Can the Superposition of Evaporative Flows Explain Broad Fe xxi Profiles during Solar Flares?

Abstract

The observation of the high-temperature (10 MK) Fe XXI1354.1 Å line with the Interface Region Imaging Spectrograph has provided significant insights into the chromospheric evaporation process in flares. In particular, the line is often observed to be completely blueshifted, in contrast to previous observations at lower spatial and spectral resolution, and in agreement with predictions from theoretical models. Interestingly, the line is also observed to be mostly symmetric and significantly broader than expected from thermal motions (assuming the peak formation temperature of the ion is in equilibrium). One popular interpretation for the nonthermal broadening is the superposition of flows from different loop strands.In this work, we test this scenario by forward-modeling the Fe XXIline profile assuming different possible observational scenarios using hydrodynamic simulations of multithread flare loops with the 1D RADYN code. Our results indicate that the superposition of flows alone cannot easily reproduce both the symmetry and the significant broadening of the line and that some other physical process, such as turbulence, or a much larger ion temperature than previously expected, likely needs to be invoked in order to explain the observed profiles.

The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. II. Hydrogen Broadening Predictions for Solar Flare Observations with the Daniel K. Inouye Solar Telescope

“…"Quiet Sun" synthesis models have started to consider the advantages of these accurate, self-consistent hydrogen broadening profiles as well (Bjørgen et al 2019;Marchenko et al 2021). Here we describe the implementation of new line profile functions in the state-of-the-art RHD code RADYN, which is widely used for simulations of a wide variety of solar flare phenomena (Simões et al 2017;Polito et al 2019;Sadykov et al 2020;Monson et al 2021), Ellerman bombs (Hong et al 2017;Reid et al 2017), nanoflares (Testa et al 2014;Polito et al 2018), and chromospheric dynamics such as spicules and shocks (Carlsson & Stein 2002;Wedemeyer-Böhm & Carlsson 2011;Guerreiro et al 2013;Molnar et al 2021). The updated hydrogen line profile functions are known as the TB09+HM88 profiles, which are extensions of the VCS unified theory calculations to higher-n j lines with self-consistent, non-ideal broadening from electron collisions and proton/ion perturbations.…”

A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS)

“…Doschek et al, 1986;Milligan, 2011). By using 1D hydrodynamic simulations of multi-thread flare loops mimicking a 3D geometry, Polito, Testa, and De Pontieu (2019) found that this scenario may not be compatible with the IRIS Fe XXI profiles, which appear to be broad but at the same time symmetric and well fitted with a single Gaussian profile (Figure 22), and they suggested that other mechanisms such as Alfvénic turbulence or non-equilibrium ionization might need to be invoked to explain the observations.…”

“…For instance, hydrodynamic models including non-equilibrium ionization for highly charged iron atoms using the HYDRAD code (Bradshaw and Mason, 2003) appear to reproduce some of the observed features, such as the broad and symmetric profiles in hot lines, assuming short flare loops and a combination of input parameters (Mandage and Bradshaw, 2020). However, simulations of longer loops with lengths 50 Mm, such as the ones observed in the 10 September 2014 flare analyzed by Polito, Testa, and De Pontieu (2019), still show profiles with blue-wing asymmetries that are not seen in the observations. Further investigation into the parameter space of these models and inclusion of additional heating mechanisms, such Alfvén waves, might be needed to solve some of these discrepancies.…”

“…In the next few years there will be exciting opportunities to investigate some of these puzzles even further, thanks to new advancements in the models, including recent multithreaded and arcade models (e.g. Polito, Testa, and De Pontieu, 2019;Kerr, Allred, and Polito, 2020;Mandage and Bradshaw, 2020), improvements in the modeling and synthesis of the chromospheric diagnostics (e.g. Zhu et al, 2019;Graham et al, 2020), as well as new approaches to the analysis of the observations based on statistical studies and machine-learning techniques (e.g.…”

Exploring mutual information between IRIS spectral lines. II. Calculating the most probable response in all spectral windows

“…Although Fe XXI has provided crucial model constraints for chromospheric evaporation, the physical mechanism responsible for the nonthermal >100 km s −1 widths at flare footpoints remains elusive. A recent study by Polito et al (2019) used hydrodynamic models to dismiss the most popular explanation of subresolution plasma flows at different velocities, showing that an additional physical mechanism, such as plasma turbulence or very large ion temperatures is necessary for a complete explanation. A speculative dismissal of the superposition scenario was already put forward several decades earlier by Antonucci et al (1986), who noted that it would be unlikely for up-and down-flowing material to exactly balance each other to produce the archetypal symmetric Fe XXI profile commonly observed.…”