The Sun at millimeter wavelengths

Henrik Eklund,Sven Wedemeyer,Mikołaj Szydlarski,Shahin Jafarzadeh

doi:10.1051/0004-6361/202140972

Abstract

Context. Interferometric observations of the Sun with the Atacama Large Millimeter/sub-millimeter Array (ALMA) provide valuable diagnostic tools for studying the small-scale dynamics of the solar atmosphere. Aims. The aims are to perform estimations of the observability of the small-scale dynamics as a function of spatial resolution for regions with different characteristic magnetic field topology facilitate a more robust analysis of ALMA observations of the Sun. Methods. A three-dimensional model of the solar atmosphere from the radiation-magnetohydrodynamic code Bifrost was used to produce high-cadence observables at millimeter and submillimeter wavelengths. The synthetic observables for receiver bands 3–10 were degraded to the angular resolution corresponding to ALMA observations with different configurations of the interferometric array from the most compact, C1, to the more extended, C7. The observability of the small-scale dynamics was analyzed in each case. The analysis was thus also performed for receiver bands and resolutions that are not commissioned so far for solar observations as a means for predicting the potential of future capabilities. Results. The minimum resolution required to study the typical small spatial scales in the solar chromosphere depends on the characteristic properties of the target region. Here, a range from quiet Sun to enhanced network loops is considered. Limited spatial resolution affects the observable signatures of dynamic small-scale brightening events in the form of reduced brightness temperature amplitudes, potentially leaving them undetectable, and even shifts in the times at which the peaks occur of up to tens of seconds. Conversion factors between the observable brightness amplitude and the original amplitude in the fully resolved simulation are provided that can be applied to observational data in principle, but are subject to wavelength-dependent uncertainties. Predictions of the typical appearance at the different combinations of receiver band, array configuration, and properties of the target region are conducted. Conclusions. The simulation results demonstrate the high scientific potential that ALMA already has with the currently offered capabilities for solar observations. For the study of small-scale dynamic events, however, the spatial resolution is still crucial, and wide array configurations are preferable. In any case, it is essential to take the effects due to limited spatial resolution into account in the analysis of observational data. Finally, the further development of observing capabilities including wider array configurations and advanced imaging procedures yields a high potential for future ALMA observations of the Sun.

Highlights

The Atacama Large Millimeter/sub-millimeter Array (ALMA) provides great advances in observing the small-scale dynamics of the solar atmosphere for the more direct measurement of chromospheric temperatures as compared to other diagnostics
The magnetic topology of the simulation from the photosphere to the chromosphere and transition region is illustrated in Fig. 1 of Jafarzadeh et al (2017), where the authors show that the height of magnetic canopies throughout the solar atmosphere depends on the strength of the magnetic fields at their footpoints in the low photosphere
Choosing the correct combination of array configuration and receiver band is essential in view of the requirements of a set science goal because it determines the achievable spatial resolution

Summary

Introduction

The Atacama Large Millimeter/sub-millimeter Array (ALMA) provides great advances in observing the small-scale dynamics of the solar atmosphere for the more direct measurement of chromospheric temperatures as compared to other diagnostics. Some of them address the small-scale dynamics, for example, that of magnetic field loop structures (Wedemeyer et al.2020), on-disk type II spicules (Chintzoglou et al 2021a), spicules at the limb (Shimojo et al 2020), and transient brightening events with corresponding brightness temperature evolution (Shimojo et al 2017; Molnar et al 2019; da Silva Santos et al 2020; Nindos et al 2020; Eklund et al 2020; Chintzoglou et al 2021b), spectral sub-bands (SB) are used to estimate the optical thickness (Rodger et al 2019), and the dynamical structure in and around sunspots (Jafarzadeh et al 2019) and of the oscillations in ALMA observations (Patsourakos et al 2020; Guevara Gómez et al 2021; Jafarzadeh et al 2021) is studied. The spatial sizes of the detected events show a distribution that increases in number toward the lower angular resolution limit (1 . 4 − 2 . 1), suggesting that there may be Article published by EDP Sciences

Methods

Results

Discussion

Conclusion