Composition of plasma in the solar corona is a tracer of the flow of plasma and energy from the solar interior. Different complex processes such as the propagation and absorption of waves, convection of hot plasma and reconnection and reconfiguration of magnetic fields can affect the flow and composition of plasma. This produces a clear and observable variation in the elemental abundances of coronal plasma across different regions of the solar atmosphere (Brooks et al. 2015). Flares, in particular, are extremely energetic events that accentuate wave propagation and plasma flow. Here, we are looking at some interesting coronal composition changes during a small flare, and how composition could give us some insight into the Alfvén wave behaviour during different events.
The way we assess composition evolution is to use first ionisation potential, or FIP. Low FIP elements like Ca and Si have a first ionisation potential of less than 10 eV. Whereas high FIP elements like S and the noble gas Ar, have >10 eV first ionisation energy. In the corona of a developed active region, the low-FIP elements usually get enhanced more, and the number of times an element gets enhanced in the corona compared to the photosphere is called the FIP bias.
FIP bias is extremely useful because it can give us some insight into the wave behaviour during different events. The ponderomotive force model by Martin Laming suggests that magnetic reconnections trigger Alfvén waves to propagate from the corona downward along loops, and they are reflected in the chromosphere (Laming 2015). Through these reflections, we have the ponderomotive force that pulls up ionised particles from the chromosphere, creating the enhancement in abundances of the low FIP elements.
Figure 1: a) HMI continuum; HMI magnetogram; AIA 193 Å; AIA 131 Å image during the flare at 14:34 UT on 2 Feb 2014. The white box indicates the EIS field of view in Figure 2. b) AIA 94 Å light curve obtained surrounding the X-shaped structure in AR 11967. Blue horizontal lines indicate EIS rastering time and duration; Red vertical lines indicate the flare time; Orange horizontal line in the zoomed section of the light curve indicates the EIS scan time over the X-shaped structure. The third flare happened during the third EIS raster, and EIS observed the X-shaped structure during its decay phase, roughly 1 minute after the peak of the flare.
In this study, we looked at a monstrous active region, AR 11967, that appeared on disk in Feb 2014. This active region created multiple flares, and we look at any changes in the coronal composition within three flares/brightenings on 2 Feb. Two sets of composition diagnostics were used. Firstly, the ratio between low FIP Si and high FIP S, at around 1.5MK. Secondly, at a higher temperature of ~2.5MK, the ratio between low FIP Ca and high FIP Ar.
In the Si X/S X composition map, FIP bias maintained around the same level. The mean FIP bias values stay around 2, prior to, and after the third flare (associated with the third composition map). However, in the second set of composition maps formed at a higher temperature, there is a very interesting and significant change. This second set of ratio maps using Ca and Ar has a mean FIP bias from ~2 to nearly 4.
Figure 2: Top to bottom: Hinode/EIS Si X 258.38Å/S X 264.23 Å composition map, Ca XIV 193.87 Å/Ar XIV 194.40 Å intensity ratio maps. In the composition and intensity ratio maps, blue represents a photospheric-like (unenhanced composition) FIP bias of 1; red represents a quiet sun composition and yellow shows a high FIP bias composition. A clear X shape of the structure could be observed in the third raster at 14:19 UT.
What caused this unexpected discrepancy between the two sets of composition maps?
Our first interpretation is the partial ionisation of different elements. Although we define S as a high-FIP element, its FIP is relatively low at ~ 10eV. Therefore, we propose that in a flare, the heating is sufficient to ionise the relatively lower FIP S i.e. more of them become ionised and the ponderomotive force brings up Si and S at the same time. On the other hand, since Ar is so hard to ionise, the small flare in this case was not energetic enough to ionise the noble gas, generating the fractionation we see in the Ca/Ar maps.
However, there could be another physical interpretation. Our flare took place between very strong magnetic fields. This has the effect of lowering the plasma fractionation height of different elements. Under this condition, S would act more like a low-FIP ion, getting pulled up by the ponderomotive force (Laming 2021).
For more details, see To, et al, ApJ, 911, 86, 2021:
The Evolution of Plasma Composition During a Solar Flare
References
Laming 2015, Living Rev. Sol. Phys., 12, 2.
Laming 2019, ApJ, 909, 17.
