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The strongly coupled hydrodynamic, magnetic, and radiation properties of the plasma in the solar chromosphere make it a region of the Sun's atmosphere that is poorly understood. We use data obtained with the high-resolution Visible Broadband Imager (VBI) equipped with an H β filter and the Visible Spectro-Polarimeter (ViSP) on the Daniel K. Inouye Solar Telescope to investigate the fine-scale structure of the plage chromosphere. To aid in the interpretation of the VBI imaging data, we also analyze spectra from the CHROMospheric Imaging Spectrometer on the Swedish Solar Telescope. The analysis of spectral properties, such as enhanced line widths and line depths, explains the high contrast of the fibrils relative to the background atmosphere demonstrating that H β is an excellent diagnostic for the enigmatic fine-scale structure of the chromosphere. A correlation between the parameters of the H β line indicates that opacity broadening created by overdense fibrils could be the main reason for the spectral line broadening frequently observed in chromospheric fine-scale structures. Spectropolarimetric inversions of the ViSP data in the Ca ii 8542 Å and Fe i 6301/6302 Å lines are used to construct semiempirical models of the plage atmosphere. Inversion outputs indicate the existence of dense fibrils in the Ca ii 8542 Å line. The analyses of the ViSP data show that the morphological characteristics, such as orientation, inclination, and length of fibrils, are defined by the topology of the magnetic field in the photosphere. Chromospheric maps reveal a prominent magnetic canopy in the area where fibrils are directed toward the observer. [ABSTRACT FROM AUTHOR]

The bright supergiant Betelgeuse (Alpha Orionis, HD 39801) experienced a visual dimming during 2019 December and the first quarter of 2020, reaching an historic minimum 2020 February 7–13. During 2019 September–November, prior to the optical dimming event, the photosphere was expanding. At the same time, spatially resolved ultraviolet spectra using the Hubble Space Telescope/Space Telescope Imaging Spectrograph revealed a substantial increase in the ultraviolet spectrum and Mg ii line emission from the chromosphere over the southern hemisphere of the star. Moreover, the temperature and electron density inferred from the spectrum and C ii diagnostics also increased in this hemisphere. These changes happened prior to the Great Dimming Event. Variations in the Mg ii k-line profiles suggest material moved outwards in response to the passage of a pulse or acoustic shock from 2019 September through November. It appears that this extraordinary outflow of material from the star, likely initiated by convective photospheric elements, was enhanced by the coincidence with the outward motions in this phase of the ∼400 day pulsation cycle. These ultraviolet observations appear to provide the connecting link between the known large convective cells in the photosphere and the mass ejection event that cooled to form the dust cloud in the southern hemisphere imaged in 2019 December, and led to the exceptional optical dimming of Betelgeuse in 2020 February. [ABSTRACT FROM AUTHOR]

We use daily observations of the Ca II K line profiles of the Sun-as-a-star taken with the Integrated Sunlight Spectrometer from 2006 December through 2011 July to deconvolve the contributions from the quiet (basal) chromosphere and with magnetic network/plage areas. The 0.5Å emission index computed from basal profiles shows a significantly reduced modulation (as compared with one derived from the observed profiles) corresponding to the Sun's rotation. For basal contribution of the Ca iiK line, the peak in power spectrum corresponding to solar rotation is broad and not well defined. Power spectra for the plage contribution show two narrow well-defined peaks corresponding to solar rotation at two distinct latitudes, in agreement with the latitudinal distribution of activity on the Sun at the end of Cycle 23 and beginning of Cycle 24. We use the lack of a signature of solar rotation in the basal (quiet Sun) component as an indication of a successful removal of the active Sun (plage) component. Even though the contribution from solar activity is removed from the basal line profiles, we find a weak dependency of intensity in the line core (K3) of basal profiles with the phase of the solar cycle. Such dependency could be the result of changes in thermal properties of basal chromosphere with the solar cycle. As an alternative explanation, we also discuss a possibility that the basal component does not change with the phase of the solar cycle. [ABSTRACT FROM AUTHOR]

We investigate the feasibility of representing a structured multi-dimensional stellar atmosphere with a single one-dimensional average stratification for the purpose of spectral diagnosis of the atmosphere's average spectrum. In particular, we construct four different one-dimensional stratifications from a single snapshot of a magnetohydrodynamic simulation of solar convection: one by averaging its properties over surfaces of constant height and three by averaging over surfaces of constant optical depth at 500 nm. Using these models, we calculate continuum and atomic and molecular line intensities and their center-to-limb variations. From an analysis of the emerging spectra, we identify three main reasons why these average representations are inadequate for accurate determination of stellar atmospheric properties through spectroscopic analysis. These reasons are nonlinearity in the Planck function with temperature, which raises the average emergent intensity of an inhomogeneous atmosphere above that of an average-property atmosphere, even if their temperature-optical depth stratification is identical; nonlinearities in molecular formation with temperature and density, which raise the abundance of molecules of an inhomogeneous atmosphere over that in a one-dimensional model with the same average properties; and the anisotropy of convective motions, which strongly affects the center-to-limb variation of line-core intensities. We argue therefore that a one-dimensional atmospheric model that reproduces the mean spectrum of an inhomogeneous atmosphere necessarily does not reflect the average physical properties of that atmosphere and is therefore inherently unreliable. [ABSTRACT FROM AUTHOR]

We have explored the impact of magnetic fields on the determination of the solar photospheric oxygen and iron abundances using three-dimensional radiation-magnetohydrodynamic (MHD) simulations of convection. Specifically, we examined differences in abundance deduced from three classes of atmospheres simulated with the MURaM code: a pure hydrodynamic (HD) simulation, an MHD simulation with a local dynamo magnetic field that has saturated with an unsigned vertical field strength of 80 G at τ = 1, and an MHD simulation with an initially imposed vertical mean field of 80 G. We use differential equivalent width analysis for diagnosing abundances derived from five oxygen and four iron lines of differing wavelength, oscillator strength, excitation potential, and Landé g-factor, and find that the morphology of the magnetic field is important to the outcome of abundance determinations. The largest deduced abundance differences are found in the vertical mean field simulations, where the O I and Fe I abundance corrections compared to the pure HD case are ∼+0.011 dex and +0.065 dex respectively. Small scale unresolved field resulting from the local dynamo has a smaller impact on abundance determinations, with corrections of –0.0001 dex and +0.0044 dex in the magnetized compared to the pure HD simulations. While the overall influence of magnetic field on abundance estimates is found to be small, we stress that such estimates are sensitive not only to the magnitude of magnetic field but also to its morphology. [ABSTRACT FROM AUTHOR]