The First Joint Observations of EUV Jets and Spicules with Solar Orbiter and BBSO

(Solar Orbiter Nugget #79 J. Lee^1,2,3, D. Longcope⁴, Y. Youn⁵, N. K. Panesar⁶, N. Huang^1,2,3 and H. Wang^1,2,3)

Introduction

Small-scale jet-like ejections and eruptions (SSEs), such as chromospheric type II spicules, mini-filaments, and transition-region or coronal jets, are widely believed to contribute substantially to the coronal energy budget, to influence the structuring of the corona, and to facilitate mass transport into the solar wind. A new opportunity for studying SSEs is available with the Solar Orbiter (SO)'s Extreme Ultraviolet Imager (EUI) through the High Resolution Imager (HRI), in collaboration with the Hα imaging from the 1.6 m, high-order adaptive optics (AO) equipped Goode Solar Telescope (GST) at the Big Bear Solar Observatory (BBSO). HRI-EUV at 174 Å offers a pixel resolution of <0.5” and the GST/VIS can achieve resolution as high as 0.1”-0.2” with the pixel size of 0.029”. Such SO-BBSO joint observation should allow us to explore the key issue of SSEs: what dictates the structure and dynamics of the SSEs and what role the magnetic reconnection has in SSEs in the photosphere, chromosphere, and the corona.

Results

SO/EUI, SDO/AIA AND GST/VIS IMAGES
Figure 1 presents SDO/AIA 171 Å image and EUI-HRI image on 2022-10-29 around 19:10 UT (a, b) and zoom-in views of the jet seen by three different instruments (c-f). The three colored curves and the warped rectangles around NOAA AR 13133 are plotted to check the different projection effects on the images. Our target is a tiny EUV jet that emerged suddenly from the dark corona within the region marked with the white box located north of the AR. The EUI-HRI image reveals the fine-scale helical pattern of EUV bright strands giving an impression of twisted field lines or flux rope (c). The GST/VIS's inverted Hα blue wing (e) and red wing (f) images exhibit the spicules in straight trajectories without any helical structure of the EUV jets. A composite image of the Hα blue wing and HRI images (g) blue wing images highlights the difference between the EUI-HRI jets and the Hα spicules. 

Figure 1. The quiet-region EUV jet.  (a) SDO/AIA 171 Å with a small jet enclosed by the box. (b) SO's EUI-HRI 174 Å of the same region. The three-color half circles are meant to show the projection effect. (c) tilt-adjusted HRI 174 Å, (d) SDO/AIA 171 Å, (e, f) Hα±0.8 Å images. As a reference, we overlay the outline of the AIA 171 Å profile (blue contour from d) on other images (c, e, f). (g) A composite image of the Hα blue wing image (grayscale) and HRI image in yellow color shades. The observation time of AIA and GST is delayed by 4.5 min from that of HRI.

EUV JETS AND Hα SPICULES

Figure 2 shows one-to-one comparison of the EUV jet images with those of Hα spicules during their evolutions. Top panel: initially (before 19:07 UT in SO time) there was a faint structure looking like a weakly curved flux tube, which develops a brightening in the middle of the tube to form a wedge-like structure and further evolves into a knot-like structure (a), and the single strand develops into multiple strands (b) and three prominent stripes at the maximum intensity (c). The jet's guiding field lines rapidly turned around (d), and the jet diminished, and a faint rapidly bending structure appeared in the flank of the EUV jet (e).
The multiple EUV stripe pattern is not exactly reproduced in the Hα images-blue wing difference Hα±0.8 Å images (middle panels), but an alternating thin blue- and red-wing feature as thin as 200 km is found near the boundary of the EUV jets (f-i) tentatively called `sheath flow,' which may be cold plasma enveloping the hot EUV jet stream. Other up-rising spicules are also in the straight trajectory in contrast with the helical EUV jet structure.  The Dopplergrams in the bottom panels show the red-shifted Hα components are timely correlated with the EUV jets and are highly concentrated in the presumed location of the EUV jet footpoint as an important component connected to the EUV coronal jets.

Figure 2. The evolution of EUI-HRI jet structure at 174 Å (top), difference between Ha±0.8 Å wing images (middle), and pseudo-Dopplergram from 5 wavelength point GST/VIS images (bottom).  They show pre-existing strand (a), knot-like structure (b), multiple stripes (b-d) and a rapidly turning or bending structure (e, j, o) as well as sheath flows (arrows in f-i, k-n). The Dopplergrams show strong redshift components concentrated under the EUV jets.

PLASMA PROPERTIES AND ENERGETICS
Since the jets under current study are faint and further occulted by a bright 171 Å EUV loop in the foreground, we utilized a deep learning model [1] based on Pix2PixCC to reproduce the five AIA channel (94, 193, 211, 131, 335 Å) images from SO's Full Sun Imager (FSI) 174 Å and 304 Å images as shown in Figure 3. We then subtract the EUI/FSI intensity at a jet quiet time (19:20:50 UT) from that at the jet time (19:10:50 UT) to obtain the net DEM of the jet to determine the DEM-weighted temperature of 2.1 MK and  total mass of the fully ionized plasma, M~ 3.7×10¹¹ g. Together with the jet speed, we find the total energy of the EUV jet therefore amounts to 1.9 ×10²⁶ erg with 87% in the thermal energy and 13% in the kinetic energy.

Figure 3. DEMs of the jet region derived from EUV images. (a) The six panels show one EUI/FSI 174 Å image and five AIA equivalent images generated by deep learning. The red box indicates the pixels used to calculate the DEM. (b) The DEM derived from the SDO/AIA images (green) with AI-generated EUV images with observed FSI 174 Å image (purple). The solid lines represent DEM profiles when the jet activity is strong (19:10:20 UT), while the dashed lines correspond to the profiles when the jet decreases (19:20:50 UT). (c) The net DEM calculated from the difference EUV images between the jet maximum and the jet quiet time.

Figure 4. The reconnection model.  (a) Five snapshots from the initial phase in which the field line (colored curve) relaxes following its formation by reconnection. The pre-reconnection field lines are shown as red and blue curves on the left-most snapshot. The reverse color scale shows EM of hot (T>10⁶ K) plasma.  A dashed diagonal line shows a constant velocity, for reference.  (b) The temperature vs. height for the five times using the same colors.  The shaded box indicates the hot plasma whose emission is shown in (a).   (c) At five later times, a spicule has been launched by the downward Alfven wave. The green region shows cool (<10⁵ K), dense plasma at each time.  (d) The vertical velocity (shaded) in space-time coordinates. A green dashed parabola shows free-fall for reference, and vertical dashed lines show the times of the snapshots.

Conclusions

The first joint observations of small-scale EUV jets using the SO's EUI-HRI and BBSO's GST/VIS during the October 2022 campaign gave us a unique opportunity to directly compare the small, short-duration EUV jets with abundant Hα spicules around in unprecedented detail. We were puzzled as to why the entangled magnetic structure exclusively appears in the EUV wavelengths and not in the Hα line. Among the alternatives, the reconnection model [2] suggests some explanations of the twisted, or non-aligned, EUV structures, straight spicules, as well as the downward Alfven wave impacting the chromosphere as illustrated in Figure 4. The model predicts the total energy of the jet as ~2.6×10²⁶ erg from the measured footpoint separation ~2'' and magnetic flux ~10¹⁷ Mx in NIRIS magnetograms, confirming our estimate of the jet energy based on the SO EUV data.
Other small jets detected by EUI-HRI include the so-called picoflare jets [3] with kinetic energies in the picoflare range and the helical EUI-HRI jets [4] interpreted as propagating torsional Alfven waves. The former jets have kinetic energies at least two-orders of magnitude lower and the latter slightly higher than the present jet energy, but in comparable morphology and lifetime. All these jets are powered by magnetic reconnection at the smallest scales of magnetic elements detectable in the quiet Sun [5].
This detection of intricate corona-chromospheric coupling highlights the power of high-resolution imaging in unraveling the mechanisms behind small-scale solar ejections across atmospheric layers and boosts SO's objective of advancing our understanding of how the Sun generates small-scale ejections in the chromosphere and corona, ultimately enhancing models of solar-heliospheric connections.

This study has been published in J. Lee et al., "Fine Structures of Tiny Quiet Sun Jets Observed by Solar Orbiter and Big Bear Solar Observatory" ApJL 992 L23 (2025), DOI: 10.3847/2041-8213/ae0df2.

Affiliations

(1) Institute for Space Weather Sciences, New Jersey Institute of Technology, University Heights, Newark, NJ 07102, USA
(2) Center for Solar-Terrestrial Research, , New Jersey Institute of Technology, University Heights, Newark, NJ 07102, USA
(3) Big Bear Solar Observatory, New Jersey Institute of Technology, 40386 North Shore Lane, Big Bear City, CA 92314, USA
(4) Dept. of Physics, Montana State University, Bozeman, MT 59717, USA
(5) School of Space Research, Kyung Hee University, Yongin 17104, Republic of Korea
(6) SETI Institute, 339 Bernardo Avenue, Mountain View, CA 94043, USA
 

Acknowledgments
We acknowledge the use of data from the GST of BBSO. BBSO operation is supported by US NSF grant AGS-2309939 and the New Jersey Institute of Technology. This work was supported by NSF grants AGS-2114201, AGS-2229064 and AGS-2309939 and NASA grants 80NSSC19K0257, 80NSSC20K0025, 80NSSC20K1282, 80NSSC24K1914, and 80NSSC24K0258.

References

[1] Youn J., Lee H., Jeong H.-J. et al. 2025 A&A 695 A125, DOI: 10.1051/0004-6361/202452304
[2] Longcope D. and Klaassen P. 2025 ApJ 989 152, DOI: 10.3847/1538-4357/adf0ef
[3] Chitta L. P., Zhukov A. N., Berghmans D. et al. 2023 Sci 381 867, DOI: 10.1126/science.ade5801
[4] Petrova E., Van Doorsselaere T., Berghmans D. et al. 2024 A&A 687 A13, DOI: 10.1051/0004-6361/202348799
[5] Lee J., Georgoulis M. K., Sharma R. et al. 2025 ApJL 988 L16, DOI: 10.3847/2041-8213/adeb54