Linking Chromospheric Conditions to Coronal Composition: IRIS and EIS Observations of the Inverse FIP Effect

 

Introduction
Combined IRIS and Hinode/EIS observations enable a multi-layer analysis of plasma composition, linking chromospheric conditions to their manifestation in the corona. Hinode/EIS provides spatially resolved measurements of elemental abundances, identifying regions of FIP and inverse FIP (IFIP) effect plasma, while IRIS probes the chromosphere where fractionation is thought to occur. Together, these observations provide a basis for relating coronal composition to underlying processes such as wave activity, turbulence, and magnetic reconnection in the lower atmosphere.

The IFIP effect, characterized by a depletion of low-FIP elements [1], is rare on the Sun but common in highly active stars whose coronae are dominated by IFIP plasma [2]. On the Sun it appears as localized patches embedded within the FIP-dominated corona, typically associated with magnetically complex active regions, ongoing flux emergence, and strong sunspot light bridges [3-8]. These locations suggest, but do not necessarily confirm, sites of enhanced wave activity and low-atmosphere reconnection [9]. Current understanding points to wave-driven processes operating below the chromospheric fractionation layer (near or even below the beta = 1 layer) as a possible origin of IFIP plasma [10].

Flares provide a means of observing this plasma by evaporating it into coronal loops, where its composition can be measured with EIS. However, its strong spatial localization and association with evolving magnetic structures imply that the conditions responsible for fractionation may already be established prior to the flare. In this sense, the flare may act primarily as a tracer of pre-existing plasma rather than the driver of the IFIP effect. Establishing the connection between chromospheric dynamics and coronal composition is therefore central to understanding the origin of IFIP plasma.

IFIP Effect in AR 11967
NOAA AR 11967 provides a useful test-bed for exploring these connections. It was a large, magnetically complex active region undergoing sustained flux emergence and producing numerous flares, including an M2.6 event. The region was characterized by coalescing sunspots and the formation of strong light bridges, indicative of continued magnetic restructuring in the lower atmosphere.

Figure 1 combines EIS composition diagnostics with IRIS and SDO context imaging to locate IFIP effect plasma within its chromospheric environment. The IFIP effect is identified using the Hinode/EIS Ar XIV/Ca XIV line ratio. Contours of strong IFIP effect plasma highlight localized patches at flare loop footpoints. The location of these IFIP effect regions is indicated by the red (C2) and cyan (C1) contours. The red contour (C2) is observed by IRIS at 21:32 UT, shortly after flare onset, while the cyan contour (C1) is observed later at 23:16 UT, during the decay phase.

Figure 1: Multi-instrument view of IFIP plasma in NOAA AR 11967. IRIS Mg II k intensity maps (panels a,b) provide the chromospheric context, while Hinode/EIS Ar XIV intensity (c) and Ar XIV/Ca XIV ratio (d) maps identify localized IFIP plasma through enhanced line ratios indicative of low-FIP depleton. Contours of strong IFIP effect plasma derived from the EIS diagnostic are overplotted, highlighting compact patches at the footpoints of flare loops. Corresponding SDO/AIA and HMI images (e-h) show that these locations coincide with sunspot umbrae and light bridges.

IRIS2+ Thermodynamic Structure
To investigate the chromospheric environment at these locations, IRIS2+ inversions are used to derive stratified profiles of electron density, temperature, and turbulent velocity [11-13]. Median profiles within the IFIP effect regions are compared with the broader field of view using Mg II k intensity deciles (Q1-Q10), providing a reference framework for typical chromospheric conditions.

In Figure 2, the IFIP effect regions correspond to the red curve (C2; rise phase) and cyan curve (C1; decay phase). The C2 site shows enhanced densities and temperatures relative to the decile profiles, consistent with flare-related heating and compression. The most distinctive signature, however, is seen in the turbulent velocity.

During the rise phase (red curve), turbulent velocity exhibits a strong enhancement peaking relatively low in the atmosphere, indicating intense localized dynamics in the mid-chromosphere. By contrast, the decay phase profile (cyan curve) shows a reduced and more uniform, nondistinct structure.

Figure 2: IRIS2+ thermodynamic structure at the IFIP effect sites. Median profiles of electron density (a,b), temperature (c,d), and turbulent velocity (e,f) are shown as a function of optical depth. The IFIP effect regions are highlighted by the red (C2) and cyan (C1) curves, corresponding to sampling during the onset and decay phases of the flare, respectively. These profiles are compared with Mg II k intensity deciles (Q1-Q10) representing typical chromospheric conditions across the field of view.

Discussion
The lower-altitude enhancement in turbulent velocity observed at C2 differs from typical flare profiles, where such peaks are generally found higher in the atmosphere. This behavior points to enhanced wave activity and energy deposition in the mid-chromosphere, consistent with processes operating below the fractionation layer. These findings suggest that sub-chromospheric dynamics, including reconnection and wave generation, may play a role in establishing the conditions required for IFIP fractionation.

This interpretation remains tentative, however, as observations capturing both the chromospheric conditions and coronal composition of IFIP effect plasma are limited. To date, only a small number of events have been observed with coordinated IRIS and EIS coverage, making this one of the few cases in which such a multi-layer comparison is possible. IFIP detections themselves are rare in the solar atmosphere, and the subset with simultaneous chromospheric diagnostics is smaller still. Expanding the sample of coordinated observations with IRIS and ground based observatories will therefore be essential for determining whether the conditions identified here are representative of IFIP effect fractionation more generally, or specific to particular magnetic and evolutionary conditions within individual active regions.

The full article will be published in the Royal Society topical issue titled "Solar Atmospheric Abundances in Space & Time" due out in early July.

References
1. Brooks DH. 2018 A diagnostic of coronal elemental behavior during the inverse FIP effect in solar flares. Astrophysical Journal 863, 140. (10.3847/1538-4357/aad415)
2. Seli B, Olah K, Kriskovics L, Kovari Z, Vida K, Balazs LG, Laming JM, van Driel-Gesztelyi L, Baker D. 2022 Extending the FIP bias sample to magnetically active stars. Challenging the FIP bias paradigm. Astronomy & Astrophysics 659, A3. (10.1051/0004-6361/202141493)
3. Doschek GA, Warren HP, Feldman U. 2015 Anomalous relative Ar/Ca coronal abundances observed by the Hinode/EUV Imaging Spectrometer near sunspots. Astrophysical Journal Letters 808, L7. (10.1088/2041-8205/808/1/L7)
4. Doschek GA, Warren HP. 2016 The mysterious case of the solar argon abundance near sunspots in flares. Astrophysical Journal 825, 36. (10.3847/0004-637X/825/1/36)
5. Doschek GA, Warren HP. 2017 Sunspots, starspots, and elemental abundances. Astrophysical Journal 844, 52. (10.3847/1538-4357/aa7bea)
6. Baker D, van Driel-Gesztelyi L, Brooks DH, Valori G, James AW, Laming JM, Long DM, Demoulin P, Green LM, Matthews SA, Olah K, Kovari Z. 2019 Transient inverse-FIP plasma composition evolution within a solar flare. Astrophysical Journal 875, 35. (10.3847/1538- 4357/ab07c1)
7. Baker D, van Driel-Gesztelyi L, Brooks DH, Demoulin P, Valori G, Long DM, Laming JM, To ASH, James AW. 2020 Can subphotospheric magnetic reconnection change the elemental composition in the solar corona?. Astrophysical Journal 894, 35. (10.3847/1538-4357/ab7dcb)
8. Baker D, van Driel-Gesztelyi L, James AW, Demoulin P, To ASH, Murabito M, Long DM, Brooks DH, McKevitt J, Laming JM, Green LM, Yardley SL, Valori G, Mihailescu T, Matthews SA, Kuniyoshi H. 2024 Searching for evidence of subchromospheric magnetic reconnection on the Sun. Astrophysical Journal 970, 39. (10.3847/1538-4357/ad4a6e)
9. Toriumi S, Cheung MCM, Katsukawa Y. 2015b Light bridge in a developing active region. II. Numerical simulation of flux emergence and light bridge formation. Astrophysical Journal 811, 138. (10.1088/0004-637X/811/2/138)
10. Laming JM. 2015 The FIP and inverse FIP effects in solar and stellar coronae. Living Reviews in Solar Physics 12, 2. (10.1007/lrsp-2015-2)
11. Sainz Dalda A, de la Cruz Rodríguez J, De Pontieu B, Gosic, M. 2019 Recovering thermodynamics from spectral profiles observed by IRIS: A machine and deep learning approach. Astrophysical Journal Letters 875, L18. (10.3847/2041-8213/ab15d9)
12. Sainz Dalda A, Agrawal A, De Pontieu B, Gosic, M. 2024 IRIS2+: A comprehensive database of stratified thermodynamic models in the low solar atmosphere. Astrophysical Journal Supplement Series 271, 24. (10.3847/1538-4365/ad1e55)
13. Sainz Dalda A, De Pontieu B. 2023 Chromospheric thermodynamic conditions from inversions of complex Mg II h & k profiles observed in flares. Frontiers in Astronomy and Space Sciences 10, 1133429. (10.3389/fspas.2023.1133429)

 

For more details, please contact: Dr. Deb Baker.

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