Evolution of Elemental Abundances in Hot Active Region Cores from Chandrayaan-2 XSM Observations

Abstract

The first ionization potential (FIP) bias, whereby elemental abundances for low-FIP elements in different coronal structures vary from their photospheric values and may also vary with time, has been widely studied. In order to study the temporal variation and understand the physical mechanisms giving rise to the FIP bias, we have investigated the hot cores of three active regions (ARs) using disk-integrated soft X-ray spectroscopic observations with the Solar X-ray Monitor on board Chandrayaan-2. Observations for periods when only one AR was present on the solar disk were used to ensure that the AR was the principal contributor to the total X-ray intensity. The average values of temperature and emission measure were ∼3 MK and 3 × 1046 cm−3, respectively. Regardless of the AR’s age or activity, the elemental abundances for the low-FIP elements Al, Mg, and Si with respect to the soft X-ray continuum were consistently higher than their photospheric values. The average FIP bias for Mg and Si was 2–2.5, whereas the FIP bias for the mid-FIP element, S, was almost unity. However, the FIP bias for the lowest-FIP element, Al, was observed to be a factor of 2 higher than Si, which, if real, suggests a dependence of the FIP bias of low-FIP elements on their FIP value. Another major result from our analysis is that the FIP bias of these elements is established within ∼10 hr of emergence of the AR and remains almost constant throughout its lifetime.

Modeling Time-variable Elemental Abundances in Coronal Loop Simulations

“…In an AR, the abundances can also vary with time, depending on the strength of heating and amount of fractionation by wave fluxes. Mondal et al (2023) examined time variation of Al, Mg, Si, and S in three ARs, finding abundance factors no lower than 2 (slightly enhanced over photospheric). Comparing the simulations here, that suggests that there may be an upper limit to nanoflare heating in those ARs of ≈0.1 erg s −1 cm −3 since the abundances never reached photospheric values.…”

“…Many recent X-ray observations of flares, however, clearly show that elemental abundances are not fixed and vary with time over the duration of the flare (Del Zanna et al 2021;Mondal et al 2021;Mithun et al 2022;Kepa et al 2023;Nama et al 2023;Rao et al 2023;Suarez & Moore 2023;Woods et al 2023). Similar observations have also found that the abundances of active regions (ARs) vary relatively quickly as well (Mondal et al 2023).…”