Small EUV Brightenings in the Quiet Solar Atmosphere: New Insights from the Solar Orbiter Mission

TL;DR

The paper surveys Solar Orbiter EUI/HRIEUV observations of ubiquitous small EUV brightenings in the quiet Sun, highlighting their impulsive, high-cadence nature and potential role in coronal heating. It shows that these events exhibit self-similar, power-law distributions (with index around $\alpha \approx 2$) across area, duration, and brightness, extending to the smallest resolvable scales ($\sim$0.01–5 $\mathrm{Mm^2}$ and a few seconds) while often remaining at transition-region temperatures ($\lesssim 1~\mathrm{MK}$). Triangulation and spectroscopy indicate most brightenings originate at heights of about $1-5~\mathrm{Mm}$ and are associated with short loops and mixed/motorized magnetic fields, with a minority showing coronal signatures. Numerical simulations support magnetic reconnection and magnetohydrodynamic processes as plausible origins, but the observed energy budget appears insufficient for corona heating alone, underscoring the need for continued multi-instrument analyses and refined classifications to understand their collective impact on solar atmospheric dynamics.

Abstract

One of the many outcomes of the Solar Orbiter mission is the evidence for the solar atmosphere being filled by highly impulsive bursts, down to about 200 km scale: the limit of the EUV instruments' spatial resolution. Small-scale events of this kind were already known, but their observation was occasional or with limited, lower resolution. Solar Orbiter has revealed that small scale, highly impulsive events are everywhere on the quiet Sun, all the time, at even smaller scales. Their similarity with known larger features, are the witnesses that the physical processes causing them are independent of the spatial scales involved. Their highly dynamic property is the signature of energy transfer and/or local dissipation. Their investigation can thus elucidate on the dominant physical processes acting on the solar atmosphere and on the possible role in the origin of the hot solar corona. In this review, we will present a summary of the observational and simulation results on this topic, led by the results from data taken by the Extreme Ultraviolet Imager (EUI)/High Resolution Imagers (HRIEUV) instrument. Here, we will cover both statistical properties and analyses of individual events.

Figures (18)

• Figure 1: The solar corona during the minimum of the solar cycle on February 10, 2020, observed by SDO/AIA in the 193 Å band (inverse colors). The QS dominates the surface, while the faint coronal holes are visible at the poles. The bright dots (dark dots in the figure) are the location of bright points mentioned in the text. Courtesy of JHelioviewer.
• Figure 2: First orbit ( dark curve) of Solar Orbiter during the nominal mission phase in a geocentric solar ecliptic coordinates (27 December 2021 -- 27 June 2022). The orange, red and blue part of the orbit mark the period when the remote sending instruments observe. The other colored curves mark to the orbits of STEREO Ahead, Parker Solar Probe and Bepi Colombo. The Earth is at 0.0, the Sun at 1.0. Courtesy of ESA (https://www.cosmos.esa.int/web/solar-orbiter).
• Figure 3: Left: the Sun observed in the 174 Å band of Solar Orbiter/FSI on 30 May 2020. The blue square locates the HRIEUV field of view, shown in detail on the right panel. Here, the blu dots represent the brightening detected within the five-minutes sequence. Reproduced with permission from Dolliou23, copyright by the author(s).
• Figure 4: Left: example of HRIEUV brightenings observed in a QS area at a distance of 0.556 AU. Right: calibrated images (L2 level) with the original sensor pixelization of an event observed from 0.323 AU; each pixel corresponds to 115 $\mathrm{km}$ on the Sun. Right: Various cuts through the middle-panel event, demonstrating that the FWHM of the feature is 1 -- 2 pixels. Reproduced with permission from Berghmans_2023, copyright by the author(s).
• Figure 5: Example of HRIEUV brightenings observed in bright-point loops. Reproduced with permission from Tiwari22, copyright by the author(s).