IMagE · Impact of Magnetic field on Emergent solar spectra

Impact of Magnetic field on Emergent solar spectra

Solar brightness varies on all measured timescales and wavelengths. While variations on timescales shorter than about 10 hours are caused mainly by the solar granulation and oscillations, variations on longer timescales are driven by the solar surface magnetic activity. The magnetic field is generated by the dynamo acting in the interior of the Sun and emerges on the solar surface forming dark (spots and pores) and bright (faculae and network) features. The magnetic field modifies the structure of the solar atmosphere and its radiative properties, and defines the brightness of the magnetic features. As a result, the magnetically driven solar irradiance variations are modelled as the combined effect of surface magnetic features. Despite the significant progress in modelling the solar brightness variations, their magnitude in the ultraviolet (UV) range remains uncertain. Evidence suggests that the solar UV irradiance variability affects the Earth’s atmosphere and climate. However, the exact mechanism through which this occurs is still poorly understood. An accurate reconstruction of the UV variability is therefore of prime importance for climate modelling. The solar UV variability is dictated by the spectral lines and continuum which are strongly influenced by the effects resulting from high temperature gradients and low density conditions in the solar atmosphere. They are called non-local thermodynamic equilibrium (non-LTE) effects. The non-LTE represents a situation where the radiation is decoupled from the local properties of the medium. Another effect which influences UV variability is the line blanketing (i.e. the reduction in the continuum caused by many overlapping spectral lines). A proper inclusion of such effects in irradiance modelling is key to understand solar UV variability and the role of the Sun in climate change. To this end, the main objective of the project was to synthesise brightness spectra of the quiet-Sun and solar magnetic features including the effects of line blanketing and non-LTE, by combining the state-of-the-art observations with fast and reliable radiative transfer (RT) calculations. The objective of the action was successfully achieved and the tools developed during the course of the project were used to model the variability of the near-UV Ca II H&K emission originating from the chromosphere, a layer of the solar atmosphere between the photosphere (the visible solar surface), and the hot outer corona. Our calculations revealed that the Ca II H&K emission variations of the Sun are completely normal in comparison to stars with solar-like magnetic activity, thus, advancing our understanding of the solar near-UV variability and solar-stellar connections.

Data: CORDIS, © European Union