High-Resolution Observations of Clustered Dynamic Extreme-Ultraviolet Bright Tadpoles near the Footpoints of Coronal Loops

(Solar Orbiter Nugget #58 by R. Wang^1,2, Y. D. Liu^1,2, L. P. Chitta³, X. Zhao^4,5 and H. Hu¹)

1. Introduction

The European Space Agency's (ESA) Solar Orbiter (SO) [1], launched in February 2020, has ushered in a groundbreaking era of solar observation by capturing high-resolution images from unprecedented proximity to the Sun. On March 26, 2022, SO reached its perihelion, i.e. the closest approach to the Sun, positioning the spacecraft within Mercury's orbit, at roughly one-third of the Sun-Earth distance. This vantage point enhanced the capabilities of the High Resolution Imager (HRI_EUV) instrument, part of the Extreme Ultraviolet Imager (EUI) suite, which boasts an angular pixel resolution of 0.492''. By March 30, 2022, HRI achieved a remarkable spatial resolution of 118 kilometers on the solar surface, among the highest ever recorded by the mission. This exceptional resolution has enabled scientists to observe intricate details of the Sun's atmosphere, revealing previously unseen small-scale structures and offering fresh insights into solar dynamics.

2. Stereoscopic Measurements

Figure 1. Stereoscopic measurements using SDO and SO. (a) Heliospheric positions of planets and spacecraft. (b) and (c) CEBTs observed from EUI/HRI_EUV and AIA views with localized zoomed inserts, respectively. The red crosses and the intersecting green and blue lines in the inserts represent stereoscopic measurements using the two instruments. Animation of the inserts is available.

Using the HRI's 174 Å channel, we identified clusters of dynamic, tadpole-like structures near the footpoints of coronal loops where magnetic field lines anchor into the solar surface. Named "Clustered Extreme-Ultraviolet Bright Tadpoles" (CEBTs) due to their elongated tails and grouped appearance (see Figure 2a) and 2b below), these features often exhibit downward motion. Observations of multi-instruments from SO indicate that they may occur above the edge of the weak magnetic field region around the coronal loops. To measure their heights, we employed a stereoscopic technique, also used to estimate the height of "campfires" [2], by combining near-simultaneous observations from SO's HRI_EUV and the Solar Dynamics Observatory's (SDO) Atmospheric Imaging Assembly (AIA) at 171 Å. This method, illustrated in Figure 1, leverages the differing perspectives of the two spacecraft to triangulate the altitudes of these structures. Analysis of six typical CEBTs revealed heights ranging from approximately 1,300 to 3,300 km, spanning the chromosphere, transition region, and lower corona.

Figure 2. Velocity measurements of D-CEBTs. (a) and (b) Typical tadpole-shaped D-CEBT structures. The times are the SO time. (c) and (d) Corresponding time-distance plots of D-CEBTs, with downward- moving velocities annotated by cyan lines.

3. Discussion and conclusions

Figure 3. The periodic measurements of dark and cooler filamentary structures associated with CEBTs. (a) Lines in different colors represent the artificial slits used for generating time-distance plots, depicted in panels (b)-(d). The directions of the slits are from bottom right to top left. Animation of (a) is available.

The discovery of CEBTs reveals new complexities in the Sun's lower atmosphere, distinguishing these features from known phenomena such as microjets or campfires. These structures are classified into upward-moving (U-CEBTs) and downward-moving (D-CEBTs) types, with D-CEBTs being more prevalent. Time-distance analyses (see Figure 3) reveal that CEBTs exhibit periodic oscillations with cycles of 3-5 minutes, resembling the behavior of Type I spicules [3] or coronal fibrils [4], jet-like features in the chromosphere known for their oscillatory motion. However, D-CEBTs display downward velocities reaching up to 70 km s^-1, far exceeding the typical speeds of Type I spicules (< 50 km s^-1) and aligning more closely with Type II spicules [3][5][6] or coronal rain descent velocities [7][8].

This intriguing combination of characteristics has sparked several hypotheses. One suggests that CEBTs may combine characteristics of both Type I and Type II spicules, displaying a hybrid nature [5][6]. Another posits a connection to micro-scale coronal rain, though the lack of the cool, dense plasma clumps typical of coronal rain challenges this idea [7]. The rapid downward motion of D-CEBTs, observed in the extreme-ultraviolet (EUV) band, is particularly intriguing. While EUV studies often highlight the upward motion of spicules [9], the downward phase has received less focus, making CEBTs potentially significant for advancing our understanding of solar atmospheric dynamics. As Solar Orbiter continues to deliver high-resolution data, future studies promise to unravel the origins and significance of these enigmatic tadpole-like structures, deepening our understanding of the Sun's complex atmosphere.

This study has been published in R. Wang, et al. 2024, Research in Astronomy and Astrophysics, 24, 125010.

Accompanying movies

Movie 1 for Figure 1

Movie 2 for Figure 3

Affiliations

(1) State Key Laboratory of Solar Activity and Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing 100190, People's Republic of China

(2) University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China

(3) Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

(4) Key Laboratory of Space Weather, National Satellite Meteorological Center (National Center for Space Weather), China Meteorological Administration, Beijing 100081, People's Republic of China

(5) School of Earth and Space Sciences, Peking University, Beijing 100871, People's Republic of China

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Acknowledgements

The research was supported by National Key R&D Program of China No. 2022YFF0503800 and No. 2021YFA0718600, the Strategic Priority Research Program of the Chinese Academy of Sciences (NO. XDB0560000), NSFC under grants 12073032, 42274201, 42150105, and 42204176, and the Specialized Research Fund for State Key Laboratories of China. We acknowledge the use of data from Solar Orbiter and SDO. 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 O ce (BELSPO/PRODEX PEA 4000134088); 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 O ce (SSO). L.P.C. gratefully acknowledges funding by the European Union (ERC, ORIGIN, 101039844). Views and opinions expressed are however those of the author(s) only and do not necessarily re ect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them.