Spectral Signatures of Chromospheric Condensation in a Major Solar Flare

Abstract

We study the evolution of chromospheric line and continuum emission during the impulsive phase of the X-class SOL2014-09-10T17:45 solar flare. We extend previous analyses of this flare to multiple chromospheric lines of Fe i, Fe ii, Mg ii, C i, and Si ii observed with the Interface Region Imaging Spectrograph, combined with radiative-hydrodynamical (RHD) modeling. For multiple flaring kernels, the lines all show a rapidly evolving double-component structure: an enhanced emission component at rest, and a broad, highly redshifted component of comparable intensity. The redshifted components migrate from 25 to 50 km s−1 toward the rest wavelength within ∼30 s. Using Fermi hard X-ray observations, we derive the parameters of an accelerated electron beam impacting the dense chromosphere, using them to drive an RHD simulation with the RADYN code. As in Kowalski et al. (2017), our simulations show that the most energetic electrons penetrate into the deep chromosphere, heating it to T ∼ 10,000 K, while the bulk of the electrons dissipate their energy higher, driving an explosive evaporation, and its counterpart condensation—a very dense (n e ∼ 2 × 1014 cm−3), thin layer (30–40 km thickness), heated to 8–12,000 K, moving toward the stationary chromosphere at up to 50 km s−1. The synthetic Fe ii 2814.45 Å profiles closely resemble the observational data, including a continuum enhancement, and both a stationary and a highly redshifted component, rapidly moving toward the rest wavelength. Importantly, the absolute continuum intensity, ratio of component intensities, relative time of appearance, and redshift amplitude are sensitive to the model input parameters, showing great potential as diagnostics.

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

“…Due to momentum conservation, the evaporation upflows are usually accompanied by downflows of chromospheric and transition-region plasma ("chromospheric condensation"), which can be observed by IRIS as red-shifts of cooler lines such as Si IV, C II, and Mg II (e.g. Tian et al, 2015;Brosius, Daw, and Inglis, 2016;Graham and Cauzzi, 2015;Graham et al, 2020). The Si IV and C II spectra sometimes show persistent red-shifts or red-asymmetries of a few tens of km s −1 during the whole impulsive phase (Warren et al, 2016;Yu et al, 2020), which provide challenging constraints to the 1D loop models (e.g.…”

“…In some cases, the IRIS chromospheric lines (including Mg II, C I, Fe I, Fe II, Si II) also show an enhanced rest component superimposed with a broad and highly red-shifted (≈ 25 -50 km s −1 ) secondary component, which rapidly decays in 30 -60 seconds (Graham et al, 2020). Such double-component profiles can be explained by RADYN hydrodynamic simulations assuming that the most energetic electrons penetrate into the deep chromosphere, while the bulk of the electrons dissipate their energy higher up into the atmosphere (Kowalski et al, 2017;Graham et al, 2020). These studies also highlight the diagnostic potential of the IRIS chromospheric lines such as Fe II, which are extremely sensitive to the model input parameters.…”

“…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. Panos et al, 2018;Panos and Kleint, 2020;Sadykov et al, 2019).…”

“…Detailed modeling of the IRIS observations of NUV properties of solar flares serve as "ground truth" for modeling approaches to these white-light properties in stellar flares. In particular, Kowalski et al (2017) and Graham et al (2020) studied IRIS, RHESSI, and Fermi/GBM data of two X-class flares to constrain RADYN simulations of the chromospheric response to the impact of moderate-to-high fluxes of electron beams and found that the NUV continuum intensity and red-shifted emission-line satellite components were rather well-reproduced by two transient flaring layers at chromospheric heights: a stationary heated layer below a chromospheric condensation (see also Section 5.1.1).…”

Multi-thermal atmosphere of a mini-solar flare during magnetic reconnection observed with IRIS

“…For multiple flaring kernels, chromospheric lines show a rapidly evolving double-component structure: an enhanced emission component at rest, and a broad, highly redshifted component of comparable intensity. Graham et al (2020) interpreted such observations by beams penetrating very deep in the atmosphere. The red-shifted components migrate from redshifts towards the rest wavelength within 30 seconds.…”