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This work presents the results of instrumentation developments and scientific studies aimed at better probing the nature of the solar chromosphere. Indeed, the sources of the energy and mass to the chromosphere and corona (and the mechanisms by which the energy is deposited there), remain one of the most mysterious aspects of the structuring of stellar atmospheres. The goal of this thesis is to apply new methods to studies of the solar chromosphere in order to advance our understanding of its structure and dynamics. I perform an analysis of imaging spectroscopy techniques, comparing the spectral fidelity achieved by different classes of instruments and evaluating a metric for the spectral measurement efficiency. I describe an upgrade of the Interferometric Bidimensional Spectrometer (IBIS) to a faster data acquisition system that better allows the application' of image reconstruction techniques and faster sampling of the rapid variations in the solar chromosphere. I use the upgraded system to obtain a unique mosaic providing a comprehensive view of the chromosphere featuring diffraction-limited resolution, full spectral information, and a large field of view. I describe two studies that used this dataset to examine the configuration of the chromospheric magnetic field, revealing that network fields in the upper atmosphere may be largely non-potential. I further examine the chromospheric diagnostics provided by several different lines, exploring the information encoded in chromospheric line widths and combining datasets to reveal different aspects of the chromospheric conditions. I find that the Ha 6563 and Call 8542 A lines, despite differences in appearances, are largely formed in the same plasma volumes.

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Total solar eclipse observations were the first to raise fundamental questions about the Sun’s atmosphere, or corona. Instruments and analysis tools have been developed (and continue to be developed) to address questions regarding the temperature and structure of the corona, the underlying mechanisms responsible for coronal activity and the effects of the Sun on the heliosphere and on Earth. The development of modern imaging instrumentation, with optics designed for a broad range of wavelengths and detector arrays, has led to new understanding of the dynamic nature of the corona. The new image processing techniques described in Chapter 2 are essential to fully exploit data from coronagraphs and EUV imagers. In order to explore the broader range of dynamic events in the corona that differ from typical coronal mass ejections (CMEs) in structure and behavior, this thesis focuses on three main areas: 1) CME-type (jet-like) events, 2) low-coronal signatures (LCSs) of ‘stealth’ CMEs and 3) dynamic events captured during total solar eclipse observations. Jets have a huge variation of observational characteristics and can arise from many different regions at the Sun. Chapter 3 describes observations of recurring jetting activity from an active region (AR). During a period of three days beginning 2013 January 17, twelve recurrent reconnection events occurred within a small region of opposing flux embedded within one footpoint of an AR, accompanied by flares and jets observed in EUV and fast and faint structureless ‘puffs’ observed by coronagraphs. During the same period a slow structured CME gradually erupts, with one end anchored close to, or within, the jetting region. Four of the jet events occur in pairs (a narrow, primary jet followed by a spray-like jet). The puffs are disturbances caused by the initial reconnection event, which also drives the first narrow jet. The primary jet, the associated flaring activity, and the puffs are all symptoms of magnetic reconnection. The source of the fast events observed in LASCO/C2 is a region of positive magnetic flux at the footpoint of a large AR. This work shows that propagating disturbances, without an accompanying flow of material, are clearly observable in the extended corona. CMEs are generally associated with LCSs such as flares, filament eruptions, EUV waves or jets. Recent published works have observed CMEs without LCSs, leading scientists to refer to them as ‘stealth’ CMEs. The study described in Chapter 4 focuses on a set of 40 stealth CMEs identified from a study by D’Huys et al. 2014. Application of advanced image processing reveals activity in the lower corona that are the LCSs associated with these so-called stealth events. Many of these LCSs were missed because vi of data and image processing limitations. Twenty-three of these events are identified as small, low-mass, unstructured blobs or puffs, often occurring in the aftermath of a large CME, but associated with LCSs such as small flares, jets or filament eruptions. Of the larger CMEs, 7 are associated with jets and 8 with filament eruptions. The main conclusion reached is that stealth CMEs are a misconception arising from observational and processing limitations. Total solar eclipse observations are essential in coronal studies because they provide a full map of the extent of the corona from the solar surface out to several solar radii. When total solar eclipse images are processed, these high resolution images reveal the finest details of coronal structures down to the spatial resolution of the instrument (1-2 arcsec). The 2012 and 2013 eclipse observations described in Chapter 5 were acquired at the peak of solar activity. The study of the dynamic events captured in the eclipse images are complemented by time-series observations from space, taken prior to and during totality. The focus of the study is on the source of ‘atypical’ large-scale structures captured in these images. The two events described show the impact of flaring activity from ARs, and their association with sprays, jets and CMEs. All three studies described in this thesis rely on new state-of-the-art image processing techniques applied mainly to satellite data from LASCO/C2, SDO, EUVI, and SWAP. Reliable image processing has opened up new venues for studying dynamic coronal events. Using multi-wavelength EUV (SDO/AIA and STEREO/EUVI), complemented by coronagraph data (LASCO/C2 and COR2) and total solar eclipse images, we have created an extensive catalog of CMEs based on their LCSs seen in EUV and their overall behavior in coronagraph and eclipse images.

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The solar corona is a hot, magnetised plasma of which several important aspects remain poorly understood. The Atmospheric Imaging Assembly (AIA) provides very high resolution images of the Sun in several extreme ultraviolet channels. AIA offers a unique chance to improve our understanding of the corona - qualitatively through detailed viewing of dynamic events and quantitatively through density and temperature diagnostics. This thesis presents a new software tool to quickly estimate coronal characteristics using AIA data. The method creates high-resolution temperature and emission measure maps of the whole solar disk within minutes. A slower but more thorough version is also developed as a comparison, and complimentary to, the main method. Both methods are tested extensively on synthetic data calculated from known temperature distributions and are then applied to real data. A prototype method for fast estimation of coronal line-of-sight emission distribution is also presented. A broad study investigates the characteristics of various coronal regions. The results are compared to previous works and found to be consistent, although the combination of values produced by the two methods reveals material cooler than that found by other studies, particularly at coronal hole boundaries. Another investigation applies the fast method to two sets of flaring active regions. A weak correlation exists between the flare size and mean temperature of the region for a small number of flares in one set. In the other set each region’s temperature variability over time is compared to a non-flaring region’s. The flaring regions’ mean temperatures are found to vary more than the non-flaring region’s - significantly more in several cases. This gives confidence in using such diagnostics as part of a future flare prediction method. The fast temperature map method presented here offers a significant speed advantage over similar methods, whilst maintaining robust results. This allows the maximum exploitation of AIA’s fine spatial and temporal resolution for temperature and emission measure studies.

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.

The solar atmosphere is a highly magnetised plasma covering a wide range of temperatures from the thousands of Kelvin to the millions. How exactly it reaches such extreme temperatures remains unknown. There are however numerous events that take place, concerned with the movement of energy and plasma throughout the solar atmosphere. This thesis makes use of four instruments that have studied the Sun in the past, and in three cases continue to do so today. Observations from these instruments are combined with synthetic observations obtained from a detailed non-equilibrium ionisation hydrodynamic radiation code to understand the nature of what was observed and deduce physical information. One study presents the replication of light curves of a loop obtained by the Atmospheric Imaging Assembly (AIA). It finds that it was a cold loop heated by a pulse of energy at its footpoint consistent with the energy of a nano are. Another study replicates line proles of a structure observed by the Extreme-Ultraviolet Imaging Spectrometer (EIS) within an outflow region. We find that it is best modelled by a long loop consisting of at least 100 strands undergoing a cyclical process of heating and cooling on timescales of approximately 80 minutes. A final study replicates line profiles from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument, and uses images from the Transition Region and Coronal Explorer (TRACE) to add context to the interpretation.