The cause of spatial structure in solar He i 1083 nm multiplet images

Abstract

Context. The He i 1083 nm is a powerful diagnostic for inferring properties of the upper solar chromosphere, in particular for the magnetic field. The basic formation of the line in one-dimensional models is well understood, but the influence of the complex threedimensional structure of the chromosphere and corona has however never been investigated. This structure must play an essential role because images taken in He i 1083 nm show structures with widths down to 100 km. Aims. To understand the effect of the three-dimensional temperature and density structure in the solar atmosphere on the formation of the He i 1083 nm line. Methods. We solve the non-LTE radiative transfer problem assuming statistical equilibrium for a simple 9-level helium atom that nevertheless captures all essential physics. As a model atmosphere we use a snapshot from a 3D radiation-MHD simulation computed with the Bifrost code. Ionising radiation from the corona is self-consistently taken into account. Results. The emergent intensity in the He i 1083 nm is set by the source function and the opacity in the upper chromosphere. The former is dominated by scattering of photospheric radiation and does not vary much with spatial location. The latter is determined by the photonionisation rate in the He i ground state continuum, as well as the electron density in the chromosphere. The spatial variation of the flux of ionising radiation is caused by the spatially-structured emissivity of the ionising photons from material at T ≈ 100 kK in the transition region. The hotter coronal material produces more ionising photons, but the resulting radiation field is smooth and does not lead to small-scale variation of the UV flux. The corrugation of the transition region further increases the spatial variation of the amount of UV radiation in the chromosphere. Finally we find that variations in the chromospheric electron density also cause strong variation in He i 1083 nm opacity. We compare our findings to observations using SST, IRIS and SDO/AIA data.

Observations of Ellerman bomb emission features in He i D3and He i 10 830 Å

“…The commonly accepted line formation mechanism for He i D 3 and He i 10830 Å in the upper chromosphere is through photoionization-recombination (Goldberg 1939;Zirin 1975;Mauas et al 2005;Andretta et al 2008;Centeno et al 2008;Leenaarts et al 2016). The neutral helium triplet levels are populated by incoming EUV radiation from the corona and transition region to the upper chromosphere, resulting in absorption lines formed by scattering of continuum photons.…”

Formation of the helium extreme-UV resonance lines

“…Modelling the the radiation in 1D would expose such a structure to EUV radiation only from the top which would result in a lower photoionization rate and fewer ions. An example of this is given in Figure 7 of Leenaarts et al (2016) where a comparison between 1D and 3D radiative transfer shows that an exposed chromospheric structure has a higher number density of He ii in the 3D case than in the 1D case.…”

“…Later he also found that the PR mechanism can not be the primary formation mechanism of He ii λ304 in quiet regions (Andretta et al 2003). The subordinate helium lines, however, have been shown to be sensitive to coronal illumination (Wahlstrom & Carlsson 1994;Avrett et al 1994;Andretta & Jones 1997;Mauas et al 2005;Centeno et al 2008;Leenaarts et al 2016).…”

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

“…Bifrost solves the equations of resistive MHD, together with non-LTE radiative losses in the photosphere and chromosphere, optically thin losses in the corona, and heat conduction along field lines. We used a snapshot from the "cb24bihe-halfxy-100" run, which was also used in Leenaarts et al (2016) and Golding et al (2017). The simulation box spans from the upper convection zone up to the lower corona, with an horizontal extent of 24×24 Mm and a vertical extent of 16.8Mm, from 2.5Mm below the photosphere to 14.3Mm above it.…”