Multi-Wavelength Studies of Dynamic Events in the Solar Corona From Space-Based and Total Solar Eclipse Observations

Abstract

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

Date of Award	2017
Original language	English
Awarding Institution	Aberystwyth University
Supervisor	Huw Morgan (Supervisor) & Xing Li (Supervisor)