Science Nugget: First coordinated observations between Solar Orbiter and the Daniel K. Inouye Solar Telescope - Solar Orbiter

First coordinated observations between Solar Orbiter and the Daniel K. Inouye Solar Telescope

(Solar Orbiter Nugget #73 by Krzysztof Barczynski^1,2, Miho Janvier³, Chris J. Nelson³, Thomas Schad⁴, Alexandra Tritschler⁵, Louise Harra^2,1, Daniel Müller³, Susanna Parenti⁶, Gherardo Valori⁷, Gianna Cauzzi⁵, and Yingjie Zhu^1,2)

Introduction

The Solar Orbiter mission [1] and US National Science Foundation Daniel K. Inouye Solar Telescope (DKIST) [2] are two of the most advanced observatories available to the solar physics community. Solar Orbiter was launched in February 2020 and follows a unique orbit that brings it as close as 0.29 astronomical units (au) from the Sun, allowing it to capture the highest-resolution images of the solar corona ever obtained.
DKIST, which began operations in 2021, is the world’s largest optical ground-based solar telescope equipped with a 4-meter primary mirror. DKIST provides detailed views of the lower solar atmosphere.
Together, these two high-resolution observatories mark a new era in solar observation. By capturing the Sun from two distinct vantage points, they offer stereoscopic views of the solar atmosphere, enable unprecedented high-resolution and for the first time stereoscopic magnetic field measurements, and provide new insights into the structure and dynamics of the corona.
The first coordinated observation campaign between Solar Orbiter and DKIST took place in October 2022. In [3], we explore three key research questions that demonstrate how joint observations from these two observatories can enhance our understanding of solar phenomena:

Q1: What are the properties of coronal loops when observed with very high spatial resolution from multiple viewing angles?
Q2: What role do small-scale brightenings, from the photosphere to the corona, play in the evolution of active regions?
Q3: What are the characteristics and origins of coronal rain, based on high-resolution, multi-perspective observations?

Coordinated Observations by Solar Orbiter and DKIST

The coordinated observation campaign focused on a solar active region and was conducted between 18 and 24 October 2022 (Figure 1). During this time, Solar Orbiter moved from a distance of 0.32 au to 0.39 au from the Sun and changed its viewing angle relative to Earth from 77° to 51°, providing stereoscopic observations with DKIST. Solar Orbiter observed the Sun with high-resolution remote-sensing instruments: the Extreme Ultraviolet Imager (EUI)[4], Polarimetric and Helioseismic Imager (PHI)[5] and Spectral Imaging of the Coronal Environment (SPICE) instrument [6][7]. EUI and PHI provided high-resolution imaging and magnetic field data, with a spatial scale between 120–140 km per pixel. SPICE observes the Sun with a temperature coverage corresponding to chromospheric and transition region structures.
DKIST observed the same active region using the Cryogenic Near-Infrared Spectropolarimeter (CryoNIRSP)[8], Visible Broadband Imager (VBI)[9] and Visible Spectropolarimeter (ViSP)[10]. From 18 to 20 October, DKIST's CryoNIRSP captured full-Stokes spectropolarimetric data (Fe XIII) and context observation (He I), both focused on the corona at the solar limb. As the active region changed its position due to solar rotation and moved toward the disk centre for an Earth observer, VBI and ViSP conducted high-resolution imaging and spectroscopy observation on 21 and 24 October 2022. VBI focused on both the photosphere (TiO and G-band) and the chromosphere (H-alpha, H-beta, Ca II K), while ViSP captured full-Stokes’s vector magnetograms of the photosphere.
In our coordinated observation campaign, other spacecraft observing the Sun e.g., Interface Region Imaging Spectrograph (IRIS) and Hinode also observed the same targets.

Figure 1: The active region (highlighted by the white arrow) selected by the coordinated observations as visible from Solar Orbiter, seen in the EUI/FSI 174 Å filter (left column), and as visible from Earth, as seen by SDO/AIA 174 Å (right column). The top row shows a plot of the Sun on the first day (18 October) of coordination, whilst the bottom row shows a plot of the Sun on the last day (24 October) of coordinated observations. The dashed line highlights the position of the limb as seen from a perspective of the coordinated instrument. The overplotted latitude and longitude grids are in heliographic Stonyhurst coordinates.

New Scientific Opportunities from Coordinated Observations

The campaign demonstrated the potential of coordinated observations between Solar Orbiter and DKIST. To demonstrate these new possibilities, our paper focused on three case studies: coronal loops, small-scale brightenings, and coronal rain (Figure 2).

Figure 2. Overview of the coordinated observations provided by Solar Orbiter and DKIST on 24 October 2022. The presented observations are used to study coronal loops physics and small-scale brightenings. (a) Position of Solar Orbiter with respect to Earth. (b) Sun as observed from the vantage point of Solar Orbiter. The coloured boxes again outline the fields of view of the coordinating instruments. (c) Sun observed with SDO/AIA, the same view as from DKIST, with the fields of view of the Solar Orbiter instruments overlaid. (d) HRI image showing the coronal emission with a high-resolution. The overlaid boxes outline the fields of view of the DKIST instruments: red is for the VBI-Red FOV, blue is for the VBI-Blue FOV, and black is for the ViSP raster FOV. (e) SPICE raster map showing the transition region emission peak intensity of the Ne VIII spectral line. (f) PHI magnetograms showing the magnetic field in range between -100 and 100 G. (g) DKIST VBI-Red band image showing the photospheric granulation in the TiO spectral line. (h) DKIST VBI-Blue band image showing the chromosphere in CaII K spectral line. (i) DKIST ViSP data showing the photospheric granulation observed in the continuum. (j) Corresponding absolute magnetic field strength.

Coronal loops

Coronal loops are arch-like magnetic structures filled with hot, dense plasma. The loop-substructures and dynamics raise an open question about properties of coronal loops (Q1). The coronal loops can also interact with small-scale features (Q2) and play a role in the formation of coronal rain (Q3).
From 18 to 20 October 2022, Solar Orbiter and DKIST observed coronal loops from complementary perspectives: Solar Orbiter viewing the disk and DKIST observing the limb. DKIST's (CryoNIRSP) offered unprecedented insight into loop dynamics and magnetic topology. Solar Orbiter’s EUI/HRI revealed complex loop topologies with high-resolution, while SPICE detected Doppler-shifted features (20–200 km/s), indicating rotational motion at loop centres [11]. Other SPICE analysis of First Ionization Potential (FIP) revealed spatially-dependent FIP bias enhancements, suggesting differences between closed and open magnetic structures [12].
The stereoscopic and magnetic data from both observatories open a new era for 3D reconstruction of coronal loops and magnetic field measurements with reduced ambiguity.

Small-scale brightenings

Small-scale brightenings (<1 Mm) are frequently observed across the solar atmosphere. Solar Orbiter’s EUI discovered numerous extreme ultraviolet (EUV) brightenings, also known as "campfires" [13]. Other similar small-scale structures e.g., miniature hot loops were known before in a plage region [14]. We observed numerous small-scale brightenings within the active region on 21 and 24 October 2022.
DKIST’s VBI and ViSP provided complementary observations of the lower atmosphere to determine whether these brightenings had chromospheric or photospheric signatures. ViSP and PHI magnetograms enabled stereoscopic analysis of the associated magnetic fields.
These observations are critical to understanding the role of small-scale events in active region evolution (Q2), and to investigating their contribution to coronal heating and energy transport.

Coronal Rain

Coronal rain consists of dense, cool plasma condensations (10³–10⁵ K) that form in the corona due to thermal instability and fall back to the solar surface along magnetic field lines. The high spatial-temporal data from Solar Orbiter allows the substructure of the solar coronal rain to be investigated [15].
On 19 October 2022, Solar Orbiter observed coronal rain events using EUI data (Figure 3). These observations provided the first views of coronal rain evolution with 5-second cadence and high spatial resolution (238 km), along with a stereoscopic perspective using DKIST and Solar Dynamics Observatory (SDO).
The underlying magnetic structures were studied using PHI and HMI, while DKIST's CryoNIRSP captured the loop system associated with the rain which was observed at the solar limb.
These coordinated observations improve our understanding of the origin, dynamics, and magnetic environment of coronal rain (Q3).

Figure 3. Coronal loops and coronal rain observed from two vantage points separated by 72°with Solar Orbiter/HRI and SDO/AIA. The upper row shows an observation obtained on 19 October 2022 at 19:00, and the bottom row shows an observation obtained 30 minutes later. Arrows highlight places where the coronal rains were observed.

Conclusions

We successfully coordinated and obtained a dataset from the first coordinated observations between DKIST and Solar Orbiter, conducted between 18 and 24 October 2022, with additional support from IRIS and Hinode. The synergy between these instruments enabled multi-perspective, high-resolution studies of the solar atmosphere—from the photosphere to the corona.
We demonstrated the scientific potential of this approach by presenting three case studies: coronal loops and their evolution, small-scale EUV brightenings and their role in active region dynamics, and coronal rain and its formation processes.
A key innovation of this campaign was the combination of the high-resolution imaging, spectroscopy observation and magnetic field measurements, allowing for 3D reconstructions of solar structures and unambiguous magnetic field analysis.
All data from this campaign are publicly available and are currently being analysed by the ISSI Team on Active Region Evolution: https://teams.issibern.ch/activeregionevolution/

Affiliations

1. ETH-Zürich, Hönggerberg campus, HIT building, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland  

2. PMOD/WRC, Dorfstrasse 33, CH-7260 Davos Dorf, Switzerland

3. European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk,  The Netherlands

4. National Solar Observatory, 22Ōhia Kū Street, Pukalani, HI 96768, USA

5. National Solar Observatory, 3665 Discovery Drive, Boulder, CO 80303, USA

6. Université Paris–Saclay, CNRS, Institut d’astrophysique spatiale, 91405 Orsay, France

7. Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

References

[1] Rimmele, T. R., Warner, M., Keil, S. L., et al. 2020, Sol. Phys., 295, 172, https://doi.org/10.1007/s11207-020-01736-7
[2] Müller, D., St. Cyr, O. C., Zouganelis, I., et al. 2020, A&A, 642, A1, https://doi.org/10.1051/0004-6361/202038467
[3] Barczynski K., Janvier M., Nelson C.J., et al., 2025, A&A, 701, A77, https://doi.org/10.1051/0004-6361/202554396
[4] Rochus, P., Auchère, F., Berghmans, D., et al. 2020, A&A, 642, A8, https://doi.org/10.1051/0004-6361/201936663
[5] Solanki, S. K., del Toro Iniesta, J. C., Woch, J., et al. 2020, A&A, 642, A11, https://doi.org/10.1051/0004-6361/201935325
[6] SPICE Consortium, Anderson, M., Appourchaux, T., et al. 2020, A&A, 642, A14, https://doi.org/10.1051/0004-6361/201935574
[7] Fludra, A., Caldwell, M., Giunta, A., et al. 2021, A&A, 656, A38, https://doi.org/10.1051/0004-6361/202141221
[8] Fehlmann, A., Kuhn, J. R., Schad, T. A., et al. 2023, Sol. Phys., 298, 5, https://doi.org/10.1007/s11207-022-02098-y
[9] Wöger, F., Rimmele, T., Ferayorni, A., et al. 2021, Sol. Phys., 296, 145, https://doi.org/10.1007/s11207-021-01881-7
[10] de Wijn, A. G., Casini, R., Carlile, A., et al. 2022, Sol. Phys., 297, 22, https://doi.org/10.1007/s11207-022-01954-1
[11] Petrova, E., Van Doorsselaere, T., Berghmans, D., et al. 2024, A&A, 687, A13, https://doi.org/10.1051/0004-6361/202348799
[12] Mzerguat et al. 2025, private communication
[13] Berghmans, D., Auchère, F., Long, D. M., et al. 2021, A&A, 656, L4, https://10.1051/0004-6361/202140380
[14] Barczynski, K., Peter, H., & Savage, S. L. 2017, A&A, 599, A137, https://10.1051/0004-6361/201629247
[15] Antolin, P., Dolliou, A., Auchère, F., et al. 2023, A&A, 676, A112, https://10.1051/0004-6361/202346016

Nuggets archive