New light on an old problem of the cores of solar resonance lines

Abstract

We re-examine a 50+ year-old problem of deep central reversals predicted for strong solar spectral lines, in contrast to the smaller reversals seen in observations. We examine data and calculations for the resonance lines of H I, Mg II and Ca II, the self-reversed cores of which form in the upper chromosphere. Based on 3D simulations as well as data for the Mg II lines from IRIS, we argue that the resolution lies not in velocity fields on scales in either of the micro- or macro-turbulent limits. Macro-turbulence is ruled out using observations of optically thin lines formed in the upper chromosphere, and by showing that it would need to have unreasonably special properties to account for critical observations of the Mg II resonance lines from the IRIS mission. The power in turbulence in the upper chromosphere may therefore be substantially lower than earlier analyses have inferred. Instead, in 3D calculations horizontal radiative transfer produces smoother source functions, smoothing out intensity gradients in wavelength and in space. These effects increase in stronger lines. Our work will have consequences for understanding the onset of the transition region, the energy in motions available for heating the corona, and for the interpretation of polarization data in terms of the Hanle effect applied to resonance line profiles.

arXiv comment12 pages, 9 figures, re-submitted to apj

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• Numerical simulations and observations of Mg II in the solar chromosphere
• Chromospheric Heating from Local Magnetic Growth and Ambipolar Diffusion Under Non-Equilibrium Conditions
• IRIS observations of chromospheric heating by acoustic waves in solar quiet and active regions
• Optimal spectral lines for measuring chromospheric magnetic fields
• Temperature diagnostics of chromospheric fibrils
• Determining the Magnetic Field in the Atmosphere of a Solar Active Region Observed by the CLASP2.1 Sounding Rocket Experiment
• Dark off-limb gap: manifestation of temperature minimum and dynamic nature of the chromosphere
• Three-dimensional non-LTE radiative transfer effects in Fe I lines IV. Line formation at high spatial resolution
• Anti-Solar Differential Rotation May Have Revived Magnetic Braking in the Subgiant 31 Aquilae