Thin Coronal Jets and Plasmoid-mediated Reconnection: Insights from Solar Orbiter Observations and Bifrost Simulations.

(Solar Orbiter Nugget #77 by D. Nóbrega-Siverio^1,2,3,4, R. Joshi^3,4,5,6, E. Sola-Viladesau², D. Berghmans⁷, D. Lim^8,7)

Introduction

Coronal Bright Points (CBPs) are ubiquitous small-scale magnetic structures in the solar corona, consisting of million-Kelvin loops rooted in the photospheric network [1]. They radiate copious amounts of energy [2,3] and cover the Sun almost uniformly [4], making them key ingredients in the quest to understand coronal heating [1]. CBPs are also textbook sites of interchange reconnection, where closed and open magnetic field lines reconnect and release plasma and energy, often producing jet-like outflows [5,6]. Yet, their contribution to the corona and solar wind has remained difficult to quantify, largely due to the lack of observations capable of resolving their rapid evolution and fine magnetic structure, as well as the limited availability of realistic numerical models.

In this work [7], we exploit the unprecedented capabilities of the High Resolution Imager at 174 Å (HRI_EUV) onboard Solar Orbiter [8,9] to reveal the previously hidden jetting activity of CBPs and to identify signatures of magnetic reconnection occurring at their current sheets. We analyze eleven datasets obtained between 2021 and 2023, capturing CBPs under diverse viewing geometries and evolutionary stages. These observations are complemented with a state-of-the-art radiative-MHD simulation [10] performed with the Bifrost code [11], which provides a physical framework to interpret the observed dynamics and reconnection signatures.
 

Results 

The analysis shows that CBPs routinely produce thin coronal jets with widths between 250 and 700 km, well below the resolution limits of previous EUV imagers. Despite their narrow shape, these jets can extend up to 22 Mm while remaining brighter than their surroundings by roughly 30–85% (see example of Fig. 1). Several events exhibit sustained, recurrent jetting that had remained hidden in AIA images, emphasizing the transformative impact of HRI_EUV’s spatial resolution.

Figure 1: Top: Thin coronal jet associated to a CBP observed with HRI_EUV. Bottom: Same region as observed by SDO/AIA 171 Å. The AIA UT time has been selected to match the corresponding HRI_EUV observation, accounting for the light travel time delay due to Solar Orbiter’s position near the Sun. The intensity of both images is given in DN.

A particularly revealing example (29 October 2022) displays a current sheet only a few megameters long, within which HRI_EUV resolves a bright, compact feature of size ≈330 km moving at ≈40 km/s. This feature matches the expected morphology and evolution of a plasmoid generated by tearing-mode reconnection. Synthetic HRI_EUV emission from a radiative-MHD Bifrost simulation of a fan-spine CBP supports this interpretation: plasmoids of comparable size appear in the model and remain detectable once degraded to HRIEUV’s resolution (see Fig. 2).

Figure 2: Plasmoid signatures observed with HRI_EUV (top) and from the Bifrost simulations after degrading the resolution to match the best HRI_EUV pixel scale in our study (108 km). From left to right, the panels are as follows. Context view showing the CBP and the current sheet within a rectangle of length, L, and width, W. Zoomed-in view of the blue rectangle, highlighting the illustrative plasmoid with a cyan arrow. Space–time plot of the current sheet, obtained by taking the maximum intensity along the W direction. The cyan dashed line indicates the trajectory of the plasmoid. Intensity profiles along the current sheet at different times, illustrating the evolution of the plasmoid.

Another event (7 April 2023) shows weak indirect imprints of plasmoid-mediated reconnection in the outflow region. There, HRI_EUV reveals intermittent, boomerang-shaped brightness enhancements with low apparent speeds. Numerical modelling indicates that such patterns could naturally arise when plasmoids erupt from a current sheet, interact with neighbouring magnetic field, and redistribute plasma along newly reconnected loops.

Conclusions

Thanks to its enhanced spatial resolution, HRI_EUV reveals that CBPs host far richer coronal jet activity than previously recognized. Earlier EUV imagers often missed these jets, particularly during the early or faint phases of CBP evolution. These results highlight the need for future statistical studies to quantify their contribution to coronal dynamics and solar wind formation.

In addition, HRI_EUV resolves a ∼330 km plasmoid forming and propagating within a CBP current sheet: the smallest plasmoid ever reported in a coronal current sheet in EUV observations. This provides direct evidence that HRI_EUV can capture fast, plasmoid-mediated reconnection at sub-megameter scales. The observed plasmoid properties closely match those produced in advanced radiative-MHD simulations, reinforcing their value for interpreting HRI_EUV’s high-resolution view of coronal reconnection.

 
This work has been published in Nóbrega-Siverio et al. 2025, A&A, 702A, 188N https://doi.org/10.1051/0004-6361/202555357
 

Affiliations

(1) Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain.
(2) Universidad de La Laguna, Dept. Astrofísica, E-38206 La  Laguna, Tenerife, Spain.
(3) Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway.
(4) Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway.
(5) Department of Physics and Astronomy, George Mason University, Fairfax, VA 22030, USA.
(6) Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
(7) Solar-Terrestrial Centre of Excellence–SIDC, Royal Observatory of Belgium, Ringlaan -3- Av. Circulaire, 1180 Brussels, Belgium.
(8) Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium.

References

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