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

Abstract

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.

Figures (10)

• Figure 1: Observation context and timeline. (\ref{['fig:context']}) Context images and pointing. (\ref{['fig:timeline']}) SOLO[] SOLO1[] SOLO-1[] s 3[] [] the -3[] The -3[] [] autoshort autolong 's relative locations and instrument timeline.
• Figure 2: (\ref{['fig:plume_light_curve']}) Intensity evolution and plume context for Plume 1 and Plume 2. (\ref{['fig:phi_maps']}) Magnetic and intensity context of plume footpoints with flux density distributions.
• Figure 3: Sample SPICE composition raster from false 22 0 Invalid month 2 2 4 at 4 ,:, 3:3,31,09,05,32 UT. First and third columns show mean images for each of the six spectral windows (window 1 to window 6), averaged along wavelength dimension. Right to each image panel we show the corresponding average spectra per window across the spatial axis.
• Figure 4: Contribution functions (in $\mathrm{erg\,cm^{3}\,s^{-1}\,sr^{-1}}$) for the SPICE[] SPICE1[] SPICE-1[] s 3[] [] the -3[] The -3[] [] autoshort autolong composition lines used in this study, computed with CHIANTI v.11 (using the IDL package of CHIANTI) for three constant electron densities of $10^{9}$, $10^{10}$, and $10^{11}\,\mathrm{cm^{-3}}$ . Each curve is labeled with the ion, wavelength (in Å) , and $T_{\mathrm{max}}$, the peak formation temperature, in K.
• Figure 5: Sample of SPICE observations from each observation set: the first set is shown in the first two columns, and the second set in the last two columns. Each column displays, from top to bottom, the S/N FIP bias maps and radiance maps for SPICE N iii(used in the FIP bias calculation; low- TR[] TR1[] TR-1[] s 3[] [] the -3[] The -3[] [] autoshort autolong ), Ne viii(top- TR[] TR1[] TR-1[] s 3[] [] the -3[] The -3[] [] autoshort autolong ), and Mg ix(corona). The regions labeled "P1" (red in the FIP bias maps and green in the radiance maps) and "P2" (blue in the FIP bias maps and light blue in the radiance maps) indicate the locations of Plume 1 and Plume 2, respectively. These regions were selected based on visual identification of the plume footpoints in one map and propagated to the others by fixing the Carrington coordinates of the region vertices.