Andy S.H. To
Elemental abundances in the solar corona are different from those in the photosphere and the differences seem to be strongly modulated by the magnetic field and magnetic activity. Elemental abundances of easy-to-ionise low-FIP elements are enhanced in active region coronae compared to their photospheric abundances, a phenomenon known as the FIP effect. Although theories have been proposed to explain this effect, the details of the evolution of solar composition remain a mystery despite almost fifty years of research. Changes in elemental abundances take place on the time scales of years (solar cycle), days or hours (magnetic flux emergence and decay), or minutes (flare). Here, we analyse an unexpected strong increase in the coronal abundances of low-FIP element calcium in a small flare observed by Hinode/EIS, which is contrary to the photospheric abundances observed using Si X/S X and predicted by current theories. The flare was studied using a combination of spectroscopic observations from Hinode/EIS, magnetic field observations from SDO/HMI and contemporaneous observations from SDO/AIA. These observations indicate that the flare occurred in an extremely stable X-shaped Quasi-Separatrix Layer (QSL) structure rooted in a highly complex active region. After studying a wavelet analysis of the flare, we propose that two different mechanisms contributed to the inconsistent composition results. Firstly, ablation of deep-chromospheric plasma caused by flare-induced heating would lead to photospheric abundances. Secondly, high-FIP S could behave more like a low-FIP than a high-FIP element in a flare. Our wavelet analysis suggests that fractionation occurs in the low chromosphere, where the background gas is neutral H. Our observations provides additional information on the ponderomotive force fractionation model and offer a new insight into the evolution of plasma abundances in the solar atmosphere during flaring.
In order to parameterise and study the coronal elemental abundances, we use the first ionisation potential (FIP) bias, defined as the ratio of an element's coronal to photospheric abundance. The FIP bias is a unique parameter which varies from solar structure to solar structure, and is closely linked to the Sun's magnetic field on all scales (Brooks et al. 2017; Baker et al. 2018), including one of the most spectacular solar events, solar flares. Solar flares are amazing results of solar magnetic reconnection, characterised by the sudden release of energy and plasma. This rapid change of magnetic configuration triggers waves and releases energy that propagate into the chromosphere, followed by the ablation of plasma into the corona and heating of the chromosphere, implying a composition change closer to photospheric abundance which has been commonly observed (e.g. Del Zanna & Woods 2013, Warren 2014 and Dennis et al. 2015). However, recent observations have found evidence of an inverse FIP (IFIP) effect, with high-FIP element abundances enhanced or low-FIP element abundances decreased during the decay phase of flares rooted in very complex active regions (e.g. Sylwester et al. 2014; Doschek et al. 2015a; Doschek & Warren 2016; Baker et al. 2019, 2020).
In this poster, we present observations of the highly complex active region, AR 11967, which have provided a unique opportunity to study the evolution of composition in flares. On 2 Feb 2014, EIS observed a small flare within the active region. The results show very different composition evolution at two different temperatures, with an unchanged composition obtained in the lower temperature Si X/S X composition map in the flaring region, while a significant FIP bias value increase was observed in the higher temperature Ca XIV/Ar XIV composition ratio.
Using a wavelet analysis approach applied to the contemporaneous AIA data, it was observed that waves were generated during the reconnection which then propagated to the chromosphere, suggesting the presence of the ponderomotive force, which is the key mechanism for elemental fractionation (Laming 2015; Laming et al. 2019). We propose two possible physical interpretations, ablation during the flare and partial ionisation of different elements that could contribute to the strange composition evolution. Firstly, in ablation, the bulk of flare plasma is ablated from deep in the chromosphere, below where fractionations occur. However, this would lead to photospheric plasma composition in both maps. Secondly, in the case of high-FIP S behaves like a low-FIP element, our wavelet analysis suggests that fractionation occurs in the low chromosphere, where the background gas is neutral H. In the strong chromospheric magnetic field of the two large spots, where this flare is taking place, the fractionation height of sulphur shifts to the low chromosphere, making S to behave more like a low-FIP element. Therefore both Si and S fractionate in this flare and the Si/S ratio does not change, while the noble gas (high-FIP) Ar does not change its behaviour. This creates a significant discrepancy that can be observed between the two sets of composition maps. These observations are consistent with the ponderomotive force interpretation for variation in FIP bias, and open up an entirely new interpretation of composition evolution during flares.