Composition mosaics from March 2022

(Solar Orbiter Nugget #34 by T. Varesano^1,2,3, D. M. Hassler², N. Zambrana Prado⁴, J. Plowman², G. Del Zanna⁵, et al.)

Introduction

The heliosphere, composed of plasma, radiation, dust, and energetic particles, is influenced by the Sun's magnetic activity. Understanding this dynamic medium is essential for grasping its effects on immersed bodies. The Solar Orbiter (SO) mission [1, 2] with its ten remote sensing and in-situ instruments, aims to investigate the solar wind's origin and acceleration. SO will provide unprecedented images of the Sun and insights into its polar regions.  

Among its instruments, the Spectral Imaging of the Coronal Environment (SPICE, [3, 4]) spectrometer captures extreme ultraviolet spectra, offering temperature coverage from the chromosphere to the corona. SPICE data can be compared with in-situ measurements from the Solar Wind Analyser [5] (SWA), aiding in understanding solar processes and heliospheric plasma phenomena. Despite known origins of the fast solar wind, the slow solar wind's formation mechanisms remain unclear. SPICE's data will help study the First Ionization Potential (FIP) effect [6], which is the enhancing of low-FIP elements (ionization energy under 10eV) in the corona. We focus our analysis on SPICE's first composition mosaic, taken on March 2, 2022 from 00:41 to 23:30 UTC while SO was very close to its first perihelion, at around a heliocentric distance of 0.54 AU. SPICE caught two active regions (AR 12957 and AR 12958, see Figure 1 with the HMI and AIA data) in its field of view. We provide details on data and results, FIP-bias measurements, and future observation plans, building on previous methodologies for interpreting SPICE data. 

Figure 1: Active Regions 12958 (a) and 12957 (b) observed on March 2nd, 2022 at 20:24 UT by SDO/AIA imager at 171 Å (representing the corona at 0.6MK), lower panel and corresponding SDO/HMI images observed on March 2nd, 2022 at 18:58 UT, upper panel. White/black areas are of positive/negative polarity.

SPICE radiance maps

We extracted radiance maps from the spectra provided by SPICE, which are the first steps to any of our diagnostics. Two wavelength ranges are observed with SPICE: 703 to 790 Å with the SW (short wavelength) detector, and 973 to 1049 Å with the LW (long wavelength) detector. Thanks to SPICE’s large temperature coverage (from 10k to 1M Kelvin), processes from the solar surface to the low corona can be observed. SPICE has a good coverage of the chromospheric region, including lines like O III 703 Å, N III 991 Å, transition region (N IV 765 Å, O VI 1032 Å) and up to the low corona (Ne VIII 770 Å, Mg IX 706 Å). 

The radiance maps showed good agreement with higher resolution images from the Extreme Ultra-Violet (EUI) instrument, even in quieter solar regions and coronal holes (see Figure 2).

Figure 2: Radiance maps superimposed on EUI/FSI images

Linear Combination Ratio method applied to SPICE data

After checking the accuracy of these radiance maps, we move on to plasma diagnostics. We use the First Ionization Potential (FIP) bias as a measure of whether the plasma shows a coronal or a photospheric composition. In order to measure the FIP bias (ratio of coronal to photospheric abundance), we use the Linear Combination Ratio (LCR) [7]. This method consists of using an optimized linear combination of spectroscopic lines of both low-FIP and high-FIP elements so that the ratio of the corresponding radiances yields the relative FIP bias. In this study, we used the sulfur (S) element as our low-FIP and nitrogen (N) as our high-FIP element. 

This allows us to investigate the mechanisms responsible for the formation and acceleration of the solar wind. From the in-situ data, a proxy of the FIP (Fe/O abundance) and the ionization level and electron temperature of the solar wind (using the O6+/O7+ ratio) can be retrieved [8].  That way, we can trace back the fractionation and the type of solar wind observed both at the surface and at the spacecraft. 

Results

We found higher FIP bias values at the footpoints of the loops, indicating that fractionation occurs – even at lower heights below the corona and in the transition region – using sulfur and nitrogen diagnostics. The behavior of the element sulfur is especially interesting because it lies in the “intermediate-FIP” category, meaning that it does not always behave as a high-FIP nor as a low-FIP element. Rather, its behavior depends on the geometry of the magnetic field [9] . From our observations, sulfur behaves as a high-FIP element in closed magnetic loops, but as a low-FIP in the solar wind (open field). 

Conclusion

Future work aims to compare in-situ and remote-sensing data to understand changes in elemental abundances, velocity, and ionization levels. Sulfur, behaving differently in closed magnetic loops versus the solar wind, may be key to identifying open fields and therefore sources of the solar wind. Indeed, as pointed out in [9] and in [10], intermediate-FIP elements as sulfur behave as high-FIP elements in closed magnetic loops, but behave as low-FIP elements in the solar wind. Moreover, sulfur fractionates only in the low chromosphere where non resonant waves on open field lines occur. Resonant Alfvèn waves can produce sufficient fractionation only in the top part of the chromosphere, since they have most of their energy trapped at higher altitudes, on top of the loops. In these conditions, the fractionation will not be significant, which is what we observe in our S/O ratio. 

The interpretation of the behavior of sulfur in remote-sensing observations might be more complex and has to be carefully considered, but will be a crucial variable to understand the solar wind mechanisms. 

Combining in-situ and different EUV spectroscopy data will be crucial for an enhance of the understanding of solar atmospheric dynamics for identifying solar wind sources.  

This article has been published with DOI: 10.1051/0004-6361/202347637   

Acknowledgements

This work at SwRI for Solar Orbiter SPICE is supported by NASA under GSFC subcontract #80GSFC20C0053 to Southwest Research Institute. The development of the SPICE instrument has been funded by ESA member states and ESA (contract no. SOL.S.ASTR.CON. 00070). The work at GSFC is supported by NASA funding for Solar Orbiter SPICE, and N. Zambrana-Prado is supported by cooperative agreement 80NSSC21M0180. The German contribution to SPICE is funded by the Bundesministerium für Wirtschaft und Technologie through the Deutsches Zentrum für Luft und Raumfahrt e.V. (DLR), grants no. 50 OT 1001/1201/1901. The Swiss hardware contribution was funded through PRODEX by the Swiss Space Office (SSO). The UK hardware contribution was funded by the UK Space Agency. S.L. Yardley would like to thank STFC via the consolidated grant (STFC ST/V000497/1). G. Del Zanna acknowledges support from STFC (UK) via the consolidated grant to the atomic astrophysics group at DAMTP, University of Cambridge (ST/T000481/1). Python modules used for this work include numpy, matplotlib, astropy, sunpy, scipy, sospice, EMToolKiT and cblind. 

Affiliations

1 Institut National Polytechnique de Grenoble, 38000 Grenoble, France 
2 Southwest Research Institute, Boulder, CO 80302, USA 
3 Department of Aerospace Engineering Sciences, University of Colorado Boulder, Boulder, CO, USA 
4 NASA Goddard Space Flight Center, Greenbelt, MD, USA 
5 DAMTP, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA, UK 
 

References 

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[7] Zambrana-Prado, N., & Buchlin, E.2019, A&A, 632, A20 https://www.aanda.org/articles/aa/pdf/2019/12/aa34735-18.pdf 

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