Prospects and challenges of numerical modeling of the Sun at millimeter wavelengths

Wedemeyer, Sven; Fleishman, Gregory D.; Rodríguez, J. de la Cruz; Gunár, S.; Santos, J. M. da Silva; Antolin, Patrick; Gómez, Juan; Szydlarski, Mikołaj; Eklund, Henrik

doi:10.3389/fspas.2022.967878

Cited by 5 publications

(1 citation statement)

References 145 publications

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“…We used the advanced radiative transfer code (ART; de la Cruz Rodríguez et al 2021), which solves the radiative transfer equation under the assumption of local thermodynamics equilibrium (LTE). In this work, ART uses the non-equilibrium electron densities provided by the Bifrost simulations, as this is important for the resulting continuum radiation at sub-mm and mm wavelengths (for an extended discussion of the use of simulations in the context of ALMA, see, Wedemeyer et al 2022). The final data product consists of two time series of continuum brightness temperatures at the frequencies of ALMA Bands 3 and 6 (see Sect.…”

Section: Simulationmentioning

confidence: 99%

The Sun at millimeter wavelengths

Gómez

Jafarzadeh

Wedemeyer

et al. 2023

A&A

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Aims. We used solar observations of a plage-enhanced network with the Atacama Large Millimeter/sub-millimeter Array (ALMA) in Band 3 and Band 6, together with synthetic continuum maps from numerical simulations with Bifrost in the same bands, to carry out a detailed study of bright small-scale magnetic features. Methods. We made use of an algorithm to automatically identify and trace bright features within the field of view (FoV) of the ALMA observations and the simulation. In particular, the algorithm recovers information of the time evolution of the shape, motion of the centre of gravity, temperature, and size for each feature. These quantities are used to determine the oscillatory properties of each feature utilising wavelets analysis. Results. We found 193 and 293 features in the Bands 3 and 6 observations, respectively. In the degraded simulation, the total number of features were 24 for Band 3 and 204 for Band 6. In the original simulation, the total number of features were 36 for Band 3 and 392 for Band 6. Based on the simulation, we confirm the magnetic nature of the features. We have obtained average oscillation periods of 30–99 s for the temperature, 37–92 s for size, and 37–78 s for horizontal velocity. There are indications for the possible presence of transverse (kink) waves with average amplitude velocities of 2.1–5.0 km s−1. We find a predominant anti-phase behaviour between temperature and size oscillations suggesting that the variations of the bright features are caused by compressible fast-sausage magnetohydrodynamics (MHD) modes. For the first time to our knowledge, we estimated the flux of energy of the fast-sausage waves at the chromospheric heights sampled by ALMA as 453–1838 W m−2 for Band 3 and 3640–5485 W m−2 for Band 6. Conclusions. We have identified MHD waves, both transverse (kink) and compressible sausage modes, in small-scale (magnetic) structures, independently, in both ALMA Band 3 and Band 6 observations, along with their corresponding synthetic images from simulations. The decrease of wave energy-flux with height (from Band 6 to Band 3) could possibly suggest energy dissipation at chromospheric heights, namely, wave heating, with the assumptions that the identified small-scale waves are typical at each band and they propagate upward through the chromosphere.

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Section: Simulationmentioning

confidence: 99%

The Sun at millimeter wavelengths

Gómez

Jafarzadeh

Wedemeyer

et al. 2023

A&A

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Comparison of chromospheric diagnostics in a 3D model atmosphere

Pandit¹,

Wedemeyer²,

Carlsson³

et al. 2023

A&A

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Context. The Hα line, one of the most studied chromospheric diagnostics, is a tracer of magnetic field structures, while the intensity of its line core provides an estimate of the mass density. The interpretation of Hα observations is complicated by deviations from local thermodynamic equilibrium (LTE) or instantaneous statistical equilibrium conditions. Meanwhile, millimetre (mm) continuum radiation is formed in LTE, and therefore the brightness temperatures from Atacama Large Millimetre-submillimetre Array (ALMA) observations provide a complementary view of the activity and the thermal structure of stellar atmospheres. These two diagnostics together can provide insights into the physical properties of stellar atmospheres, such as their temperature stratification, magnetic structures, and mass density distribution. Aims. In this paper, we present a comparative study between synthetic continuum brightness temperature maps at mm wavelengths (0.3 mm to 8.5 mm) and the width of the Hα 6565 Å line. Methods. We used the 3D radiative-transfer codes Multi3D and Advanced Radiative Transfer (ART) to calculate synthetic spectra for the Hα line and the mm continua, respectively, from an enhanced network atmosphere model with non-equilibrium hydrogen ionisation generated with the state-of-the-art 3D radiation magnetohydrodynamics (rMHD) code Bifrost. We use a Gaussian point spread function (PSF) to simulate the effect of ALMA’s limited spatial resolution and calculate the Hα versus mm continuum correlations and slopes of scatter plots for the original and degraded resolution of the whole box, quiet sun, and enhanced network patches separately. Results. The Hα linewidth and mm brightness temperatures are highly correlated and the correlation is highest at a wavelength of 0.8 mm, that is, in ALMA Band 7. The correlation systematically increases with decreasing resolution. On the other hand, the slopes decrease with increasing wavelength. The degradation of resolution does not have a significant impact on the calculated slopes. Conclusions. With decreasing spatial resolution, the standard deviations of the observables, Hα linewidth, and brightness temperatures decrease and the correlations between them increase, but the slopes do not change significantly. These relations may therefore prove useful in calibrating the mm continuum maps observed with ALMA.

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Science development study for the Atacama Large Aperture Submillimeter Telescope (AtLAST): Solar and stellar observations

Wedemeyer,

Barta,

Brajša

et al. 2024

Open Res Europe

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Observations at (sub-)millimeter wavelengths offer a complementary perspective on our Sun and other stars, offering significant insights into both the thermal and magnetic composition of their chromospheres. Despite the fundamental progress in (sub-)millimeter observations of the Sun, some important aspects require diagnostic capabilities that are not offered by existing observatories. In particular, simultaneous observations of the radiation continuum across an extended frequency range would facilitate the mapping of different layers and thus ultimately the 3D structure of the solar atmosphere. Mapping large regions on the Sun or even the whole solar disk at a very high temporal cadence would be crucial for systematically detecting and following the temporal evolution of flares, while synoptic observations, i.e., daily maps, over periods of years would provide an unprecedented view of the solar activity cycle in this wavelength regime. As our Sun is a fundamental reference for studying the atmospheres of active main sequence stars, observing the Sun and other stars with the same instrument would unlock the enormous diagnostic potential for understanding stellar activity and its impact on exoplanets. The Atacama Large Aperture Submillimeter Telescope (AtLAST), a single-dish telescope with 50m aperture proposed to be built in the Atacama desert in Chile, would be able to provide these observational capabilities. Equipped with a large number of detector elements for probing the radiation continuum across a wide frequency range, AtLAST would address a wide range of scientific topics including the thermal structure and heating of the solar chromosphere, flares and prominences, and the solar activity cycle. In this white paper, the key science cases and their technical requirements for AtLAST are discussed.

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Prospects and challenges of numerical modeling of the Sun at millimeter wavelengths

Cited by 5 publications

References 145 publications

The Sun at millimeter wavelengths

The Sun at millimeter wavelengths

Comparison of chromospheric diagnostics in a 3D model atmosphere

Science development study for the Atacama Large Aperture Submillimeter Telescope (AtLAST): Solar and stellar observations

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