Periodic Reporting for period 1 - IMagE (Impact of Magnetic field on Emergent solar spectra)
Berichtszeitraum: 2019-01-01 bis 2020-12-31
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.
To achieve the objectives of the action, we used state-of-the-art solar observations, simulations of the evolution of magnetic field on the solar surface, SATIRE model for solar irradiance variability, and RT codes RH and NESSY. We synthesised spectra of the quiet-Sun and magnetic features using the RT code NESSY in an optimal non-LTE setup. We tested this setup to reproduce the observations from Atacama Large Millimeter/submillimeter Array and James Clerk Maxwell Telescope. This led to the development of a new model for the sunspot umbra (dark central area of a sunspot) with a better constrained chromosphere for the first time. We also modelled the solar chromospheric emission variations in the near-UV Ca II H&K spectral lines. For a long time, Ca II emission has served as the best proxy for solar and stellar magnetic activity. However, many aspects of this emission remain unexplored. We used the observed surface distribution of solar magnetic features in combination with their non-LTE spectra, in SATIRE model for solar irradiance variability and reproduced the observed variability of the near-UV emission on timescales of the solar activity cycle and the solar rotation. Using this model along with simulations of magnetic field evolution on the surface, we showed that the Sun’s seemingly strong Ca II variability is a bias introduced by the Sun being observed from its near-equatorial plane and during a period of relatively high magnetic activity (Fig. 1). Consequently, there is nothing unusual in the variability of solar Ca II emission. The announcement of the third early release of the data from Gaia space mission and the anticipation of the full data release led to a strong interest in the role of stellar magnetic activity in detecting exoplanets using astrometry technique, which measures the stellar wobbles created by the star-planet interaction. We used the tools developed during the action to calculate the jitter due to magnetic activity (i.e. the displacement of the star’s brightness centre caused by magnetic activity) and showed that the magnitude of the jitter for the Sun observed from different vantage points is comparable to the astrometric signal generated by the Earth (Fig. 2). These results further our understanding of the solar near-UV variability, solar-stellar connection and the role of stellar magnetic activity in the search for exoplanets. The action and the newly fostered collaborations through it has led to two peer-reviewed publications (available on arXiv with free access) and five more are in preparation. In addition, the action results were widely disseminated to the scientific community through seminars, oral and poster presentations at international conferences.
The main outcomes of the action are the newly developed models that have advanced our understanding of a) solar irradiance variability in the near-UV b) solar-stellar connection particularly with respect to non-LTE chromospheric emissions c) potential impact of magnetic activity on the detection of Earth-like planets around stars with solar-like magnetic activity. The model of the sunspot umbra that we developed has a better constrained chromosphere. This goes beyond the existing umbral models which either lack the chromosphere or have a poor representation of it. This model is anticipated to be useful for analysing the data from future missions such as SUNRISE-III and Aditya-L1 to further advance our understanding of the solar UV irradiance variability. The model to compute near-UV Ca II emission variations helped us to partly remove important biases in the solar-stellar comparison. Several studies found that the variability of solar Ca II emission is higher than in stars with near-solar magnetic activity, indicating that solar activity cycle is anomalous with respect to stars with near-solar magnetic activity. We showed that the Sun’s seemingly strong variability in Ca II is a bias introduced due to the Sun being observed from the ecliptic plane and during a period of high magnetic activity, thus, unambiguously showing that the Sun has an absolutely normal variability in Ca II emission. Our results are expected to motivate further studies on the chromospheric emissions and UV variability of Sun and other stars. We used the tools developed to study the influence of stellar magnetic activity on the detection of exoplanets with the astrometry technique. We showed that the astrometric jitter of the Sun observed from different vantage points is comparable to the astrometric signal that Earth going around the Sun would generate. Further work in this regard will be of significance to using astrometric measurements from Gaia and upcoming Small-JASMINE missions for exoplanet detections.