Observations of solar chromospheric heating at sub-arcsec spatial resolution

Abstract

A wide variety of phenomena such as gentle but persistent brightening, dynamic slender features (∼100 km), and compact (∼1 ) ultraviolet (UV) bursts are associated with the heating of the solar chromosphere. High spatio-temporal resolution is required to capture the finer details of the likely magnetic reconnection-driven, rapidly evolving bursts. Such observations are also needed to reveal their similarities to large-scale flares, which are also thought to be reconnection driven, and more generally their role in chromospheric heating. Here we report observations of chromospheric heating in the form of a UV burst obtained with the balloon-borne observatory, Sunrise. The observed burst displayed a spatial morphology similar to that of a large-scale solar flare with circular ribbon. While the co-temporal UV observations at 1.5 spatial resolution and 24 s cadence from the Solar Dynamics Observatory showed a compact brightening, the Sunrise observations at diffraction-limited spatial resolution of 0.1 at 7 s cadence revealed a dynamic sub-structure of the burst that it is composed of extended ribbon-like features and a rapidly evolving arcade of thin (∼0.1 wide) magnetic looplike features, similar to post-flare loops. Such a dynamic sub-structure reveals the small-scale nature of chromospheric heating in these bursts. Furthermore, based on magnetic field extrapolations, this heating event is associated with a complex fan-spine magnetic topology. Our observations strongly hint at a unified picture of magnetic heating in the solar atmosphere from some large-scale flares to small-scale bursts, all being associated with such a magnetic topology.

Solar Ultraviolet Bursts

“…4.4), and EBs will be prime targets that offer the opportunity to study the reconnection process in unprecedented detail. A very recent study of a UV burst at 75 km resolution (Smitha et al 2018) with the Ca II H line from the SUNRISE observatory suggests bursts may have a complex, flare-like structure when observed at ultra-high resolution.…”

Impulsive coronal heating during the interaction of surface magnetic fields in the lower solar atmosphere

“…Since the ratio of their filter responses peaks sharply at around 1550 Å, the intensity ratio of the 1600 Å to 1700 Å filters (henceforth, UV ratio) traces the signal mostly from the C iv doublet. Signatures of UV bursts and rapid moss variability in the lower atmosphere can be traced in these UV ratio maps (Chitta et al 2018;Smitha et al 2018). We use these UV ratio maps in the second step to identify the footpoints of hot loops in the lower atmosphere.…”

An observationally constrained model of strong magnetic reconnection in the solar chromosphere

“…UVBs (Peter et al 2014) are compact transient brightenings observed in UV bands, with very complex Si iv profiles, and do not generally show a co-spatial brightening in the coronal channels of the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) instrument. Similar targets have been studied with recent simulations (Danilovic 2017;Hansteen et al 2017Hansteen et al , 2019Libbrecht et al 2020) and observations (Socas-Navarro et al 2006;Toriumi et al 2017;Libbrecht et al 2017;Guglielmino et al 2018;Smitha et al 2018;Vissers et al 2019;Yadav et al 2019;Ortiz et al 2020;da Silva Santos et al 2020). Microflares or stronger events show a visible response in coronal channels and they would originate from or extend into higher layers.…”

“…Bi-directional flows at these locations are commonly reported in both observations and simulations (Peter et al 2014;Vissers et al 2019;Hansteen et al 2017). They usually have time scales of a few minutes (Vissers et al 2019;Smitha et al 2018), however, they can live up to several hours (Guglielmino et al 2018;Chitta et al 2017). Therefore, differences in their lifetime, size, location, etc., are used to distinguish them.…”