Studies of coronal mass ejection and the solar wind based on multi-instrument and total solar eclipse observations

Crynodeb

This thesis is concerned with studies and analysis methods for white-light and extreme ultraviolet observations of the solar corona, including coronal mass ejections (CMEs) and other dynamic activity therein. The solar corona is a hot (∼1MK) magnetised plasma, and is host to many dynamic processes. CMEs are the largest and most dynamic phenomena that originate from the Sun, and can be observed in the extended corona by white light coronagraphs. CMEs are huge eruptions of magnetised plasma which possess a broad range of masses and outflow speeds. These eruptions and their associated bursts of energetic particles can cause adverse space weather at Earth. The 3-part appearance of many CMEs arising from erupting filaments emerges from a large magnetic flux tube structure, consistent with the form of the erupting filament system. Other CMEs arising from erupting filaments lack a clear 3-part structure and reasons for this have not been researched in detail. Chapter 3 aims to further establish the link between CME structure and the structure of the erupting filament system and to investigate whether CMEs which lack a 3-part structure have different eruption characteristics. A survey is made of 221 near-limb filament eruptions observed from 2013/05/03 - 2014/06/30 by EUV imagers and coronagraphs. 92 filament eruptions are associated with 3-part structured CMEs, 41 eruptions are associated with unstructured CMEs. The remaining 88 are categorized as failed eruptions. For 34% of the 3-part CMEs, processing applied to EUV images reveals the erupting front edge is a pre-existing loop structure surrounding the filament, which subsequently erupts with the filament to form the leading bright front edge of the CME. This connection is confirmed by a flux-rope density model. Furthermore, the unstructured CMEs have a narrower distribution of mass compared to structured CMEs, with total mass comparable to the mass of 3-part CME cores. This study supports the interpretation of 3-part CME leading fronts as the outer boundaries of a large pre-existing flux tube. Unstructured (non 3-part) CMEs are a different family to structured CMEs, arising from the eruption of filaments which are compact flux tubes in the absence of a large system of enclosing closed field. Chapter 4 presents a new, automated method of detecting CMEs in three dimensions for the LASCO C2 and STEREO COR2 coronagraphs. By triangulating isolated CME signal iv from the three coronagraphs over a sliding window of five hours, the most likely region through which CMEs pass at 5R is identified. The centre and size of the region gives the most likely direction of propagation and approximate angular extent. The Automated CME Triangulation (ACT) method is tested extensively using a series of synthetic CME images created using a
wireframe flux rope density model, and on a sample of real coronagraph data; including halo CMEs. The accuracy of the angular difference (σ) between the detection and true input of the synthetic CMEs is σ=7.14◦, and remains acceptable for a broad range of CME positions relative to the observer, the relative separation of the three observers, and even through the loss of one coronagraph. For real data, the method gives results that compare well with the distribution of low coronal sources and results from another instrument and technique made further from the Sun. The true 3D-corrected kinematics and mass/density are discussed. The results of the new method will be incorporated into the CORIMP database in the near future, enabling improved space weather diagnostics and forecasting. Chapter 5 concerns observations of the extended white light corona during a total solar eclipse. Using data from the total solar eclipse of 2015 March 20 at 10:10:40 UT on the Norwegian archipelago of Svalbard, a temperature map of the solar corona is created, which accurately calculates the temperature of the solar corona out to 2R. This is achieved using ratios of the Fe XI (789.2 nm) and Fe XIV (530.3 nm) spectral lines, which are cross-calibrated with the total brightness observations of the Large Angle Spectrometric Coronagraph (LASCO) instrument C2 on board the Solar and Heliospheric Observatory (SOHO). This is a powerful tool for diagnostics of the coronal holes present during the eclipses. The temperature at the base of the corona is an important parameter for the modelling of the solar wind. Using the map, a temperature of 1.2 × 106 K is observed within the northern polar coronal hole. By applying a spherically symmetric inversion of brightness along the line of sight to the eclipse total brightness observations, the electron density as a function of height is derived. By combining the eclipse observations with LASCO C2 observations, electron density is found to be 7 × 107 cm−3 at the base of the corona, descending to 1 × 104 cm−3 at 6R. Considering conservation of mass flux, the velocity and acceleration of the solar wind outflow are also derived. Using these values estimates are also made for proton temperature within the coronal hole. The outflow velocity is found to peak at 430 km.s−1 , and acceleration at 96 m.s−2 . Fitting the density and velocity results to a Gaussian plus a 1st order degree polynomial function of height gives a robust estimate for proton temperature. Fitting returns a high peak in proton temperature of 2.5 MK, implying that wave-particle interactions are also working to drive the solar wind from the coronal hole as well as the thermal pressure gradient. An overview of the method and implementation is given. Under certain conditions large enough CMEs may cause widespread damage to satellites and other important infrastructure. The ability to predict the occurrence of a CME, and to v predict their subsequent evolution, depends critically on understanding the dominant physical processes which occur in the corona. The work presented in this thesis aims to enable the improvement of space weather diagnostics and forecasting capabilities

Dyddiad Dyfarnu	2017
Iaith wreiddiol	Saesneg
Sefydliad Dyfarnu	Prifysgol Aberystwyth
Goruchwyliwr	Huw Morgan (Goruchwylydd) & Xing Li (Goruchwylydd)