HIGH-RESOLUTION OBSERVATIONS OF SMALL-SCALE ACTIVITY IN CORONAL HOLE PLUMES

Introduction

Coronal hole plumes are long-lived, ray-like structures embedded in coronal holes and extending far into the outer solar atmosphere. Coronal holes are widely recognized as the source regions of the fast solar wind, with open magnetic field lines allowing plasma to escape into interplanetary space. However, the specific contribution of plumes to the outflowing solar wind remains uncertain, as they occupy only a small fraction of coronal hole volume and mass [1] and exhibit a wide range of reported flow speeds depending on the observation and diagnostic method [2–4].

Recent high-resolution observations have revealed that plumes are highly structured and dynamic, consisting of fine-scale features such as plumelets, jetlets and brightenings [5]. Propagating disturbances (PDs) are also frequently observed along plume structures, but their physical nature remains debated, with interpretations ranging from quasi-periodic flows [6] to magnetosonic waves [7], or a combination of both. These findings motivate detailed investigations of the small-scale activity at plume footpoints and its possible role in forming plumes and contributing to the solar wind.

In this study, using high-resolution data from the High Resolution Extreme Ultraviolet telescope (HRIEUV), which is part of the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter, we focus on three plumes observed within an equatorial coronal hole and analyze the characteristics and movements of the dynamic base brightenings. We further compare the properties of base brightenings with PDs observed along plume structures in order to assess whether the two phenomena are physically connected.

Observations

We select an equatorial coronal hole observed by EUI on 13 October 2022, located south of NOAA 13105 and west of NOAA 13107. Three plumes are present within the field of view of HRIEUV. At the time of observation, Solar Orbiter was at 0.29 au from the Sun. HRIEUV (17.4 nm) recorded a 30-minute sequence from 17:00 to 17:30 UT with a 5 s cadence, binned to 20 s during analysis to reduce noise. The pixel scale of 0.492” corresponds to about 100 km pixel^-1.

Figure 1. Overview of coronal hole observations. (a) Full-disk image of the Sun provided by Full Sun Imager (FSI) of EUI. The box indicates the region covered by HRIEUV. (b) Image taken by HRIEUV with three boxes showing the base regions of the three plumes shown in panels (c)–(e). Both (a) and (b) are plotted in logarithmic scale in brightness. (c)–(e) Zoom-in normalized images of the base regions of the three plumes.

As shown in Fig. 1, the coronal hole is located in the southern hemisphere, with the three plumes positioned close to disk center (about 25 degrees heliocentric angle). HRIEUV clearly shows that all plumes exhibit highly dynamic bases, with recurrent brightenings forming near their footpoints and propagating outward along the plume structures. We detect small-scale base brightenings using both visual identification and automatic methods based on enhanced image intensity thresholds and feature connectivity. In total, 50 events are identified visually, which also helps to set thresholds for the automatic method, where 451 brightenings are detected across the three plume bases (see Fig. 2).

Results

For all base brightenings selected with both methods, we investigate their properties statistically. Normalized distributions of mean intensity, lifetime, area, length-to-width ratio, and velocity are shown in Fig. 3.

These brightenings are found to be typically small (with an area of less than 1.3 Mm²), short-lived (lasting less than five minutes), and often show slightly elongated morphologies. It is interesting that their plane-of-sky motions are generally slow, with most events exhibiting speeds below 10 km s^-1.

These velocities are significantly lower than those of PDs (80–160 km s^-1) observed higher in plumes. Fig. 4 shows examples of the PDs, whose apparent speeds are measured based on the time-slice plot.

Data from the Polarimetric and Helioseismic Imager aboard Solar Orbiter are used as photospheric boundary conditions for 3D magnetic field extrapolations with a potential field model, to de-project the measured brightening velocities at plume bases. However, their real velocities are still found to be significantly lower than, and hard to reconcile with, the high speeds observed in PDs at greater heights within the plumes.

Discussion and conclusion

The physical interpretation of PDs in plumes provides a useful reference for understanding the less explored base brightenings. By analyzing the intensity evolution along plume structures, we find that the EUV emission initially increases with height before decreasing, while the intensity peak propagates outward with time (see Fig. 5(b)). These behaviors are inconsistent with a hydrostatic equilibrium scenario.

In addition, only the fastest PDs exhibit apparent speeds approaching the local acoustic speed of 150 km s⁻¹ at coronal temperatures of 10⁶ K. Although projection effects may cause similar PDs to appear at different speeds, interpreting the observed velocity range of 80 to 160 km s⁻¹ would require unrealistically large differences in inclination angles. Overall, the PD properties are more consistent with mass flows than with waves.

Assuming PDs are mass flows, we investigate whether base brightenings could represent their low-altitude counterparts. Assuming a constant mass flux from base brightenings evolving into plume outflows, the observed increase in velocity (from about 30 to 100 km s⁻¹) together with a widening of the outflow cross-section by about a factor of 9 (see Fig. 5(c)) implies that the density must decrease by roughly a factor of 27 with height to satisfy mass conservation. This is inconsistent with observed EUV intensity. These results therefore do not support a direct link between base brightenings and PDs.

The origin of base brightenings is likely more complex. One possibility is interchange reconnection, which has been widely proposed to explain jets or outflows in coronal holes [5,8]. However, it is not strictly required by recent MHD models [9], and alternative mechanisms may also contribute. For example, the slow and irregular motions of the majority of base brightenings, together with their periodicity and association with chromospheric network concentrations, could point to a link with lower-atmosphere features such as Type I spicules.

In conclusion, our results do not support a direct link between base brightenings and PDs. PDs are consistent with high-speed plasma outflows along plume structures, whereas base brightenings trace small-scale activity at plume footpoints that may be related to interchange reconnections or Type I spicules. A direct evolutionary link between the two is not favored, though cannot be conclusively ruled out by the present observations. Future high-resolution, multi-instrument measurements, including simultaneous magnetic and, most importantly, spectroscopic diagnostics (such as those that will be provided by the forthcoming EUVST spectrograph aboard the Solar-C mission of JAXA), are required to fully determine the physical origin of base brightenings and their role in supplying mass and energy to the fast solar wind.

This science nugget is based on the paper Huang et al., A&A, 708, A180 (2026).

Affiliations

(1) Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

(2) Institut für Sonnenphysik (KIS), Georges-Köhler-Allee 401A, 79110 Freiburg, Germany

(3) Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstrasse 3, 38106 Braunschweig, Germany

Acknowledgements

Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. The EUI instrument was built by CSL, IAS, MPS, MSSL/UCL, PMOD/WRC, ROB, LCF/IO with funding from the Belgian Federal Science Policy Office (BELSPO/PRODEX PEA C4000134088); the Centre National d’Etudes Spatiales (CNES); the UK Space Agency (UKSA); the Bundesministerium für Wirtschaft und Energie (BMWi) through the Deutsches Zentrum für Luft- und Raumfahrt (DLR); and the Swiss Space Office (SSO). Ziwen Huang conducted the work in this paper in the framework of the International Max Planck Research School (IMPRS) for Solar System Science at the Technical University of Braunschweig. L.P.C. gratefully acknowledges funding by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council (grant agreement No 101039844). Neither the European Union nor the granting authority can be held responsible for them.

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