Dottorato in ASTRONOMIA

Titolo della tesi: Photospheric Magnetic Field Variability as a Driver of Solar Activity

The Sun exhibits significant variability driven by its magnetic field that influence both the radiative and the particle output across various temporal and spatial scales. Photospheric magnetic fields serve as a crucial boundary condition for the magnetic fields in the upper atmospheric layers. In this PhD project, I investigated the role of photospheric magnetic fields in causing some manifestations of solar activity, namely Coronal Holes (CHs) and Total Solar Irradiance (TSI) variations. CHs are regions of the solar atmosphere characterized by a reduced emissivity in the Extreme Ultra Violet (EUV) and soft X-ray. They are the source regions of the fast solar wind, which has a pivotal role in Space Weather. TSI is the electromagnetic energy emitted by the Sun per unit time and per unit area that falls outside the Earth’s atmosphere at a distance of one astronomical unit, integrated over the whole spectrum. TSI represents the forcing of terrestrial climate models. In this thesis, firstly, I studied the properties of the photospheric magnetic fields associated to CHs. CH boundaries have been defined from images of the solar corona in the EUV and projected onto magnetograms, i.e. maps of the photospheric magnetic fields of the Sun. I analyzed then the properties of the photospheric magnetic fields of 60 CHs, compared with the properties of the fields of 60 regions not associated to CHs (NCHs), used as a control group. I studied the distributions of the photospheric fields of all the events, confirming that the photospheric fields of CHs are imbalanced in the polarity: photospheric fields of CHs are characterized by a dominant polarity. I used the sign singularity analysis to characterize the scaling behaviours of spatial oscillations of the fields, finding that the imbalanced of the fields of CHs is associated mainly to the supergranular convective scales. This conclusion has been sustained by simple magnetic pattern simulations of the regions of interest considered in the previous analysis. I further studied the diffusivity properties of the photospheric magnetic fields of a statistics of 17 CHs. In particular I analyzed the evolution in time of photospheric magnetic flux concentrations (magnetic elements) of the different regions considered, tracking their position in time. From this analysis I have found that magnetic elements which have the dominant polarity of the CHs diffuse in a less efficient way with respect to those having the non-dominant polarity. This result sustains what found with the previous analysis, i.e. the fact that CH imbalance emerges mainly at the supergranular scales. In the second part of the thesis I investigated the role of the photospheric fields in modulating the TSI. I contributed to the reconstruction of the TSI from 1513 to 2001. The model used here incorporated the effect on TSI variability of sunspots, faculae, and a long-term component associated to the quiet magnetic network, revealing a TSI variation of 2.5 Wm^-2 between the Maunder minimum and the present epoch. Additionally, relying on the connection between photospheric fields and TSI variability, I contributed to the prediction of the TSI intensity during the 25° solar cycle. The analysis extended to studying the evolution of a small pore, i.e. small regions of the solar atmosphere which if observed in the visible appear like small dark feature. In this analysis I related the evolution of its area and emissivity (in the visible) to the evolution of the associated photospheric magnetic fields. This study revealed the role of the magnetic fields in modulating plasma radiative properties, inhibiting locally convective motions and then reducing the efficiency of the transport of the heat. In conclusion, this thesis underscores the central role of photospheric magnetic fields in shaping and affecting the solar atmosphere, modulating the radiative emissivity and the particle output of the Sun.