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

Abstract

Plumes have been proposed to channel 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 that could be related to the PDs. We study three plumes within an equatorial coronal hole observed by the EUV High Resolution Imager of the Extreme Ultraviolet Imager on board Solar Orbiter. The properties of the small-scale brightenings at the plume bases are investigated to interpret their nature and possible relation with PDs. We process images with the Difference of Gaussians method to highlight the target brightenings, which are further identified with two different approaches. In the 30-min observation, 50 brightenings are visually selected, which also help set thresholds for automatic detection, where we find 451 brightenings. Their properties, including velocities on the plane of sky (PoS), are analyzed statistically. Potential field extrapolation based on the magnetic field data from the Polarimetric and Helioseismic Imager on board Solar Orbiter is used for correcting the PoS velocity to the real velocity along the magnetic field. We observe that the majority of the base brightenings are small-scale, short-lived, and slightly elongated at the plume bases. They display intricate movements, with most exhibiting velocities in the PoS of less than 10 km/s. Their 3-dimensional velocities are found to be substantially lower than (and difficult to reconcile with) the speeds of PDs. A direct link between base brightenings and PDs remains inconclusive. 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.

Figures (11)

• Figure 1: Overview of coronal hole observations. (a) Full-disk image of the Sun provided by 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. (The associated movie is available online.)
• Figure 2: Dynamics at the base of plumes. (a) A sample light curve obtained from the base region of plume 2 (see Figure \ref{['obs_fig1']} (d)). The whole time period (30 min) is divided into 4 sections (partitioned at orange dashed lines). (b) - (e) Images of the base region of plume 2 acquired at the middle instance from each temporal section in (a). Brightenings identified using chosen thresholds are marked by the contours (colors indicate time progression; as defined by the color bars). The red boxes highlight several examples of the ‘gradient patterns’, which are formed by contours from successive frames being adjacent to or overlapping one another, thereby illustrating the movement of the brightenings. Note that, even though we have divided the entire observation into four sub-periods, the contour of the brightenings that appears later will inevitably overlap the ones that appear earlier. For example, a discrete contour could be either a brightening that only appears in one frame, or a brightening that does not show a significant movement.
• Figure 3: Zoom into the plumes bases observed on 13 October 2022 17:10 UT. The intensity-weighted center of each base brightening along the evolution is overplotted on the images. The color represents the time of the first appearance of each base brightening (see the color bar).
• Figure 4: Distributions of the properties of the base brightenings selected with the visual identification method (50 base brightenings). (a) shows the mean intensity of the base brightenings without subtracting the local background while (b) shows the mean intensity with the local background removed. (c) - (f) show the lifetime, area, ratio of length to width and velocity. The velocity here is a projection on the PoS and is calculated using the first and last positions of the intensity-weighted center of the base brightening.
• Figure 5: Normalized distributions of the properties (mean intensity, lifetime, area, ratio of length to width and velocity) of the base brightenings detected with both visual identification method (50 base brightenings) and automatic method (451 base brightenings). The grey bars show the results from the automatic method and the blue line-filled bars show the results from the visual identification method.