Transient small-scale brightenings in the quiet solar corona: A model for campfires observed with Solar Orbiter

Abstract

Context. Recent observations by the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter have characterized prevalent small-scale transient brightenings in the corona above the quiet Sun termed campfires. Aims. In this study we search for comparable brightenings in a numerical model and then investigate their relation to the magnetic field and the processes that drive these events. Methods. We used the MURaM code to solve the 3D radiation magnetohydrodynamic equations in a box that stretches from the upper convection zone to the corona. The model self-consistently produces a supergranular network of the magnetic field and a hot corona above this quiet Sun. For the comparison with the model, we synthesized the coronal emission as seen by EUI in its 174 Å channel, isolated the seven strongest transient brightenings, and investigated the changes of the magnetic field in and around these in detail. Results. The transients we isolated have a lifetime of about 2 min and are elongated loop-like features with lengths around 1 Mm to 4 Mm. They tend to occur at heights of about 2 Mm to 5 Mm above the photosphere, a bit offset from magnetic concentrations that mark the bright chromospheric network, and they reach temperatures of above 1 MK. As a result, they very much resemble the larger campfires found in observations. In our model most events are energized by component reconnection between bundles of field lines that interact at coronal heights. In one case, we find that untwisting a highly twisted flux rope initiates the heating. Conclusions. Based on our study, we propose that the majority of campfire events found by EUI are driven by component reconnection and our model suggests that this process significantly contributes to the heating of the corona above the quiet Sun.

The magnetic drivers of campfires seen by the Polarimetric and Helioseismic Imager (PHI) on Solar Orbiter

“…The second category of the campfires is for events that could not be linked to clear magnetic footpoints. However, we find in some of these cases opposite polarity features distributed in a larger area around the campfires, the arrangement of which could be similar to the magnetic configuration reported in the simulations of Chen et al (2021). This could indicate that they might undergo component reconnection in the corona following the different magnetic field-line configurations described in Chen et al (2021).…”

“…The spatial distribution of these magnetic features is similar to group (ii) of the categorised magnetic environment around the simulated campfires of Chen et al (2021).…”

“…In an attempt to investigate how campfires relate to the magnetic field in the lower photosphere, Chen et al (2021) used MU-RaM (Vögler et al 2005) simulations to model the photosphere, chromosphere, and lower transition region of a quiet-Sun network region. They then synthesized the emission in the working wavelengths of SO/EUI and degraded their simulations to match the spatial resolution of the SO/EUI observations.…”

Coronal microjets in quiet-Sun regions observed with the Extreme Ultraviolet Imager onboard Solar Orbiter

“…Thus, we conclude that the identified coronal microjets are a peculiar subset of coronal campfires. Different from the suggestion that many campfires might correspond to apexes of low-lying small-scale network loops heated at the coronal heights by component reconnection between different loops (Chen et al 2021), the coronal microjets are more likely to be generated by magnetic reconnection between small-scale magnetic loops and the ambient network field at lower heights.…”

“…The primary data 1 we used were taken by the two High Resolution Imagers (HRI) of EUI, the passbands of which are centered at 174 Å (HRI EUV : dominated by Fe x and Fe ix lines) and 1216 Å (HRI Lyα : dominated by the Lyα line of Hydrogen), respectively (Berghmans et al 2021). The HRI EUV passband has a peak response at the temperature of ∼1 MK (Chen et al 2021), and thus samples plasma in the corona. We mainly used HRI EUV images to search for coronal microjets.…”