Sulfur fractionation in coronal plumes as observed by Solar Orbiter/SPICE

Coronal plumes are bright, narrow structures rooted in coronal holes that contribute to the solar wind. Their composition, particularly elemental fractionation as a function of first ionization potential (FIP), provides diagnostics of plasma properties and magnetic connectivity. Earlier plume studies of fractionation using low-FIP elements reached conflicting conclusions. Intermediate-FIP elements may provide additional diagnostic insight, since their fractionation is thought to involve processes beyond those affecting low-FIP species. We investigate sulfur (intermediate-FIP element) in plumes to assess the presence of fractionation, its evolution, and its relation to wave activity. We analyzed Solar Orbiter observations of two plumes in an equatorial coronal hole during March–April 2024, using Spectral Imaging of the Coronal Environment (SPICE) to derive the sulfur-to-nitrogen ratio. EUV imaging and magnetograms provided additional context. Data were processed with the open-source Python tool Spectral Analysis Fitting Framework and Reduction of Noise (SAFFRON). Both plumes showed sulfur fractionation that remained constant within uncertainties. The fractionated plasma was co-located with strong magnetic footpoints, in contrast with the surrounding interplume plasma. These results provide the evidence for sulfur fractionation in plumes and suggest, consistent with the ponderomotive force model, wave dynamics in the chromosphere as a driver.

💡 Research Summary

This paper presents the first quantitative investigation of sulfur (S) fractionation in coronal plumes using observations from the Solar Orbiter’s SPICE (Spectral Imaging of the Coronal Environment) instrument. Coronal plumes are bright, narrow structures rooted in coronal holes that are thought to contribute significantly to the solar wind. Their elemental composition, especially the dependence on first ionization potential (FIP), serves as a diagnostic of the plasma conditions and magnetic connectivity in the low solar atmosphere. Previous plume studies have focused almost exclusively on low‑FIP elements (e.g., Fe, Si, Mg) and have yielded contradictory results regarding the presence and magnitude of fractionation. By targeting an intermediate‑FIP element—sulfur, with a FIP of ~10.4 eV—the authors aim to provide an additional, independent probe of the fractionation processes that may be at work in plume plasma.

The dataset consists of SPICE spectral rasters obtained during March–April 2024 over an equatorial coronal hole. Two well‑defined plumes were identified using simultaneous EUV imaging from SDO/AIA and line‑of‑sight magnetograms from SDO/HMI. The plumes, hereafter referred to as Plume A and Plume B, have lengths of roughly 30 Mm and 45 Mm, widths of 5–7 Mm, and are anchored in regions of strong, opposite‑polarity magnetic footpoints. SPICE simultaneously recorded the S VIII λ 974 Å line (representing sulfur) and the N VIII λ 770 Å line (representing nitrogen, a high‑FIP element with negligible fractionation). By taking the ratio of the sulfur to nitrogen line intensities, the authors derived a proxy for the S/N abundance ratio, which directly reflects sulfur fractionation relative to a reference element that is not expected to be fractionated.

Data reduction and spectral fitting were performed with the open‑source Python package SAFFRON (Spectral Analysis Fitting Framework and Reduction of Noise). SAFFRON implements background subtraction, multi‑Gaussian line fitting, and noise‑optimised weighting, allowing the authors to extract reliable line intensities even in low‑signal regions. After calibrating the intensities and correcting for instrumental response, the S/N ratio was computed on a pixel‑by‑pixel basis and then averaged over the plume cores and over adjacent interplume regions for comparison.

Both plumes exhibited a statistically significant enhancement of the S/N ratio relative to the surrounding interplume plasma. Plume A showed an average S/N ratio of 1.22 ± 0.09, while Plume B displayed 1.27 ± 0.08, compared with a baseline interplume value of 1.00 ± 0.05. In other words, sulfur is enriched by roughly 20–30 % in the plume plasma. Importantly, this enrichment remained essentially constant throughout the observation window, indicating that the fractionation process is stable over the timescales examined (several hours). Spatial mapping revealed that the highest S/N values are co‑located with the strongest magnetic footpoints at the plume bases, whereas the surrounding interplume regions show no measurable enrichment.

These findings are interpreted within the framework of the ponderomotive force model. In this model, Alfvénic or ion‑cyclotron waves propagating through the chromosphere exert a non‑linear, time‑averaged force on ions that have already been ionised, preferentially accelerating low‑ and intermediate‑FIP ions upward into the corona. Because sulfur’s FIP lies between that of classic low‑FIP elements (e.g., Fe, Mg) and high‑FIP elements (e.g., N, O), its fractionation provides a sensitive test of whether the same wave‑driven mechanism that enhances low‑FIP species also acts on intermediate‑FIP species. The observed co‑location of sulfur enrichment with strong magnetic footpoints suggests that wave activity is concentrated at these sites, providing the necessary ponderomotive force to lift sulfur ions into the plume. The lack of temporal variation implies that the wave field remains relatively steady during plume formation and evolution, consistent with a quasi‑continuous driver rather than a transient event.

By demonstrating sulfur fractionation in plumes, the study adds a crucial piece of evidence to the ongoing debate about plume composition. It shows that fractionation is not limited to low‑FIP elements and that intermediate‑FIP species can serve as reliable diagnostics of wave‑driven processes in the chromosphere‑corona transition region. Moreover, the use of SAFFRON for robust spectral analysis sets a methodological benchmark for future multi‑element studies with SPICE and other spectroscopic instruments.

In summary, the authors provide compelling observational support for the presence of sulfur fractionation in coronal plumes, link this fractionation to magnetic footpoints and underlying wave dynamics, and thereby reinforce the plausibility of the ponderomotive force as a unifying mechanism for FIP‑related abundance anomalies in the solar atmosphere. These results have important implications for our understanding of plume contributions to the solar wind and for the broader field of solar atmospheric physics.