Observation and characterization of the solar turbulent convection

The turbulent solar magneto-convection is one of the most intriguing phenomena observed in our star. The convection zone regulates the energy transport from the radiative zone to the photosphere (where the photons are free to escape) by means of the bulk displacement of the solar plasma. The motions of convective cells is established by the gravitational field and the vertical temperature gradient and it basically consists in the upflow of the hotter and brighter plasma elements (the so-called granules) and the downflow of the colder and darker plasma elements (the intergranular lanes). The latter are turbulent due to the high stratification in density of the solar convective plasma and they are considered the drivers of the solar convection. This mere scenario is considerably complicated by the presence of the photospheric magnetic fields, which interact with the convection pattern and alter it, originating a large variety of phenomena, such as the appearance of magnetic bright points in the intergranular lanes or the onset of micropores. These phenomena are rather well explained by radiative magnetohydrodynamical (MHD) numerical simulations, which combine the properties of the solar plasma with the presence of the interacting magnetic fields and with the radiative transfer of the light. Despite the good agreement between radiative MHD simulations and high resolution observations, there are still several scientific open questions to be addressed and several processes that are not completely clear. High resolution spectro-polarimetry is the more suitable methodological tool that can be used to infer the physical conditions of the solar convective plasma. In fact, the polarization states at different wavelengths of the incoming solar light own the imprinting of the physical parameters of the solar atmosphere, which can be extracted using inversion techniques. To do this, we need high spatial (< 100 km on the solar surface), spectral (R > 200.000) and temporal (few tenth of seconds) resolution spectro-polarimetric data acquired with top level technology instruments and telescopes. In this PhD thesis, of the PhD program in Astronomy, Astrophysics and Space Science" of the jointly collaboration between University of Rome La Sapienza, University of Rome Tor Vergata and Istituto Nazionale di Astrofisica, I am interested in the study of the physical properties of the solar turbulent magneto-convection using two complementary approaches: data analysis of high-resolution spectro-polarimetric dataset, and design, development and realization of instrumentation for Solar Physics applications. In the Introduction, I present the current knowledge on turbulent solar magneto-convection, the parameters used to describe the convective plasma and the observation evidences compared to the theoretical approach of radiative MHD simulations. Then, I discuss on the open scientific questions on solar convection the instruments and methods needed and the organization of the manuscript. The Second Chapter is devoted to the theory of solar spectro-polarimetry, the radiative transfer and the analysis methods used in this thesis, the Center of Gravity Method (CoG) and the Inversion Techniques. After describing the dataset, I introduce my contribution to the data analysis part of this manuscript. I present a comparison analysis between the CoG method and the inversion techniques, showing evidences that the inversion techniques tend to overestimate weak magnetic fields in Quiet Sun regions. After that, I use the same dataset to evaluate the vertical heat flux maps, a proxy of the entropy production rate, which can be used as a clue to study the solar convection. With this analysis, I obtain strong evidenced that the solar turbulent convection satisfies the simmetry conjecture predicted by the Gallavotti-Cohen Fluctuation Theorem, analyzing the solar convection as a non-equilibrium stationary-state system. The Third Chapter is dedicated to the spectro-polarimetric instrumentation required for the observation of the solar convection. After describing the theory behind the operation of a Fabry-Perot Interferometer (FPI), I present three instrumental activities. I partecipated in the design, assembly and test phases of a FPI prototype controlled with one of the first digital controller, featuring its electronical noise and resulting spectral stability. This kind of digital control could substitute the old analog ones and they will be of fundamental importance for the next generation spectro polarimetric imaging instruments based on FPIs. After that, I realized a feasibility study of a narrow band imager based on large diameter FPIs and off-axis parabolic mirrors, starting from the conceptual design of Greco and Cavallini, using Zemax software. I implemented a new 3D version of the optical scheme, pointing out the improvements and the tolerance problem, and suggesting possible solutions to overcome these instrumental issues. At the end of this Chapter, I present the optical scheme of a full-disk solar synoptic telescope based on the Magneto-Optical Filters (MOF) technology that I entirely designed with Zemax. The new Tor vergata Solar Synoptic Telescope (TSST) will consist in this MOF-based telescope coupled with an Halpha solar telescope, and it will be used for large scale patterns studies, Space Weather applications and are forecasting. The last Chapter summarizes the results that I obtained during my PhD, discussing the achieved scientific impact and instrumental improvements. The thesis is concluded by discussions on future developments of the work done.