Hinode EIS Observations of Plasma Composition Evolution and Radiative Cooling of Flare Loops

Nugget
Number:	519
1st Author:	Teodora Mihăilescu
2nd Author:	Peter R. Young et al.
Published:	March 16, 2026
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Previous Nugget:	When Magnetic Field Lines Stretch, Snap, and Expand: A New Look at Solar Flares with L-maps

Overview

The chemical composition of an astronomical objects produces abundant spectroscopic signatures. In particular the EIS instrument on the Hinode satellite can generate EUV spectroscopy resolved in space and time, as well as wavelength. This makes it possible to study flares, for example.

Plasma composition in flaring regions has been shown to have significant spatial and temporal variations, likely driven by dynamical processes that take place as a consequence of the sudden energy release at the reconnection site. The origins of these variations, as well as the effects they might, in turn, have on flare loops dynamics are not yet fully understood. In this work (Ref. [1]) we investigate the plasma composition evolution and its link to plasma cooling times in the loops formed following the M-class flare SOL2022-04-02T13:56 using high cadence Hinode EIS spectroscopic observations.

Hinode EIS Observations

Hinode EIS observed the evolution of this flare's coronal loops using a "sit-and-stare" study (a fixed single slit position) which captured a portion of the flare loop arcade top and one of the footpoints (see Figure 1). This allowed us to study plasma composition using the Ca XIV 193.866 Å /Ar XIV 194.401 Å diagnostic and a DEM analysis. Given the narrow temperature distribution of the structure, a Gaussian shape was assumed for the DEM function. Observations indicate a systematic difference in plasma composition along the loop, with the loop top exhibiting a FIP bias of 2.8±0.2, higher than the 2.4±0.2 value observed at the footpoint (see Figure 2). FIP bias values higher than 1 indicate an overabundance in low-FIP elements in the corona.

Figure 1: Context AIA 94 Å image of the flare loops associated with the flare. The solid white rectangle indicates the position of the EIS slit for the Fe XII 195.119 Å window, and the dashed white rectangle shows the loop arcade studied in this work.

Figure 2: Evolution of Gaussian DEM centroid temperature (top) and associated FIP bias (bottom) for the loop top (blue) and loop footpoint (red).

The evolution of line intensities covering a wide temperature range (log(T) = 5.72 to 7.25) was used to track the cooling times, revealing shorter cooling times at the loop top compared to the loop footpoint (see Figure 3). The evolution of the Gaussian centroid temperature also shows a steeper slope for the loop top than footpoint, providing additional indication that the flare loop top is cooling faster than the footpoint.

Figure 3: Radiative energy losses, Q(T,Ne), as a function of electron temperature in the case of plasma with FIP bias = 2.4 (red) and FIP bias = 2.8 (blue), assuming the default CHIANTI ionisation file and an electron density of 10¹⁰ cm^-3.

EBTEL Simulations

The radiative loss function depends on plasma composition, particularly in the temperature range where emission is dominated by low-FIP elements (Ref. [2]; see the example in Figure 3 here). We investigate the effect the spatial variation in FIP bias observed along the flare loop could have on the loop cooling times using simulations from the EBTEL 0D hydrodynamic model. The EBTEL simulations show that the observed difference in FIP bias between the loop top and footpoint is sufficiently high to produce a noticeable difference in cooling times, with the higher FIP bias leading to a faster radiative cooling rate (see Figure 4).

Figure 4: Intensity evolution comparison between observations and simulations. Left: Averaged Hinode EIS spectra intensity evolution in the loop top (blue) and footpoint (red). The peak emission times for each feature in each line are indicated by the vertical dashed lines. Right: Synthetic intensity evolution computed using an EBTEL simulation of a heating event in a single loop strand, assuming a plasma with a FIP bias = 2.8 (blue) and a FIP bias = 2.4 (red) respectively. Lifetimes are indicated in the top right corner of each plot.

Conclusions

The observed FIP bias values are higher than those reported in some previous studies which found photospheric composition during flares. While several aspects need to be considered when comparing our results to previous ones, the relatively high FIP bias values we observe suggest that, in this particular event and flare stage captured, the energy deposition and consequent chromospheric evaporation take place above the fractionation level in the chromosphere.

The consistently higher FIP bias and shorter cooling times at the loop top compared to the loop footpoint suggest a faster radiative cooling rate. Previous studies have reported even stronger spatial variations in FIP bias along flare loops (e.g., Ref. [3]), suggesting that the influence of plasma composition on heat transport and loop cooling may be even more significant in more energetic events.

Acknowledgments

Co-authors: Peter R. Young, David H. Brooks, Deborah Baker, Lucie M. Green, David M. Long, Lidia van Driel-Gesztelyi.

References

[1] "Hinode EIS Observations of Plasma Composition Evolution and Radiative Cooling of Flare Loops"

[2] "Effect of Coronal Elemental Abundances on the Radiative Loss Function"

[3] "Photospheric and Coronal Abundances in an X8.3 Class Limb Flare"