Radiative hydrodynamic simulations of FIP fractionation in solar flares

TL;DR

Addresses variability of elemental abundances in solar flares and tests how depth of footpoint fractionation and evaporation shape coronal composition. Introduces a spatiotemporal abundance factor $f(s,t)$ and couples it to radiative losses via CHIANTI in HYDRAD, then tests two fractionation-depth scenarios with footpoint spikes (100 km narrow, 500 km wide) and impulsive electron-beam heating. Hydrodynamic simulations of a 50 Mm loop heated by an electron beam ($E_c=15$ keV, $δ=5$), across four energy fluxes, show that narrow spikes produce apex-localized $f$ enhancements and coronal rain across heating rates, while wide spikes can fully fractionate the corona at low heating and require stronger heating to induce localized rain. The work predicts a direct link between coronal rain and the combination of fractionation depth and flare heating, offering testable observational strategies and, importantly, providing publicly available simulation data on Zenodo.

Abstract

Elemental abundances in solar flares are observed to vary both spatially and temporally, but the underlying mechanisms remain poorly understood. There is an interplay between advection and the preferential acceleration of low first ionization potential (FIP) elements that likely shapes the observed abundance distributions. Models of the FIP effect predict enhancements near loop footpoints that diffuse upward over time. We simulate strong evaporation events that advect this low-FIP enhancement into the corona. When the enhancement is sharply peaked, the corona does not become fractionated, exhibiting only a localized abundance peak near the loop apex that facilitates coronal rain formation. In contrast, a broad enhancement with relatively weak heating yields a uniformly fractionated corona, which is not sufficient for coronal rain formation. As the heating rate increases, the low-FIP enhanced plasma is increasingly compressed toward the loop apex, and rain is able to form. These results suggest a potential observational correlation between the presence and amount of coronal rain, the strength of flare heating, and the fractionation process itself.

Figures (6)

• Figure 1: Simulations with narrow spikes in the abundance factor $f$ at the footpoints of the loops. Each set of five plots shows one simulation. The panel at left in each case shows the abundance factor, from times 0 to 300 s in the simulation at a 10-second cadence (blue to green to yellow in time). The panels on the right show time-distance plots of the electron temperature, electron density, bolometric radiative loss rate, and the bulk flow velocity (where blue means towards the apex, red away from the apex), tracking the evolution of the loop in each simulation. In all cases, a spike in $f$ forms near the apex, enhancing the radiative loss rate there and causing the formation of a coronal rain event (seen as high density, low temperature regions late in the simulations). (Figure continued on next page.)
• Figure 2: Continued for stronger heating cases.
• Figure 3: Similar to Figure \ref{['fig:narrow']}, with wide spikes in the abundance factor $f$ at the footpoints of the loops. In this case, since the enhancement contains a much deeper portion of the chromosphere, the evaporation event carries up significantly more low-FIP enhanced plasma. With weak heating, this creates a fully fractionated corona. As the heating rate grows stronger, the evaporative flows compress more material near the apex, causing a slight localized enhancement there. Only with the strongest heating do we see the formation of coronal rain. (Figure continued on next page.)
• Figure 4: Continued for stronger heating cases.
• Figure 5: Similar to Figure \ref{['fig:narrow']}, showing two examples with asymmetric heating, with the right-hand leg of the loop receiving half the energy as the left-hand leg. The top simulation shows an example of two narrow spikes at the footpoints, while the bottom simulation shows a case with wide spikes at the footpoints. The asymmetry in the heating affects whether rain forms where on the loop it forms. The supplemental material includes plots and simulations with the other energy fluxes considered.