Solar magnetic flux emergence, solar jets and coronal mass ejections

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 λ 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 stud- ied 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 200.97Å, 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 4 × 10^9 ‑ 2 × 10^19 cm^-3 (using the ratio of Ca XV 181.90Å over Ca XV 200.97Å). 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 id 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.