that are below the resolution limit of present-day instruments. So far only detections of individual events with energy down to the "high-end" limit of Parker's estimate have been achieved (e.g., Berghmans et al 1998;Parnell & Jupp 2000;Aschwanden et al 2000;Winebarger et al 2013;Régnier et al 2014;Joulin et al 2016;Subramanian et al 2018). It has been argued that these small events could heat the upper atmosphere if the energy released during various types of flarelike activity follows a power-law frequency distribution with energy that has an index α ≥ 2 (e.g., Hudson 1991).…”

Flows in Enthalpy Based Thermal Evolution of Loops

“…This may be explained as follows. During the impulsive phase, conductive losses dominate and radiative losses are negligible (see also Rajhans et al 2021;Subramanian et al 2018;Cargill et al 1995). Therefore, the sum of enthalpy and kinetic energy flux across coronal base γ γ−1 P 0 v 0 + 1 2 µn 0 v 2 0 is approximately equal to the conduction flux (F 0 ) across it.…”