Abstract
The Sun, our nearest star, serves as an invaluable reference for the study of solar-like stars, due to the availability of many spatiotemporally resolved solar spectra. Amongst several spectral lines, some of the strongest chromospheric diagnostics are the Ca II H & K lines which can be used to gauge the temperature stratification of the atmosphere as the line core and wings are formed at different heights of the solar atmosphere. Furthermore, the Hα line is a tracer for the magnetic structures and its line core provides an estimation of the mass density.
Complementing these diagnostics are brightness temperatures derived from observations made by the Atacama Large Millimeter (mm)/ submillimeter (sub-mm) Array (ALMA). These data open up fresh perspectives on stellar atmospheric activity and thermal structures. First, we compare synthetic Ca II and Hα spectra with their corresponding millimetre continuum maps to gain deeper insights into the stellar structure.
To obtain synthetic spectra for the Ca II lines, Hα line, and mm-sub mm continuum, this work employs radiative transfer codes such as RH1.5D, ART, and Multi3D. These simulations are based on an enhanced network atmosphere model created using the state-of-the-art Bifrost code. The resulting activity indices are then used to compare solar-like stars with the solar spectrum. These indices reveal crucial details about temperature stratification, magnetic structures, and mass density distribution in stellar atmospheres. Notably, we find strong correlations between the Hα linewidth and ALMA Band 7 data, the Ca II s index and the lower chromosphere, and the Ca II Infrared triplet (IRT) index and an even lower atmospheric layer near the upper photosphere.
Furthermore, we undertake long-term full-disk (FD) total power (TP) observations from ALMA and compare them with other diagnostic data sources, including UV-EUV data from Solar Dynamics Observatory Atmospheric Imaging Assembly (SDO/AIA) and microwave data from the Radio Polarimeter from the Nobeyama observatory, as well as s index data. The first long-term solar activity variation using ALMA TP data is observed in this work. There exists notable short-term variations as well. As a result, a systematic utilisation of a combination of the SDO/AIA 304, 1600, and 1700, channels, which originate from comparable altitudes and display strong correlations with ALMA data, might be worth considering for enhancing the TP calibration process.
In summary, this PhD project revolves around a comparative exploration of solar and stellar activity among Sun-like stars. The investigations are based on observations from ALMA, in conjunction with other solar and stellar observational and synthetic data.