Unit:
Department of PhysicsLibrary of the School of Science
Author:
Sinnis Charalampos
Dissertation committee:
Νεκτάριος Βλαχάκης, Καθηγητής, τμήμα Φυσικής, ΕΚΠΑ
Κανάρης Τσίγκανος, Ομότιμος Καθηγητής, τμήμα Φυσικής , ΕΚΠΑ
Ιωάννης Κοντόπουλος, Διευθυντής Ερευνών, ΚΕ.Α.Ε.Μ, Ακαδημία Αθηνών
Θεοχάρης Αποστολάτος, Καθηγητής, τμήμα Φυσικής, ΕΚΠΑ
Στυλιανός Καζαντζίδης, Επίκουρος Καθηγητής, τμήμα Φυσικής, ΕΚΠΑ
Κωνσταντίνος-Νεκτάριος Γουργουλιάτος, Αναπληρωτής Καθηγητής, τμήμα Φυσικής, Πανεπιστήμιο Πατρών
Μαρία Πετροπούλου, Επίκουρη Καθηγήτρια, τμήμα Φυσικής, ΕΚΠΑ
Original Title:
Stability Analysis of Relativistic Magnetized Astrophysical Jets
Summary:
One special trait of astrophysical jets is their enhanced structural stability, as their total length can be many times their initial radii. Although it is established from previous studies that instabilities develop along their flow, it is not fully understood how they affect the outflow properties. This thesis focuses on this specific scientific question, thus the stability properties of astrophysical jets are studied.
The term astrophysical jet involves a family of outflows that share the same formation and propagation mechanisms, i.e. an accretion disk is formed around a massive object which accretes into this central engine, leading to the creation of these cosmic outflows. The physical quantities describing the jets span over an extended value range. For example, there are jets originated from protostars (YSO jets) which are non-relativistic with total lengths of ~pc. On the contrary, there are jets originated from the accretion of matter into a supermassive black hole (AGN jets) which are relativistic and they travel distances ~kpc.
Throughout this thesis the dynamics of these outflows are described by the relativistic magnetohydrodynamic (RMHD) set of equations which consists of the equations of Maxwell coupled with three continuity equations for mass, momentum and energy and Ohm's law in the case of infinite conductivity. Finally, in order for the system to close an equation of state for the plasma is included. In order to study the stability properties of the jets the linear stability analysis methodology is utilized. This methodology requires to insert small perturbative terms in the set of equations that describe the dynamics of the outflow and expand the equations regarding terms up to the first order. The resulting linearized system of equations is essentially a boundary conditions problem. The thesis focuses on the unstable solutions of this system and their properties.
In terms of outflow configuration, the main point of interest is around mildly relativistic magnetized astrophysical jets. These jets are typical in the case of AGN outflows. They carry magnetic field usually characterized by a helical topology, and their rotation is slight or non-existent. Two main kinds of instabilities emerge from this type of jets. The first one is related to shear in the velocity profile or when two fluids with different velocity values are in contact. This is the Kelvin-Helmholtz instability. The second instability is associated with the existence of the magnetic field in the outflow, they are called current-driven instabilities. Depending on the jet configuration these two types of instability may emanate, evolve and affect the initial configuration. The effect on the outflow varies, the initial outflow may be disrupted and evolve into a new quasi-steady state or be destroyed entirely.
The thesis at hand studies the stability properties of relativistic magnetized astrophysical jets. The configurations that are probed include cylindrical outflows carrying helical magnetic fields with bulk flow velocities corresponding mainly to mildly relativistic jets. The first configuration for which the stability properties are presented is a two-component jet, most commonly known as a spine-sheath outflow. Alongside the results from the linear stability analysis, there are also numerical simulations that examine the non-linear evolution of the perturbed outflow. The results from the two different methodologies are in agreement, while the most important parameter affecting the intensity of the instabilities is magnetization. The second configuration is a single-component magnetized mildly relativistic jet. The stability profile of this specific configuration is characterized by the existence of the magnetized Kelvin-Helmholtz instability which is generalized in the relativistic regime for a cylindrical outflow geometry. The stability analysis successfully identifies the regulating parameters of the mode's behavior, while it is shown that under specific circumstances the Kelvin-Helmholtz instability of a cylindrical jet can be approximated by the corresponding results of a Cartesian counterpart.
Main subject category:
Science
Keywords:
instabilities, magnetohydrodynamics (MHD), methods: analytical, jets and outflows, galaxies: jets, relativistic processes
Number of references:
116
