Combined Metis and EUI observations for streamer characterization.

(Solar Orbiter Nugget #85 by L. Abbo¹, R. Susino¹, S. Parenti², F. Auchère², V. Andretta³, D. Spadaro⁴, M. Romoli^5,6, S. Fineschi¹, R. Lionello⁷, S. Giordano¹, V. Da Deppo⁸, C. Grimani^9,10, P. Heinzel^11,12, G. Naletto¹³, G. Nicolini¹, M. Stangalini¹⁴, L. Teriaca¹⁵, M. Uslenghi¹⁶, Y. De Leo^4,17, F. Landini¹, G. Jerse¹⁸, M. Pancrazzi¹ and C. Sasso³ )

Introduction

Comprehensive observations of the solar atmosphere, ranging from the limb of the Sun out into the extended corona, are fundamental for understanding the physical connections between the Sun and the heliosphere (e.g.[1-2] and references therein). In particular, determining the electron temperature structure in corona is crucial to constrain theoretical models and characterize the specific mechanisms responsible for heating the plasma and accelerating the solar wind. 

While physical parameters in the lower corona have been extensively investigated using ultraviolet spectroscopy (e.g., [3], [4]), obtaining electron temperature measurements in the 'middle corona', specifically within the heliocentric range of 3 to 8 Solar Radii (R_☉)_,has historically posed a significant observational challenge (e.g., [5-7], and references therein). Coronal electron temperatures have primarily been inferred from remote-sensing spectroscopic data and imaging observations using various diagnostic methods; furthermore, they have often been derived from electron density profiles obtained via visible-light brightness measurements, assuming the plasma is in hydrostatic equilibrium (e.g., [8-9]).

To address this gap, this work explores the feasibility of a novel diagnostic method by exploiting the unprecedented capabilities of the Solar Orbiter mission. The study relies in fact on the synergistic analysis of multi-band observations acquired by two specific instruments: the Metis coronagraph [10],[11] and the Extreme Ultraviolet Imager (EUI) [12].

Results

We analyzed a dataset acquired on March 21, 2021, during the Solar Orbiter mission's cruise phase. The EUI instrument used its Full Sun Imager (FSI) telescope in a special "coronagraphic mode" for the first time, where an occulting disk blocked the light of the solar disk, allowing the observation of ultraviolet emission (at 17.4 nm, produced by iron ions FeIX/FeX) in the extended corona up to a heliocentric distance of about 4.5 R_☉, a region usually not observable in EUV imagers due to stray light [13]. 

Figure 1. Top: Composite of FSI 17.4 nm images obtained on March 21, 2021, on disk (at 02:46:43 UT, below 1.85 R_☉) and in coronagraphic mode (at 00:45:45 UT, between 1.85 and 4.45 R_☉), and Metis polarized-brightness (pB, left) and HI Lyman-α (right) images acquired at 03:00 UT, above 4.45 R☉. The dotted white circles mark the inner (4.05 R_☉) and outer (4.45 R_☉) limits of the overlapping portion of FSI and the Metis fields of view. The streamer regions considered in the analysis are shown by the two angular sectors delimited by the dotted lines.
Bottom-Left: Metis pB image (red) and EUI/FSI images (yellow on disk and in inner corona), enhanced with a wavelet-optimized-whitening filter (WOW; [14]) to enhance the contrast of the features. Bottom-Right: Extrapolation of the magnetic field lines with the 3D MHD model developed by Predictive Science Inc. (PSI) for the date of March 21, 2021, as seen from Solar Orbiter’s point of view.

Almost simultaneously, the Metis instrument observed the corona above 4 R_☉ in polarized visible light (pB), arising from the Thomson scattering of sunlight by free electrons in the corona, and neutral hydrogen UV Lyman-α (121.6 nm), not directly used for this analysis. The top panels of Figure 1 report composites of FSI 17.4 nm images acquired on disk and in coronagraphic mode, and Metis pB (left) and HI Lyman-α (right) images, showing the electron and neutral hydrogen plasma components of the outer corona above 4.25 R☉. In the bottom-left panel of Figure 1, in particular, a wavelet-optimized-whitening filter (WOW; [14]) has been applied to highlight coronal structures in the Metis pB image, while the bottom right panel shows the magnetic extrapolation of the field lines obtained by applying the 3D MHD MAS model developed by Predictive Science Incorporated (PSI) for the date of March 21, 2021 (see the full publication for more details).

The intensities measured by Metis and EUI/FSI at different wavelengths originate from physical processes that depend in a distinct way on electron density and temperature. Under proper assumptions, it is therefore possible to exploit co-spatial data from the two instruments to derive information and/or place constraints on the physical properties of the emitting plasma. The combined method presented in this work exploits the differing dependencies of these emissions on physical parameters: visible light is primarily a function of electron density, while ultraviolet emission depends on both density and temperature.

Figure 2, Top: Latitudinal profiles (in arbitrary units) of Metis pB (red line), of HI Lyman-α intensity (blue line) and of FSI 17.4 nm intensity (grey line) obtained by averaging the data in the overlapping region. The profiles are normalised to their maximum value (the values including the error bars are greater than 1). Bottom: Radial profiles of Metis pB (red lines), HI Lyman-α  intensity (blue lines), FSI FeΙΧ/FeΧ 17.4 nm intensity (thick grey lines), and the square root of FSI intensity (thick black lines) for the eastern streamer (left panel) and the western streamer (right panel). In order to be comparable, all profiles are normalized to their average value in the overlapping region between 4.05 and 4.45 R_☉, indicated in plots by the vertical green arrows.

Our work focused on the analysis of two prominent streamer regions located on the Sun’s streamer belt above the eastern and western limbs, specifically within the overlapping field of view of the Metis and EUI instruments. This region covers a heliocentric distance ranging from approximately 4 to 4.5 R_☉ in the considered data set, allowing for a direct comparison of the measured intensities. The top panel of Figure 2 shows the latitudinal profiles of Metis pB (red line), of HI Lyman-α intensity (blue line) and of FSI 17.4 nm intensity (grey line) obtained by averaging the data in the overlapping region. The bottom panels of Figure 2 show the radial profiles of Metis pB (red lines), of HI Lyman-α intensity (blue lines), of FSI 17.4 nm intensity (thick grey lines), and of the square root of FSI intensity (thick black lines) for the eastern streamer (left) and the western streamer (right). 

By first inverting the pB data acquired by Metis, we derived the electron density (ne) profiles for the two streamers. The resulting density values were found to be consistent with various historical models and measurements for equatorial streamers during solar minimum as shown in Figure 3. The electron density has been then used to calculate the plasma emission measure (EM) all over the field of view of Metis, by integrating the density profiles along a line of sight (l.o.s.) extending ±10 R_☉ from the plane of the sky.

Figure 3. Electron density radial profiles obtained from this analysis, compared with the streamer profile obtained from Metis first-light data (green by [15]) and with other density profiles for equatorial and midlatitude streamers reported in the literature, as indicated in the legend [16-21].

When comparing the ultraviolet emission expected in the FSI 17.4 nm passband (calculated for various electron temperatures using the EM derived from Metis data) with the actual intensity measured by the instrument, it is possible to infer two distinct temperature values for the plasma at the representative height of 4.25 R_☉. This duality arises from the specific analytical shape of the FSI response function, which allows for both a "cold" and a "hot" intersection with the observed data as shown in Figure 4a,b). For the eastern streamer, the derived temperatures were approximately 530,000 Kelvin for the cold solution and 1.4 million Kelvin for the hot solution. Similarly, the western streamer yielded a cold solution of roughly 570,000 Kelvin and a hot solution of 1.4 million Kelvin. The electron temperature values derived for the streamers region are shown in the Figure 4c). The thick coloured lines show the electron temperature profiles derived from the inversion of the electron density curves obtained from Metis data assuming hydrostatic equilibrium. The inferred values are compared with others from the literature, as reported in the plot legend [9,21-24].

Figure 4. a-b): Comparison between the count rates measured in the FSI 17.4 nm passband (horizontal grey bands) and the expected count rates (red bands) calculated from the EM in the eastern (left panel) and western (right panel) streamers (the values are shown with the corresponding uncertainties). c): Electron temperature values derived for the western (red) and eastern (orange) streamers region (at the representative height of 4.25 R_☉), obtained with the method described in this work (open circles: cold solutions; filled circles: hot solutions, see the paper for details). The thick coloured lines show the electron temperature profiles derived from the inversion of the ne curves obtained from Metis data assuming hydrostatic equilibrium. The inferred values are compared with others from the literature, as reported in the plot legend [9,21-24]. In particular, we show T_e profiles for equatorial (E), mid-latitude (ML) streamers, and the streamer and/or coronal-hole boundary (S/CH). Profiles with H in the legend were derived from the corresponding density profiles assuming hydrostatic equilibrium.

A notable finding from the analysis is that the derived temperature values remain relatively constant across the width of the streamers, suggesting uniform thermal conditions within the denser plasma of these structures. In particular, the "hot" solution of 1.4 million Kelvin appears particularly significant. This value aligns well with previous observations, such as those from the UVCS/SoHO instrument and recent eclipse data, which indicate that the quiescent corona remains almost isothermal, maintaining temperatures around 1.4 to 1.6 million Kelvin extending out to distances of 4 R_☉ (see the full publication for details).

Conclusions

This work highlights how the combined use of Metis and EUI data is a promising approach for probing the physical conditions of the middle corona, a region otherwise difficult to explore.  It is worth noting that the method described in this work relies on certain physical assumptions. The first and more critical one is the ionization equilibrium which implies that atoms have time to adjust to local conditions. Although calculations show that the plasma is at the limit of this equilibrium at the explored heights, the approximation is considered reasonable for providing a valid estimate. In the case of non-ionization equilibrium, the results obtained in this analysis can be considered as an upper limit of the local electron temperature. The other underlying assumption in the method is that of isothermal plasma along the line of sight. However, since the focus of the study regards the core of two bright equatorial streamers, it is likely that the bulk of the emission, both in the VL and EUV, is coming from the denser plasma confined in the portion of the l.o.s. crossing the streamer.

We also note that there are more recent datasets of combined observations that also include the He II 304 narrow-band channel of FSI. The analysis of these observations in three bands (Metis VL and FSI 174, 304) could improve the application of the method helping to distinguish between the hot and cold temperature solutions.

This nugget is based on the following work: Abbo et al., A&A, 702, A254, (2025) doi: 10.1051/0004-6361/202347599

Affiliations

(1) INAF – Astrophysical Observatory of Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
(2) Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, F-91405, Orsay, France
(3) INAF – Astronomical Observatory of Capodimonte, Salita Moiariello 16, I-80131 Napoli, Italy
(4) INAF – Astrophysical Observatory of Catania, Via Santa Sofia 78, I-95123 Catania, Italy
(5) University of Firenze, Department of Physics and Astronomy, Via Giovanni Sansone 1, I-50019 Sesto Fiorentino, Italy
(6) INAF – Arcetri Astrophysical Observatory, Largo Enrico Fermi 5, 50125 Florence, Italy
(7) Predictive Science Inc., San Diego, CA 92121, USA
(8) CNR, Institute for Photonics and Nanotechnologies, Via Trasea 7, I-35131 Padova, Italy
(9) University of Urbino Carlo Bo, Department of Pure and Applied Sciences, Via Santa Chiara 27, I-61029 Urbino, Italy
(10) INFN, Section in Florence, Via Bruno Rossi 1, I-50019 Sesto Fiorentino, Italy
(11) Czech Academy of Sciences, Astronomical Institute, Friˇcova 298, CZ-25165 Ondˇrejov, Czech Republic
(12) University of Wroclaw, Centre of Scientific Excellence Solar and Stellar Activity, ul. Kopernika 11, PL-51-622 Wrocław, Poland
(13) University of Padova, Department of Physics and Astronomy, Via Francesco Marzolo 8, I-35131 Padova, Italy
(14) ASI, Via del Politecnico snc, I-00133 Roma, Italy
(15) Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, D-37077 Gottingen, Germany
(16) INAF, Institute of Space Astrophysics and Cosmic Physics of Milan, Via Alfonso Corti 12, I-20133 Milano, Italy
(17) Institute of Physics, University of Graz, Universitätsplatz 5, 8010, Graz, Austria
(18) INAF – Astronomical Observatory of Trieste, Località Basovizza 302, I-34149 Trieste, Italy

Acknowledgements

Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. Metis was built and operated with funding from the Italian Space Agency (ASI), under contracts to the National Institute of Astrophysics (INAF) and industrial partners. Metis was built with hardware contributions from Germany (Bundesministerium für Wirtschaft und Energie through DLR), from the Czech Republic (PRODEX) and from ESA. Metis team thanks the former PI, Ester Antonucci, for leading the development of Metis until the final delivery to ESA. Moreover, the authors thanks EA for useful comments and suggestions to improve the discussion of the results. The EUI instrument was built by CSL, IAS, MPS, MSSL/UCL, PMOD/WRC, ROB, LCF/IO with funding from the Belgian Federal Science Policy Office (BELSPO/PRODEX PEA C4000134088); 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 Office (SSO). CHIANTI is a collaborative project involving George Mason University, the University of Michigan (USA), University of Cambridge (UK) and NASA Goddard Space Flight Center (USA). P.H. was supported by the Czech Funding Agency with grant No. 22-34841S and by the program “Excellence Initiative – Research University” for years 2020-2026 at University of Wrocław, project no. BPIDUB.4610.96.2021.KG.

References

[1] Parenti, S., Réville, V., Brun, A. S., et al. 2022, ApJ, 929, 75, https://doi.org/10.3847/1538-4357/ac56da 
[2] West, M. J., Seaton, D. B., Wexler, D. B., et al. 2023, Sol. Phys., 298, 78, https://doi.org/10.1007/s11207-023-02170-1 
[3] Wilhelm, K., Dwivedi, B. N., Marsch, E., & Feldman, U. 2004, Space Sci. Rev.,111, 415, https://doi.org/10.1023/B:SPAC.0000032695.27525.54 
[4] Wilhelm, K., Marsch, E., Dwivedi, B. N., & Feldman, U. 2007, Space Sci. Rev., 133, 103, https://doi.org/10.1007/s11214-007-9285-0 
[5] Kohl, J. L., Noci, G., Cranmer, S. R., & Raymond, J. C. 2006, The Astronomy and Astrophysics Review, 13, 31, https://doi.org/10.1007/s00159-005-0026-7 
[6] Del Zanna, G. & Mason, H. E. 2018, Living Reviews in Solar Physics, 15, 5, https://doi.org/10.1007/s41116-018-0015-3 
[7] Parenti, S., Chifu, I., Del Zanna, G., et al. 2021, Space Sci. Rev., 217, 78, https://doi.org/10.1007/s11214-021-00856-1 
[8] Munro, R. H. & Jackson, B. V. 1977, ApJ, 213, 874, https://doi.org/10.1086/155220
[9] Gibson, S. E., Fludra, A., Bagenal, F., et al. 1999, J. Geophys. Res., 104, 9691, https://doi.org/10.1029/98JA02681
[10] Antonucci, E., Romoli, M., Andretta, V., et al. 2020, A&A, 642, A10, https://doi.org/10.1051/0004-6361/201935338 
[11] Fineschi, S., Naletto, G., Romoli, M., et al. 2020, Experimental Astronomy, 49, 239, https://doi.org/10.1007/s10686-020-09662-z 
[12] Rochus, P., Auchère, F., Berghmans, D., et al. 2020, A&A, 642, A8, https://doi.org/10.1051/0004-6361/201936663
[13] Auchère, F., Berghmans, D., Dumesnil, C., et al. 2023a, A&A, 674, A127, https://doi.org/10.1051/0004-6361/202346039 
[14] Auchère, F., Soubrié, E., Pelouze, G., & Buchlin, É. 2023b, A&A, 670, A66 https://doi.org/10.1051/0004-6361/202245345 
[15] Romoli, M., Antonucci, E., Andretta, V., et al. 2021, A&A, 656, A32, https://doi.org/10.1051/0004-6361/202140980 
[16] Saito, K., Poland, A. I., & Munro, R. H. 1977, Sol. Phys., 55, 121, https://doi.org/10.1007/BF00150879
[17] Withbroe, G. L. 1988, ApJ, 325, 442, https://doi.org/10.1086/166015
[18] Guhathakurta, M., Fludra, A., Gibson, S. E., Biesecker, D., & Fisher, R. 1999, J. Geophys. Res., 104, 9801, https://doi.org/10.1029/1998JA900082
[19] Hayes, A. P., Vourlidas, A., & Howard, R. A. 2001, ApJ, 548, 1081, https://doi.org/10.1086/319029
[20] Frazin, R. A., Cranmer, S. R., & Kohl, J. L. 2003, ApJ, 597, 1145, https://doi.org/10.1086/378558
[21] Vásquez, A. M., van Ballegooijen, A. A., & Raymond, J. C. 2003, ApJ, 598, 1361, https://doi.org/10.1086/379008
[22] Gopalswamy, N., Newmark, J., Yashiro, S., et al. 2021, Sol. Phys., 296, 15, https://doi.org/10.1007/s11207-020-01751-8 
[23] Spadaro, D., Susino, R., Ventura, R., Vourlidas, A., & Landi, E. 2007, A&A, 475, 707, https://doi.org/10.1051/0004-6361:20077873
[24] Susino, R., Ventura, R., Spadaro, D., Vourlidas, A., & Landi, E. 2008, A&A, 488, 303, https://doi.org/10.1051/0004-6361:200809713