Properties and correlation of flares and coronal mass ejections and their possible relevance on the southern night sky background: a statistical study

Abstract

This Thesis is devoted to a statistical study of solar phenomena (flares and coronal mass ejections) and their possible effects on the Earth. The Sun, our star, can be considered a huge laboratory where we can study the interaction of a ionized gas with magnetic fields. In particular, the solar atmosphere (the outer layers of the Sun, those which are accessible to observations) is characterized by phenomena that due their existence to localized magnetic fields: sunspots, faculae, filaments, active regions, bright points, coronal holes, etc. The occurrence of these phenomena is variable, depending on the so-called activity cycle, characterized by a period of 11 years. Moreover, in some situations, the magnetic field that permeates the active regions, from an initial potential field configuration (characterized by the minimum energy content), can slowly store energy, changing its configuration to more complex and more energetic ones. When the magnetic field configuration is not able to maintain its equilibrium, the stored magnetic energy is abruptly released, giving rise to phenomena that are generally termed as solar eruptions but that, depending on their characteristics, are distinguished between flares, filament eruptions and coronal mass ejections. In the last decades, the possibilities offered by new computer capabilities, new instruments and by satellite observations, have allowed us to understand many of the characteristics of these eruptions, as well as their effects on the Earth magnetosphere and ionosphere. However, despite the progress in our comprehension of these phenomena, there are still many aspects that need to be clarified: how and where is the energy stored, what causes the trigger of the eruption, how the different phenomena are related to each other and how they can affect our environment, to cite only a few. In this scenario, the work carried out in this Thesis has been motivated by three main questions: -- Are there preferential locations on the Sun where the magnetic field is prone to produce eruptive events ? -- What kind of correlation exists between flares (mainly confined to the solar atmosphere) and coronal mass ejections that, by definition, expel magnetized clouds into the interplanetary space ? -- Can the charged particles emitted during these events and arriving to the Earth ionosphere have a role in the observed variations of the night-sky background? The attempt to provide answers to the previous questions has been faced in this Thesis from an observational / statistical point of view. More precisely, the dataset that have been used in order to answer the first two questions have been retrieved from public archives of flares and coronal mass ejections relevant to the last two solar cycles (23 and 24), while in order to provide an answer to the third question, also data acquired by the Pierre Auger Observatory have been used. The main results obtained in this Thesis can be summarized as follows: -- The spatial and temporal distribution of the flares analyzed show persistent domains of occurrence within well defined belts of longitude, with a behavior similar to the one observed for other activity phenomena, like the sunspots. -- There is a temporal correlation between flares and CMEs for the 60 % of the events analyzed; the time interval (between 10-130 minutes) however depends on the dataset used. Moreover, the majority of CMEs with highest velocities show a clear temporal correlation with flares. -- The variations in the night-sky background analyzed for the nights when a major impact of charged particles associated to CMEs was expected, could not be clearly correlated to these events.