Browsing by Author "Lepreti, Fabio"

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Effects of the solar activity on space weather and earth's climate
(2017-06-13) Alberti, Tommaso; Carbone, Vincenzo; Lepreti, Fabio
The large variability of the physical conditions of the Sun, over a wide range of spatial and temporal scales, represents the primary source which determines global and local changes inside the heliosphere and, what is perhaps more interesting, in the near Earth space. However, due to the extreme complexity of the system, nonlinear interactions among di erent parts of the Sun{Earth system play a key role, enormously increasing the range of physical processes involved. Indeed, uctuations in the magnetic eld within the solar atmosphere act as a complex modulation of plasma conditions in the interplanetary space, producing sudden enhancements of the solar energetic particles (SEP) uxes and cosmic rays, as well as sudden coronal mass ejections (CMEs), or solar irradiance changes in several spectral ranges (from UV to visible). These events are associated with the origin of geomagnetic storms, which have important e ects on our technological society, and possibly on global changes in the climate conditions through complex interactions with the Earth's atmosphere. The investigation of the physical processes which mainly a ect solar and interplanetary space conditions, and the observation and understanding of the interactions of the solar wind with the Earth's magnetosphere are crucial to be able to predict and mitigate those phenomena that a ect space and ground infrastructures or impair the human health. This thesis addresses, through both data analysis and theoretical models, some of the main issues concerning the nature of the variability of solar activity which a ect Space Weather and Earth's climate. The solar wind{magnetosphere coupling during geomagnetic storms is investigated considering the two events occurred on March 17, 2013 and the same day of 2015, well{known as St. Patrick's Day storms. To this purpose, we analyze interplanetary magnetic eld and energy transfer function (i.e., known as Perreault{Akasofu coupling function) time series to study the solar wind variability, as well as geomagnetic indices, related to the ring current and auroral electrojets activity, to investigate their response to solar wind variations. Through the Empirical Mode Decomposition (EMD) we identify the intrinsic oscillation timescales in both solar and magnetospheric time series. A clear timescale separation between directly driven processes, through which solar wind a ects magnetospheric current systems, and loading{unloading processes, which, although triggered by solar wind variations, are related to the internal dynamics of the magnetosphere, is found. These results are obtained by the combined analysis between EMD and information theory (i.e., Delayed Mutual Information analysis) allowing us to investigate linear and non{linear coupling mechanisms, without any assumptions on the linearity or stationarity of the processes. By using both geostationary and ground{based observations of the Earth's magnetic eld, we investigate, then, the role of the ionosphere into the variations of the geomagnetic eld, during both quiet and disturbed periods. We also provide a separation of both magnetospheric and ionospheric signatures in the geomagnetic eld as well as the large{timescale contribution which could be useful to de ne a new local index to monitor geomagnetic activity, since it is free from any magnetospheric or ionospheric contribution. In the framework of the short{term e ects of solar activity on Earth's environment, we investigate the occurrence of SEP events in both solar cycles 23 and 24 and we validate a short{term prediction model (termed ESPERTA) on a new database, di erent from that on which it was previously evaluated. We found a reduction of SEP events occurrence of 40%, suggesting that several di erences can be found between the latter two solar cycles. Although these di erences, the performance of the ESPERTA model are quite similar in both periods, con rming the robustness and e ciency of the model. Concering solar{terrestrial relations on larger timescales we propose two di erent climate models to investigate the role of solar irradiance changes on the stability of the Earth climate as well as the e ects of greenhouse variations on the planetary surface temperature. We nd that the greenhouse e ect plays a key role into the stabilization and self{regulation properties of the Earth climate and that solar irradiance changes could a ect the evolution of Earth's climate. Interestingly, for the present conditions of solar irradiance an oscillatory behavior is found with temperature uctuations T 3 K and oscillations on 800{yr timescale that needs to be investigated with more accuracy because it can reproduce several quasi{periodic behaviors observed in climatic time series. Moreover, by analyzing the time{behavior of oxygen isotope 18O during the last glacial period (i.e., 20{120 kyr before present) we nd that the climate variability is governed by physical mechanisms operating at two di erent timescales: on 1.500{yr timescale, climate dynamics is related to the occurrence of fast warming events, known as Dansgaard{Oeschger (DO) events, while on multi{millennial timescales, climate variations are related to the switch between warming/cooling periods. While DO events can be seen as uctuations within the same climate state, warming/cooling phases are associated to uctuations between two climate states, characterized by global increase/decrease of temperature. Finally, the results of cross{correlation analysis show that Antarctic climate changes lead those observed in Northern Hemisphere with a time delay of 3 kyr, which could be related to the oceanic thermohaline circulation.
Particle acceleration at shocks and magnetic turbulence in the interplanetary space
(2019-06-14) Chiappetta, Federica; Carbone, Vincenzo; Lepreti, Fabio
The solar wind is a supersonic and super-alfvenic flow of plasma that propagates in space up to the Earth and throughout the heliosphere reaching speeds of about 400 􀀀 800 km s􀀀1. It permeates the heliosphere and is a fantastic laboratory for plasma physics, since it is the only astrophysical environment in which spacecrafts can provide in situ measurements of the relevant physical parameters. Embedded within the solar wind plasma is the interplanetary magnetic field. The interaction of the interplanetary field with the magnetic field of the Earth determines the formation of a magnetosphere, in which the magnetic field of the Earth is confined, bounded by a discontinuity between the two fields, called magnetopause. Magnetic field is at the origin of most of the phenomena that are observed in the various layers of the solar atmosphere, called solar activity. Flares and coronal mass ejections (CMEs) are some of the most spectacular and interesting manifestations of solar activity that can generate shock waves in interplanetary space. A shock is a discontinuity, characterized by a sudden change in pressure, temperature and density of the medium. Solar flares and CMEs can release energetic particles (Solar Energetic Particles - SEPs) that travel faster than the particles already present in the interplanetary space plasma. SEPs, following the interplanetary magnetic field, can reach the Earth in an hour or less and are of particular interest because they can cause damage to the electronic instruments on board the space probes, influence communications and navigation systems and endanger astronauts’ life in orbit, especially the particles with energy greater than 40 MeV. During its expansion, the solar wind develops a strong turbulent character, which evolves towards a state similar to that of hydrodynamic turbulence, described by Kolmogorov (1941). The low frequency fluctuations are generally described by magnetohydrodynamics (MHD). The magnetohydrodynamic turbulence in the solar wind has been studied in great detail in recent years, thanks to the numerous spacecrafts that have been launched in the interplanetary space since the beginning of the space age. This work concerns the study of energetic protons at interplanetary shocks, the related acceleration mechanisms and the connection to magnetic turbulence in the upstream and downstream regions of the shocks. In particular, we performed a correlation analysis between the particle flux enhancements and the magnetic field turbulence observed in the upstream and downstream regions of interplanetary shocks. The data used in the analysis are taken by the Stereo Ahead spacecraft and cover a period from 2009 to 2016. The interplanetary shocks selected are divided into two lists: the first contains 24 events that show an increase of the proton flux close to the shock itself; instead, the second includes 14 events that present flux enhancements more distant from the shocks. In order to quantify the magnetic field turbulence, we used the total wave power, calculated from the standard spectral analysis methods. Because of the low correlation obtained, in the case of the first list we separated shocks occurring on the wake of a SEP event from NO SEP events. On the contrary, this is not possible for the shocks of the second list due to the smaller number of events. We also performed a parametric and non-parametric correlation analysis to study the degree of compressibility in the upstream and downstream regions of interplanetary shocks for both lists of selected events, using the variance of the magnetic field. Moreover, in order to have information on the propagation and acceleration of particles in the interplanetary space, we studied the evolution of the particle energy spectra for shocks associated with the SEP events of the first list. In particular, we identify two types of distribution that well fit the spectra: a Weibull functional form, obtained for quasi-perpendicular shocks and a double power law in the case of quasi-parallel shocks. Thanks also to the combined study of the proton flux enhancements with the Mach number and the shock angle, we identify the shock surfing acceleration as the acceleration mechanism suitable to explain the particle spectra at interplanetary quasi-perpendicular shocks. Finally, concerning fluctuations of the magnetic field in the interplanetary space, we studied high-frequency dynamics, a problem that is still open and not entirely clear. Unlike magnetic fluctuations in the range of kinetic scales, those at low frequencies have been extensively investigated and show a universal scaling behavior, described in the nonlinear turbulent energy cascade framework At small scales (high frequencies), instead, the plasma dynamics in the interplanetary space is extremely complex, since it exhibits simultaneously a dispersive and dissipative character. Therefore, we introduced a Brownian approach that provides a simple description of the high-frequency dynamics of magnetic fluctuations, which is able to successfully reproduce the spectra of the fluctuations observed at high frequencies. This framework allows an interpretation of the observed high frequency magnetic spectra with no assumptions about dispersion relations from plasma turbulence theory