Multi-spacecraft analysis of the solar coronal plasma

The thesis "Multi-spacecraft analysis of the solar coronal plasma" deals with two different approaches in the analysis of the solar corona: an observational and a theoretical one.

The first approach aims at the reconstruction of the 3D structure of phenomena in the solar corona using data obtained by multiple spacecraft. We used observations from three spacecraft: Solar Terrestrial Relation Observatory (STEREO) A and B and Solar Dynamic Observatory. The observed and analyzed solar phenomena were prominences and CMEs. For the analysis of the observed phenomena we extended and applied a 3D stereoscopic reconstruction method, called MBSR (Multi-view B-spline Stereoscopic Reconstruction) which was developed as part of this thesis. The MBSR method has a large spectrum of possible applications to solar phenomena, from coronal loops to coronal mass ejections (CME). We applied the MBSR method to two eruptive prominences which evolved into CMEs.

In one of the events a bright patch of low polarized radiation was observed in coronagraph images of the CME core, which was presumably caused by a Hα resonant scattering. This effect is not common since at the usual coronal temperatures at the height of the analyzed CME, one expects the plasma to be fully ionized. The polarization ratio method failed to retrieve a meaningful location of the bright patch. Therefore, we applied the MBSR method and determined its probable 3D position in the CME core. For the second event we make use of simultaneous data from three space probes to reconstruct the 3D location of the highest ridge of a rising prominence and the core and leading edge of the CME which evolved from it. We follow the evolution of the eruption from the time of the initial rise of the prominence until the CME core leaves the field of view of the COR1 coronagraph. We calculate various parameters which characterize the 3D curves, such as the propagation direction, the rise velocity, the angular width of the prominence and of the CME core and their rotation.

The second approach is related to the extrapolation of the coronal magnetic field from a photospheric magnetogram using the NLFFF (non-linear force-free field) model. It is generally accepted that coronal loops observed in EUV images outline magnetic field lines. The results from many conventional magnetic field extrapolations show, however, large discrepancies between the extrapolated magnetic field lines and the observed coronal loops, typically they deviate by angles of the order of 20 degrees. We therefore introduced an additional observational constraint to the extrapolation scheme by requiring the field also reproduces 3D reconstructed coronal loops. This is achieved by minimizing the local angles between the extrapolated magnetic field and the tangents to the coronal loops. We call this new method stereoscopic - nonlinear force-free field (S-NLFFF) extrapolation method because the shape of the coronal loops is reconstructed from EUV images by stereoscopy. In the thesis we present the S-NLFFF method and tests of it with synthetic data.

The thesis is structured in six chapters: in Chapter 1 we give an introduction to the studied solar coronal phenomena; in Chapter 2 we present the methods which we devel- oped for analyzing prominences, coronal loops and CMEs as well as those for computing the coronal magnetic field. Already existing methods employed here are also described. in Chapter 3 we present the spacecraft and instruments which we have used for our data analysis. Chapter 4 presents the application of the MBSR code and the analysis of two coronal events. In Chapter 5 we present the tests for S-NLFFF model. Chapter 6 contains conclusions and a brief outlook.