Solar Orbiter's Closest Look Reveals Myriad Tiny Flares
One of the most enduring mysteries in solar physics is the coronal heating problem: why is the Sun's outer atmosphere, the corona, hundreds of times hotter than its visible surface? One leading theory suggests that the corona is heated by a constant storm of tiny, impulsive energy releases, often called “nanoflares".
A new study published in the Astronomy & Astrophysics led by Dr. Nancy Narang from the STCE, provides compelling new evidence to support this theory, thanks to unprecedented observations from the Solar Orbiter spacecraft.
The Closest View Ever
Using the High Resolution EUV Imager (HRIEUV) of the Extreme Ultraviolet Imager (EUI) onboard Solar Orbiter spacecraft, scientists took advantage of Solar Orbiter's closest perihelion passes in 2022 and 2023. From a distance of just 0.293 AU (less than one-third of the distance from the Sun to Earth), EUI captured the highest-resolution images of the quiet solar corona ever obtained.
These unique observations had an image scale of just 105 km per pixel and were taken at a rapid cadence of 3 seconds, revealing the solar atmosphere in stunning, fine detail.
Top Panels: EUI/FSI (Full-Sun Imager aboard EUI) images of the full Sun with the HRIEUV field-of-view overlaid.
Bottom Panels: Detailed HRIEUV observations with detected EUV brightenings marked in cyan.
The animations are available as movie1 and movie2. Courtesy: Narang et al. 2025
Unprecedented Findings
The STCE researchers used a sophisticated wavelet-based algorithm to automatically detect and characterize thousands of transient EUV brightenings, often called "campfires," in the HRIEUV data obtained at the closest-perihelion of Solar Orbiter. The results were staggering:
- Record-Breaking Detections: The study reports the detection of the smallest and shortest-lived EUV brightenings to date. These events were observed with sizes down to 10000 km² and lifetimes as short as 3 seconds—the very limit of the instrument's detection capability.
- A Staggering Occurrence Rate: The analysis revealed that these tiny events are far more numerous than previously thought. The team estimates that, on average, about 3600 EUV brightenings appear per second across the entire surface of the Sun. This rate is more than an order of magnitude higher than previously reported values.
- The Small Dominate: The statistical analysis showed a clear power-law distribution, confirming that smaller and shorter-lived events are vastly more common than their larger, and longer-lived counterparts.
Close-up view of tiniest EUV brightenings. Courtesy: Narang et al. 2025
Heating of the Solar Corona
This incredibly high number of tiny, ubiquitous energy releases provides strong support for the nanoflare heating model. While the energy contributed by each individual EUV brightening is small, their collective effect across the entire Sun could be a significant source of the energy required to maintain the million-degree corona (Narang et al. 2025).
The study concludes that these EUV brightenings observed by the HRIEUV are the most prevalent, localized, and finest-scale transient events ever detected in the quiet solar corona.
While these results are a major step forward, the researchers at STCE note that the next crucial step is to determine the thermal energy of this newly-discovered population of tiny events. Active research is being performed at STCE to explore the thermal nature of these tiniest EUV brightenings, that will put another step forward to fully understand their contribution in solving the coronal heating mystery.
The research that led to these results was subsidized by the Belgian Federal Science Policy Office through the contract B2/223/P1/CLOSE-UP.
Based on the paper:** Narang, N., Verbeeck, C., Mierla, M., Berghmans, D., Auchère, F., Shestov, S., Delouille, V., Chitta, L. P., Priest, E., Lim, D., Dolla, L. R., & Kraaikamp, E. (2025). Extreme-ultraviolet transient brightenings in the quiet-Sun corona. Astronomy & Astrophysics, 699, A138. https://doi.org/10.1051/0004-6361/202554650
