High-resolution observations of small-scale activity in coronal hole plumes

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

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 5 September 2025
Accepted: 1 February 2026

Abstract

Context. Largely radial ray-like structures, termed plumes, are often observed in coronal holes. Plumes have been proposed to channel magnetohydrodynamic (MHD) waves and the solar wind into the heliosphere. High-speed propagating disturbances (PDs), though well detected in plumes, cannot yet be clearly assigned to MHD waves or to mass flows. Additionally, plume bases as observed in the extreme ultraviolet are riddled with small-scale transients, which could be related to PDs.

Aims. We studied three plumes within an equatorial coronal hole observed by the EUV High Resolution Imager (HRI_EUV) of the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter. The properties of small-scale brightenings at the plume bases were investigated to interpret their nature and the possible relation with PDs in plumes.

Methods. A 30 Mm × 30 Mm sub-region is selected for each plume base. We processed the images with the difference of Gaussians (DoG) method to highlight the target brightenings, which were further identified with two different approaches. In the 30 minute observation, 50 brightenings were visually selected, which also helps to set thresholds for automatic detection, where we found 451 brightenings. Their properties, including apparent velocities on the plane of sky (PoS), were analyzed statistically. Potential field extrapolation based on the magnetic field data from the Polarimetric and Helioseismic Imager on board Solar Orbiter was used for correcting the PoS velocity to the real velocity along the magnetic field.

Results. We observe that the majority of base brightenings are small scale (with an area of less than 1.3 Mm²), short lived (lasting less than five minutes), and slightly elongated at the plume bases. They display intricate movements, with most showing velocities less than 10 km s⁻¹ on the PoS. The velocity distribution of the base brightenings selected via visual identification shows a peak at 6–7 km s⁻¹, while that of the automatically detected base brightenings peaks at a lower velocity of less than 5 km s⁻¹. Their 3D velocities are found to be substantially lower than (and difficult to reconcile with) the speeds detected at greater heights in the plumes.

Conclusions. A direct link between base brightenings and PDs remains inconclusive due to the huge difference between their velocities. We propose two possibilities for base brightenings: they may be related to wave-driven Type I spicules or originate from interchange reconnections. Further investigation is required to validate these hypotheses.