OBSERVATIONS AND SIMULATIONS OF THE Na i D₁ LINE PROFILES IN AN M-CLASS SOLAR FLARE

Abstract

We study the temporal evolution of the Na I D 1 line profiles in the M3.9 flare SOL2014-06-11T21:03UT, using observations at high spectral resolution obtained with the Interferometric Bidimensional Spectrometer instrument on the Dunn Solar Telescope combined with radiative hydrodynamic simulations. Our results show a significant increase in the intensities of the line core and wings during the flare. The analysis of the line profiles from the flare ribbons reveals that the Na I D 1 line has a central reversal with excess emission in the blue wing (blue asymmetry). We combine RADYN and RH simulations to synthesize Na I D 1 line profiles of the flaring atmosphere and find good agreement with the observations. Heating with a beam of electrons modifies the radiation field in the flaring atmosphere and excites electrons from the ground state 3s 2 S to the first excited state 3p 2 P, which in turn modifies the relative population of the two states. The change in temperature and the population density of the energy states make the sodium line profile revert from absorption into emission. Furthermore, the rapid changes in temperature break the pressure balance between the different layers of the lower atmosphere, generating upflow/downflow patterns. Analysis of the simulated spectra reveals that the asymmetries of the Na I D 1 flare profile are produced by the velocity gradients in the lower solar atmosphere.

Modeling of the Hydrogen Lyman Lines in Solar Flares

“…This was also found at the coreformation height of the Na I D 1 in an F11 simulation by Kuridze et al (2016). Because of this, the line core is not Doppler-shifted; instead, it is centrally reversed as S ν peaks deeper in the atmosphere.…”

“…Synthetic Hα and Ca II 8542 Å lines from RADYN were compared to IBIS observations of an M class flare by Rubio da Costa et al (2015). IBIS observations were again compared to synthetic Na I D 1 profiles by Kuridze et al (2016). Simões et al (2016) studied the formation of the He lines, and Kerr et al (2016) investigated the formation of the Mg II h and k and Ca II 8542 Å lines.…”

Spectropolarimetric Inversions of the Ca ii 8542 Å Line in an M-class Solar Flare

“…The Stokes I profile is centrally reversed and shows excess emission in the red wing (red asymmetry) with a blueshifted line center (top left panel of Figure 3). The asymmetry appears to be due to the blueshifted line core of Stokes I, which in turn is a consequence of upflows (visible in our inversions in the bottom right panel of Figure 3) at the height of line core formation (Carlsson & Stein 1997;Abbett & Hawley 1999;Kuridze et al 2015Kuridze et al , 2016Kuridze et al , 2017. The flare pixel has a higher chromospheric temperature at logτ∼−2.5 and−5.5 compared to the quiet-Sun temperature profile, which is overplotted as a blue dashed line in the bottom left panel of Figure 3.…”

Mapping the Magnetic Field of Flare Coronal Loops

“…We note that line profiles of bright loop top seen in Ca ii and Hβ images (Figure 3, 4, 8, 11b) have a very strong central reversal. This bright top is formed through the accumulation of evaporated plasma and it can have higher density and optical depth compared to loop legs, which explains why its line profiles show a central reversal (Kuridze et al 2015(Kuridze et al , 2016(Kuridze et al , 2017. We will investigate the density structure of this loop top and formation of central reversal in the follow-up study.…”