Accessing the fine temporal scale of EUV brightenings and their
quasi-periodic pulsations: 1-second cadence observations by Solar Orbiter/EUI

(Solar Orbiter Nugget #80 D. Lim^1,2, T. Van Doorsselaere², N. Narang¹, L. A. Hayes³, E. Kraaikamp¹, A. Joshi², K. Loumou¹, C. Verbeeck¹, D. Berghmans¹, and K. Barczynski^4,5)

Introduction

The coronal heating problem remains one of the most persistent and fundamental challenges in astrophysics [1]. The nanoflare heating theory, one of the leading mechanisms proposed to address this issue, posits that the solar corona may be heated by a sufficiently large number of small-scale energy release events [2,3]. Imaging observations in extreme-ultraviolet (EUV) wavelengths have consistently revealed ubiquitous small, transient brightenings in the corona, which may correspond to such small-scale flaring events [4-14].

A central question in the nanoflare heating scenario is whether these numerous weakest events are sufficiently abundant to contribute significantly to coronal heating. Addressing this requires not only identifying smaller-scale events [15], but also uncovering those that remain hidden due to limited temporal resolution. 

The Solar Orbiter/Extreme Ultraviolet Imager (EUI) not only provides the high spatial resolution, but also allows for unprecedented temporal resolution. In particular, the EUI High Resolution Imager at 17.4 nm (HRIEUV) allows EUV imaging at cadences as short as 1 s, opening a new observational regime for investigating short-lived EUV brightenings and their quasi-periodic pulsations (QPPs), an intrinsic feature of impulsive energy release events such as flares [16,17].
 

Results

Figure 1. Left: Logarithmic histograms of EUV brightening lifetimes. Events detected in ARs and the QS are shown in blue and orange, respectively. Right: Logarithmic histograms of the periods of detected QPPs. Periods below 5 s are excluded, as we applied a conservative criterion requiring detected periods to be at least five times the observational cadence of 1 s. The dashed lines represent normal distributions in logarithmic space.

During dedicated high cadence observing campaigns, the HRIEUV telescope of Solar Orbiter/EUI was operated at a cadence of 1 s on 19 October 2024 (~174 km/pixel) and 19 March 2025 (~140 km/pixel). These observations captured both active regions (ARs) and quiet Sun (QS) areas within the same field of view.

These datasets revealed EUV brightenings spanning a wide range of spatial and temporal scales. The lifetime distribution extends down to the observational limits imposed by the temporal resolution of 1 s in both ARs and the QS regions (left panel of Figure 1). From the resulting event catalogue, we selected 64526 events from ARs and 28354 from QS regions brightenings with lifetimes equal to or greater than five time frames. For each event, light curves were constructed by summing the intensity over the area corresponding to the event’s maximum spatial extent. Representative examples of EUV brightenings and their integrated light curves are shown in Figure 2.

Figure 2. Examples of EUV brightenings detected with HRIEUV (top) alongside their corresponding light curves (bottom), showing the normalised integrated brightness over the entire brightening regions. The two left panels show events observed in the AR on 19 October 2024, while the two right panels show events observed in the QS on 19 March 2025. 

To maximise the detection of QPPs, we applied two complementary analysis techniques, a Fourier analysis with the AFINO routine [18] and ensemble empirical mode decomposition [19]. Using these methods, QPPs were identified in 11% of the AR events and 9% of the QS events. The detected QPP periods range from 5 s to about 500 s (right panel of Figure 1), comparable to period ranges previously reported for both solar [20,21] and stellar flares [22]. The relationship between QPP period and event length scale shows a trend consistent with P = 20L, assuming a standing wave with a phase speed of 100 km/s. Interpreting this speed as the sound speed implies an inferred plasma temperature of approximately 0.4 MK, comparable to temperatures previously measured for EUV brightenings from Solar Orbiter spectroscopic observations [23]. 

Figure 3. Relationship between QPP period (P) and lifetime (τ) across different event scales. Blue, orange, and green dots represent QPPs detected in EUV brightenings (this study), GOES X-ray solar flares (Hayes et al. 2020), and TESS stellar flares (Joshi et al. 2025), respectively. The dashed black line shows a power-law fit to the combined dataset.

To investigate whether a unified scaling law might apply across different flare magnitudes, we compared our results with QPPs previously detected in GOES X-ray solar flares [21] and TESS stellar flares [22]. As shown in Figure 3, the combined dataset exhibits a consistent scaling trend, analogous to the well-established temperature emission measure scaling from EUV brightenings to stellar flares [24].

Conclusions

Thanks to the high cadence, we were able to detect a significant population of very short-lived brightenings (lifetime < 3 s), highlighting the importance of temporal resolution for capturing fine-scale coronal dynamics. By combining our results with previously reported QPPs in GOES solar flares and TESS stellar flares, we found that the period-lifetime relation follows a single power-law scaling across more than three orders of magnitude. The best-fit exponent of approximately 0.39 indicates that the observed relation is consistent with a universal, scale-invariant behaviour, suggesting a shared physical origin. These findings further support the interpretation that small-scale EUV brightenings may be manifestations of flare-like processes.

This nugget is based on the following paper: Lim et al. 2025, A&A, 704, A58, https://doi.org/10.1051/0004-6361/202557135
 

Affiliations

(1) Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium, Ringlaan -3- Av. Circulaire, 1180 Brussels, Belgium
(2) Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
(3) Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dublin, D15 XR2R, Ireland
(4) ETH-Zurich, Wolfgang-Pauli-Str. 27, 8093 Zurich, Switzerland
(5) Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, 7260 Davos Dorf, Switzerland
 

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