Dissertation committee:
Καθηγητής Κανάρης Τσίγκανος (ΕΚΠΑ), Δρ. Κωνσταντίνος Γοντικάκης (Ακαδημία Αθηνών, ερευνητής Β), Lecturer Βασίλης Αρχοντής (University of St. Andrews)
Summary:
This is the PhD Thesis of Petros Syntelis on the topic of “Magnetic flux
emergence, solar Jets
and Coronal Mass Ejections”. The PhD program is offered by the National and
Kapodestrian University of Athens, in the School of Science, at the Faculty of
Physics.
In the first part of this dissertation, we study the emergence of a non-twisted
flux tube from the solar interior into the solar atmosphere. We investigate
whether the length of the buoyant part of the flux tube (i.e. λ) affects the
emergence of the field and the dynamics of the evolving magnetic flux system.
We perform three-dimensional (3D), time-dependent, resistive, compressible MHD
simulations using the Lare3D code. We find that there are considerable
differences in the dynamics of the emergence of a magnetic flux tube when λ is
varied. In the solar interior, for larger values of λ, the rising magnetic
field emerges faster and expands more due to its lower magnetic tension. As a
result, its field strength decreases and its emergence above the photosphere
occurs later than in the smaller _ case. However, in both cases, the emerging
field at the photosphere becomes unstable in two places, forming two magnetic
bipoles that interact dynamically during the evolution of the system. Most of
the dynamic phenomena occur at the current layer, which is formed at the
interface between the interacting bipoles. We find the formation and ejection
of plasmoids, the onset of successive solar jets from the interface, and the
impulsive heating of the plasma in the solar atmosphere. We discuss the
triggering mechanism of the jets and the atmospheric response to the emergence
of magnetic flux in the two cases.
The second part of this dissertation deals with the study of Coronal Mass
Ejections (CMEs). In our first study on CMEs, we investigate the initiation and
formation of CMEs via a detailed two-viewpoint analysis of low corona
observations of a relatively fast CME acquired by the SECCHI instruments aboard
the STEREO mission. The event which occurred on 2 January 2008, was chosen
because of several unique characteristics. It shows upward motions for at least
four hours before the flare peak. Its speed and acceleration profiles exhibit a
number of inflections which seem to have a direct counterpart in the GOES light
curves. We detect and measure, in 3D, loops that collapse toward the erupting
channel while the CME is increasing in size and accelerates. We suggest that
these collapsing loops are our first evidence of magnetic evacuation behind the
forming CME flux rope. We report the detection of a hot structure which becomes
the core of the white light CME. We observe and measure unidirectional flows
along the erupting filament channel which may be associated with the eruption
process. Finally, we compare these observations to the predictions from the
standard flare-CME model and find a very satisfactory agreement.
In our second study on CMEs, we made a spectroscopic analysis of the
pre-eruptive configuration of active region NOAA 11429, prior to two very fast
CMEs on March 7, 2012 associated with this active region. We study the thermal
components and the dynamics associated with the ejected flux ropes. Using
Differential Emission Measure (DEM) analysis of Hinode/EIS and SDO/AIA
observations, we identify the emission components of both the flux rope and the
host active region. We then follow the time evolution of the flux rope emission
components by using AIA observations. The plasma density, Doppler and
non-thermal velocities associated with the flux ropes are also calculated, from
the EIS data. The East and West part of the active region, from which the two
different fast CMEs originated during two X-class flares, were studied
separately. In both regions we identified an emission component in the
temperature range of log T = 6.8- 7.1 associated with the presence of flux
ropes. The time evolution of the East region showed an increase of the mean DEM
in this temperature range by an order of magnitude, 5 hours prior to the first
CME. This was associated with a gradual rise and heating of the flux rope as
manifested by blue-shifts and increased non-thermal velocities in Ca XV
200.97A, respectively. An overall upward motion of the flux ropes was measured
(relative blue-shifts around 12 km/s ). The measured electron density, was
found to be 4x10^9 – 2x 10^10 cm^-3 (using the ratio of Ca XV 181.90A over Ca
XV 200.97A). We compare our findings with other works on the same AR to provide
a unified picture of its evolution.
In our third study on eruptions, we report on three-dimensional MHD simulations
of recurrent small scale CME -like eruptions using flux-emergence simulations
and study their formation and eruption mechanism. These eruptions have the size
and energies of small prominence eruptions. The erupting flux ropes are formed
due to the reconnection of J-loops (formed by shearing and rotation) and are
located inside torus unstable magnetic envelope field. The flux ropes eruptions
are triggered by the action of a tension removal mechanism, such as the typical
tether-cutting where the envelope field reconnects with itself. Another side
tether-cutting is also found. There, the envelope field reconnected with the
J-loops. We report that the different tension removal mechanisms produce
different temperature, density and velocity distributions inside the erupting
structures. Simulations with smaller magnetic field strength indicate that the
torus unstable flux ropes lead to confine eruptions. Extrapolations of the
erupting structures show that these eruptions have the potential to have the
size of small CMEs. Simulations of higher magnetic energy flux tubes showed
that the kinetic energies of these eruptions can also increase to reach the
energies of small sized CMEs.
The present thesis is structured as follows. Chapter 1 presents a brief
introduction to Solar Physics and introduces the topic of the thesis. Then, the
thesis is divided in two parts. Part I has to do mostly with flux emergence.
Chapter 2 gives a brief introduction to the theory of magnetohydrodynamics and
then chapter 3 introduces the theoretical basis of magnetic flux emergence. In
chapter 4 we present our results on the non-twisted flux tube emergence. Part
II is dedicated to CMEs and solar eruptions. In chapter 5 we give a brief
introduction to the observations and theory of CMEs. In chapter 6 we present
our results on the initiation of a CME from NOAA 10980 using STEREO data. In
chapter 7 we present our results on the pre-eruptive configuration of NOAA
11429 prior to the ejection of two very fast CMEs. Finally, in chapter 8 we
present our results on the nature of recurrent eruptions.
Keywords:
Magnetic flux emergence, Flux tubes, Solar jets, Coronal mass ejections, MHD
