Statistical Properties of Cavitons and Spontaneous Hot Flow Anomalies in a Global Hybrid-Vlasov Magnetosphere Simulation

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http://urn.fi/URN:NBN:fi:hulib-202004281952
Title: Statistical Properties of Cavitons and Spontaneous Hot Flow Anomalies in a Global Hybrid-Vlasov Magnetosphere Simulation
Author: Tarvus, Vertti
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2020
Language: eng
URI: http://urn.fi/URN:NBN:fi:hulib-202004281952
http://hdl.handle.net/10138/314588
Thesis level: master's thesis
Degree program: Alkeishiukkasfysiikan ja astrofysikaalisten tieteiden maisteriohjelma
Master's Programme in Particle physics and Astrophysical Sciences
Magisterprogrammet i elementarpartikelfysik och astrofysikaliska vetenskaper
Specialisation: Astrofysikaaliset tieteet
Astrophysical Sciences
De astrofysikaliska vetenskaperna
Discipline: none
Abstract: The magnetic field of Earth interacts with the supersonic solar wind that emanates from the outer part of the Sun’s atmosphere. The interaction results in the formation of Earth’s magnetosphere with a bow shock and a foreshock upstream of it. Together, they form a complex system that hosts a large number of different phenomena, ranging from aurorae visible with the naked eye from Earth’s surface to magnetic waves and transient structures only observable by spacecraft with in-situ measurements. In addition to spacecraft measurements, numerical simulations performed with computers have become increasingly important in space research with the constantly growing amount of available computing power. The topic of this thesis, two types of transient structures found upstream of the bow shock in the foreshock, cavitons and spontaneous hot flow anomalies (SHFAs), are examples of phenomena that have been discovered and studied with the combination of numerical simulations and spacecraft observations. These transient types are related, as cavitons can evolve into SHFAs. In this thesis, cavitons and SHFAs are studied with the global hybrid-Vlasov simulation Vlasiator. The transients are studied statistically in a global simulation for the first time, granting the largest statistical sample up to date. The approach taken in this study is to track individual transients in time, for which purpose a tracking algorithm was developed as a part of this thesis. With this method, the first detailed investigation of the evolution of cavitons and SHFAs is conducted. The statistical results obtained in this work indicate that cavitons and SHFAs form in a uniform region near the bow shock. There is a distinct distance to the shock within which cavitons can become SHFAs, and it is found that SHFAs can either form independently, or evolve from cavitons. The properties of the transients are found to have some dependence on the transients’ location relative to the bow shock. The propagation velocity of the transients is measured, and is found to agree with prior spacecraft observations.


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