Comparison of Solar Fine Structure Observed Simultaneously in Lyα and Mg ii h

Abstract

The Chromospheric Lyman Alpha Spectropolarimeter (CLASP) observed the Sun in H I Lyα during a suborbital rocket flight on 2015 September 3. The Interface Region Imaging Telescope (IRIS) coordinated with the CLASP observations and recorded nearly simultaneous and co-spatial observations in the Mg II h and k lines. The Mg II h and Lyα lines are important transitions, energetically and diagnostically, in the chromosphere. The canonical solar atmosphere model predicts that these lines form in close proximity to each other and so we expect that the line profiles will exhibit similar variability. In this analysis, we present these coordinated observations and discuss how the two profiles compare over a region of quiet Sun at viewing angles that approach the limb. In addition to the observations, we synthesize both line profiles using a 3D radiation-MHD simulation. In the observations, we find that the peak width and the peak intensities are well correlated between the lines. For the simulation, we do not find the same relationship. We have attempted to mitigate the instrumental differences between IRIS and CLASP and to reproduce the instrumental factors in the synthetic profiles. The model indicates that formation heights of the lines differ in a somewhat regular fashion related to magnetic geometry. This variation explains to some degree the lack of correlation, observed and synthesized, between Mg II and Lyα. Our analysis will aid in the definition of future observatories that aim to link dynamics in the chromosphere and transition region.

Three-dimensional modeling of chromospheric spectral lines in a simulated active region

“…The main problem in modeling both long fibrils and flare ribbons is that the radiation originating from the solar chromosphere forms under complex non-Local Thermodynamic Equilibrium (non-LTE) conditions. On the one hand, this makes observations of the chromosphere difficult to interpret; on the other hand, the forward synthesis of chromospheric lines becomes quite complex and computationally expensive (Leenaarts et al 2012;Štěpán et al 2015;Schmit et al 2017;Leenaarts et al 2013b;Bjørgen et al 2018;Sukhorukov & Leenaarts 2017) if one wants to reproduce the spectral lines in detail.…”

The Response of the Lyα Line in Different Flare Heating Models

“…The line center is formed around the transition region while the line wings are formed in the mid-chromosphere (Vernazza et al 1981). Schmit et al (2017) studied the correlation of the Lyα line with the Mg II h line which is formed in the mid-upper chromosphere. They found that for both observations and simulations, there is a good correlation between the core intensities of the two lines.…”

The dynamic chromosphere: Pushing the boundaries of observations and models

“…Bjørgen et al (2018) use similar models to study the formation of the Ca ii H and K lines and to help interpret the observations of CHROMIS, a new imaging spectrometer at the SST. On a similar vein, 3D simulations were used to study the polarisation of chromospheric lines such as Lyα ( Štěpán et al, 2015;Schmit et al, 2017) and Ca ii 854.2 nm ( Štěpán and Trujillo Bueno, 2016). These studies provide unique insight into the formation mechanism of different spectral lines, are invalu-able for observers, and are a model for successfully understanding future solar observations.…”