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Characterization of the dynamics of magnetic substorms in the Earth’s magnetosphere

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2019-06
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2019-07-03
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If modern societies were divided into eras based on their technological and scientifc improvements, there is little doubt that these days would belong to the space industry and its contributions to our daily lives. Nowadays, space is the most proftable feld of investment for agencies and private companies. In the light of this, thorough comprehension of the conditions in which satellites work, as well as the external agents afecting facilities and services on ground, is required. Research on space environment is playing a main role, mostly focused on the understanding of the multiple threats appearing in this harsh environment, and the mech- anisms to mitigate the damages derived from them. Among the principal hazards menacing the correct functioning of the technological devices operating in Earth, the efects of the magnetosphere dynamics deserve particular attention. If the dynamics of the magnetosphere could be characterized, consistent methods for prediction and counteraction of their efects would be implemented. On the one hand, geomagnetic storms, occurring at the day side of the magnetosphere, are already known to be a direct consequence of solar activity and their dynamics are fairly well understood. On the other hand, substorms and the dynamics governing the tail of the magnetosphere still remain unknown. According to the principal studies conducted in the feld, two main theories emerge to explain the processes taking place within the magnetotail and the driver causing these dynamics: the forced criticality based on an external driver (solar activity), proposed by T. Chang, and the more standard SOC, where the dynamics of the magnetotail are originated by the complex interactions inherent to this region of the magnetosphere. Lack of consensus on the theoretical models attempting to describe magnetotail dynamics has lead to the implementation of new techniques in the search of answers to this phenomena. Interested in fnding the driver governing these dynamics, the present project has focused on the analysis of time series (signals describing solar wind, solar activity, and magnetic storms and substorms related indexes) by means of a recent statistical tool, called transfer entropy, capable of detecting causal relationships between signals as it has been also demonstrated within the project. Starting from the beginning, due to the novelty of transfer entropy, scarce studies conducted using this tool were found. So, prior to the analysis of the transfer entropy in the real system, a synthetic model designed to contain similar features had to be created for comparison purposes and to test robustness of the tool. Furthermore, due to the intrinsic complexity of the signals to be studied, several considerations had to be taken into account. Noise and abnormal values in the net transfer entropy provoked by periodicities within the signals resulted to be crucial concerns to be carefully analyzed, so the synthetic model allowed us to grasp the response of the tool to these perturbations. Therefore, the frst contribution of the work consisted on the study of the mechanisms and singularities of this statistical technique. Concerning the main objective of the project, the characterization of the intricate dynamics of the magnetotail, entropy transfers were quantifed at diferent timescales to study the evolution of the interaction between solar activity and magnetosphere dynamics. After this analysis, very interesting results have been achieved, and promising features have been revealed for future studies. At short time frames (∼−1h), solar wind afects both the D and AE indexes (although ST efect on D is signifcantly higher), governing the dynamics of the magnetosphere for ST short periods of time. For this timescale, D and AE indexes show similar features, ST responding to the collision of the magnetic feld embedded in the plasma ejected from the Sun with the Earth’s magnetosphere. For the times close to 100h, the time taken for the CME to reach the magnetosphere, high entropy magnitudes appear for the analysis between D index and solar activity, ST while the infuence of solar wind on the magnetotail dynamics signifcantly decreases. Finally, increasing the timescale considered, transfer entropy between the solar activity and the AE index keep diminishing consistently, pointing out a change in magnetotail dynamics with respect to short time frames. And, more precisely, evidencing a switch in the driver forcing the system. The whole picture shows that the response of the magnetotail at short times is governed by the solar wind, which shakes the magnetosphere and triggers disturbances in the entire structure. And, for larger timescales, the intrinsic interactions within the magnetotail are the ones conditioning its dynamics, revealing features more consistent with a self organized criticality system (SOC). All in all, the physics ruling the space environment are very complex. Even though this work has successfully shed some light on this issue, full understanding of the magnetosphere dynamics remains as a challenge for the present and the future of astrophysics.
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Magnetosphere, Magnetotail, Self Organized Criticality, Forced systems, Driven systems, Transfer Entropy, Substorm dynamics, Synthetic model
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