Intriguing Plasma Composition Pattern in a Solar Active Region: A Result of Nonresonant Alfvén Waves?

Abstract

The plasma composition of the solar corona is different from that of the solar photosphere. Elements that have a low first ionization potential (FIP) are preferentially transported to the corona and therefore show enhanced abundances in the corona compared to the photosphere. The level of enhancement is measured using the FIP bias parameter. In this work, we use data from the EUV Imaging Spectrometer on Hinode to study the plasma composition in an active region following an episode of significant new flux emergence into the preexisting magnetic environment of the active region. We use two FIP bias diagnostics: Si x 258.375 Å/S x 264.233 Å (temperature of approximately 1.5 MK) and Ca xiv 193.874 Å/Ar xiv 194.396 Å (temperature of approximately 4 MK). We observe slightly higher FIP bias values with the Ca/Ar diagnostic than Si/S in the newly emerging loops, and this pattern is much stronger in the preexisting loops (those that had been formed before the flux emergence). This result can be interpreted in the context of the ponderomotive force model, which proposes that the plasma fractionation is generally driven by Alfvén waves. Model simulations predict this difference between diagnostics using simple assumptions about the wave properties, particularly that the fractionation is driven by resonant/nonresonant waves in the emerging/preexisting loops. We propose that this results in the different fractionation patterns observed in these two sets of loops.

Identifying Plasma Fractionation Processes in the Chromosphere Using IRIS

“…As noted by Mihailescu et al (2023), resonant waves fractionating plasma in the upper chromosphere should produce comparable FIP bias enhancements using both the Si X/S X and Ca XIV/Ar XIV diagnostics, whereas nonresonant waves fractionating plasma in the lower chromosphere should produce significantly higher Ca XIV/Ar XIV values compared to Si X/ S X. The FIP bias diagnostic values presented here are consistent with nonresonant waves fractionating plasma in the following polarity region and resonant waves fractionating the plasma in the leading polarity and emerging flux regions.…”

“…Very little other work has been done on identifying signatures of fractionation in IRIS observations, with most work focusing on coronal-(e.g., Baker et al 2015;Brooks et al 2015) or ground-based chromospheric (e.g., Stangalini et al 2021;Murabito et al 2024) observations of FIP bias evolution or associated Alfvén waves. In spite of this, there has been some recent work updating the ponderomotive force model to try and explain observations of differing FIP bias values in different loop populations within the same active region (Mihailescu et al 2023). In this case, the suggestion is that fractionation by the ponderomotive force is being driven at different heights in the chromosphere by resonant or nonresonant waves, with resonant waves acting near the top of the chromosphere, producing a milder fractionation signature, while nonresonant waves act lower in the chromosphere and produce a much stronger fractionation signature.…”

“…The fractionation process should occur in the chromosphere, with Mihailescu et al (2023) suggesting that the ponderomotive force proposed by Laming (2004Laming ( , 2009Laming ( , 2015 as the driver of plasma fractionation should be induced in the upper or lower chromosphere if driven by resonant or nonresonant waves, respectively. Despite a thorough analysis of IRIS observations of this region, no clear signature of this process could be identified here.…”

“…Despite being a static model, the ponderomotive force model has also been used by To et al (2021) to explain differences in the FIP bias measured using two different composition diagnostics in a small solar flare. The waves, which can induce the ponderomotive force, have also been observed in the chromosphere using spectropolarimetric data from the Interferometric Bidimensional Spectrometer instrument and related to coronal FIP bias measurements (Baker et al 2021;Stangalini et al 2021), while both Mihailescu et al (2023) and Murabito et al (2024) have noted a relationship between resonant waves and increased FIP bias via the ponderomotive force. Detections of the IFIP effect in the solar wind also seem to be consistent with the ponderomotive force model of abundance variations, due to chromospheric fastmode waves (Brooks et al 2022).…”

“…The turbulence velocity derived from the IRIS 2 inversions has previously been used by Testa et al (2023) to probe the fractionation process and relationship with FIP bias in outflow regions. Mihailescu et al (2023) have suggested that the ponderomotive force could be acting in the mid or upper chromosphere depending on the presence of resonant versus nonresonant waves. This is consistent with the suggestion of Martínez-Sykora et al (2023) that in the normal collisional environment of the chromosphere, if the ponderomotive force is the dominant force in fractionation, waves should propagate from the chromosphere upward (a suggestion supported by observations made by Murabito et al 2024).…”