Quasi-Periodic Pulsations in EUV Brightenings

(Solar Orbiter Nugget #65 by D. Lim^1,2, T. Van Doorsselaere¹, D. Berghmans², L. A. Hayes³, C. Verbeeck², N. Narang², M. Dominique², and A. R. Inglis^4,5

1. Introduction

Extreme-ultraviolet (EUV) brightenings are ubiquitous, small-scale, short-lived intensity enhancements in the solar corona, observed with imaging instruments [1–4]. These brightenings have been the focus of considerable interest as potential small-scale flare events that could play a role in coronal heating [5,6]. Although observations and simulations increasingly point towards magnetic reconnection as a likely driving mechanism [7–9], the physical origin of these brightenings has yet to be conclusively determined.

A distinctive signature of solar and stellar flares is the presence of quasi-periodic pulsations (QPPs) [10,11]. If EUV brightenings share the same underlying mechanisms as larger-scale flares, it is reasonable to expect that QPPs might also be present in these smaller events.

2. Results

At perihelion (0.29 au from the Sun), the HRIEUV telescope of Solar Orbiter/EUI [12] observed two quiet Sun regions on 12 October 2022 and 6 October 2023 with its highest spatial resolution (~105 km/pixel) and a cadence of 3 seconds. These observations revealed EUV brightenings spanning a wide range of spatial and temporal scales [13]. From the resulting EUV brightening event catalogue, we selected 22623 events and analysed light curves generated by summing the brightness over the area corresponding to each event’s maximum spatial extent. Figure 1 shows two examples of EUV brightenings along with their integrated light curves.

Figure 1. Examples of EUV brightenings detected with HRIEUV (left) alongside their corresponding light curves (right), showing the normalised integrated brightness over the entire brightening regions observed on 12 October 2022. Dashed lines indicate the start, peak, and end times of each EUV brightening. Each brightening image corresponds to its peak time.

Using Fourier analysis with the AFINO routine [14], we detected 602 stationary QPPs. The periods of these QPPs range from 15 s to 255 s (Figure 2), which is comparable to the QPP period ranges reported in both solar [15,16] and stellar flares [17].

Figure 2. Logarithmic histograms of the periods of detected QPPs. Note that periods below 15 s are excluded, as we applied a conservative criterion requiring detected periods to be at least five times the observational cadence of 3 s. The dashed red line represents the normal distribution in log space, with a mean of 21.6 s of QPP periods observed in standard solar flares (Hayes et al. 2020).

We further investigated potential relationships between QPP periods and the characteristics of EUV brightenings. First, we found a negligible correlation between QPP period and peak brightness, with a Pearson correlation coefficient (CC_P) of 0.02 (left panel of Figure 3). A similar lack of correlation between flare magnitude and QPP period has been reported in previous studies of larger solar [14–16,18] and stellar flares [17,19].

Figure 3. Scatter plots of the QPP period versus the peak brightness (left panel) and lifetime (right panel) of EUV brightenings. Period uncertainties were estimated from the detection model. The colour scales in each panel represent the logarithmic values of lifetime and peak brightness. The Pearson correlation coefficient (CC_P) and Spearman’s rank correlation (CC_R) are indicated.

Likewise, the relationship between QPP period and event lifetime (right panel of Figure 3) shows no significant correlation (CC_P of 0.11). This result stands in contrast to the strong correlation between QPP period and flare duration observed in GOES X-ray flares [16,20] and in white-light stellar flares [17].

3. Conclusions

The light curves of EUV brightenings exhibit clear signatures of QPPs, with detected periods comparable to those observed in both solar and stellar flares. As with standard solar flares, no correlation was found between the QPP period and peak brightness. However, unlike in solar flares, no correlation was observed between the QPP period and event lifetime. The results demonstrate the robust presence of QPPs in EUV brightenings and support the interpretation that such events may represent the small-scale counterparts of flares.

This nugget is based on the following paper: Lim et al. 2025, A&A, 698, A65

Affiliations

1. Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
2. Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium, Ringlaan -3- Av. Circulaire, 1180 Brussels, Belgium
3. Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dublin, D15 XR2R, Ireland
4. Solar Physics Laboratory, Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
5. Physics Department, The Catholic University of America, Washington, DC 20064, USA

References 

[1] Berghmans, D., Clette, F., & Moses, D. 1998, A&A, 336, 1039

[2] Harrison, R. A., Lang, J., Brooks, D. H., & Innes, D. E. 1999, A&A, 351, 1115

[3] Chitta, L. P., Peter, H., & Young, P. R. 2021, A&A, 647, A159

[4] Berghmans, D., Auchère, F., Long, D. M., et al. 2021, A&A, 656, L4

[5] Aschwanden, M. J., & Parnell, C. E. 2002, ApJ, 572, 1048

[6] Purkhart, S., & Veronig, A. M. 2022, A&A, 661, A149

[7] Chen, Y., Przybylski, D., Peter, H., et al. 2021, A&A, 656, L7

[8] Panesar, N. K., Tiwari, S. K., Berghmans, D., et al. 2021, ApJ, 921, L20

[9] Kahil, F., Hirzberger, J., Solanki, S. K., et al. 2022, A&A, 660, A143

[10] Van Doorsselaere, T., Kupriyanova, E. G., & Yuan, D. 2016, Sol. Phys., 291, 3143

[11] Zimovets, I. V., McLaughlin, J. A., Srivastava, A. K., et al. 2021, Space Sci. Rev., 217, 66

[12] Rochus, P., Auchère, F., Berghmans, D., et al. 2020, A&A, 642, A8

[13] Narang, N., Verbeeck, C., Mierla, M., et al. 2025, A&A, Accepted, arXiv:2505.03656

[14] Inglis, A. R., Ireland, J., Dennis, B. R., Hayes, L., & Gallagher, P. 2016, ApJ, 833, 284

[15] Dominique, M., Zhukov, A. N., Dolla, L., Inglis, A., & Lapenta, G. 2018, Sol. Phys., 293, 61

[16] Hayes, L. A., Inglis, A. R., Christe, S., Dennis, B., & Gallagher, P. T. 2020, ApJ, 895, 50

[17] Joshi, A.,  Van Doorsselaere, T., Lim, D., & Fritzewski, D. J. 2025, A&A, Submitted

[18] Pugh, C. E., Nakariakov, V. M., Broomhall, A. M., Bogomolov, A. V., & Myagkova, I. N. 2017b, A&A, 608, A101

[19] Pugh, C. E., Armstrong, D. J., Nakariakov, V. M., & Broomhall, A. M. 2016, MNRAS, 459, 3659

[20] Pugh, C. E., Broomhall, A. M., & Nakariakov, V. M. 2019, A&A, 624, A65