Theses and Dissertations at Montana State University (MSU)
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Item Long-term variability of the sun in the context of solar-analog stars(Montana State University - Bozeman, College of Letters & Science, 2017) Egeland, Ricky Alan; Chairperson, Graduate Committee: Dana W. LongcopeThe Sun is the best observed object in astrophysics, but despite this distinction the nature of its well-ordered generation of magnetic field in 11-year activity cycles remains a mystery. In this work, we place the solar cycle in a broader context by examining the long-term variability of solar analog stars within 5% of the solar effective temperature, but varied in rotation rate and metallicity. Emission in the Fraunhofer H & K line cores from singly-ionized calcium in the lower chromosphere is due to magnetic heating, and is a proven proxy for magnetic flux on the Sun. We use Ca H & K observations from the Mount Wilson Observatory HK project, the Lowell Observatory Solar Stellar Spectrograph, and other sources to construct composite activity time series of over 100 years in length for the Sun and up to 50 years for 26 nearby solar analogs. Archival Ca H & K observations of reflected sunlight from the Moon using the Mount Wilson instrument allow us to properly calibrate the solar time series to the S-index scale used in stellar studies. We find the mean solar S-index to be 5-9% lower than previously estimated, and the amplitude of activity to be small compared to active stars in our sample. A detailed look at the young solar analog HD 30495, which rotates 2.3 times faster than the Sun, reveals a large amplitude ~12-year activity cycle and an intermittent short-period variation of 1.7 years, comparable to the solar variability time scales despite its faster rotation. Finally, time series analyses of the solar analog ensemble and a quantitative analysis of results from the literature indicate that truly Sun-like cyclic variability is rare, and that the amplitude of activity over both long and short timescales is linearly proportional to the mean activity. We conclude that the physical conditions conducive to a quasi-periodic magnetic activity cycle like the Sun's are rare in stars of approximately the solar mass, and that the proper conditions may be restricted to a relatively narrow range of rotation rates.Item Connecting coronal holes and open magnetic flux through observation and models of solar cycles 23 and 24(Montana State University - Bozeman, College of Letters & Science, 2015) Lowder, Christopher Alan; Chairperson, Graduate Committee: Jiong Qiu; Robert Leamon (co-chair); Jiong Qiu, Robert Leamon and Yang Liu were co-authors of the article, 'Coronal hole detection and open flux measurements' in the journal 'The astrophysical journal' which is contained within this thesis.; Jiong Qiu and Robert Leamon were co-authors of the article, 'Coronal holes and open flux in solar cycles 23 and 24' submitted to the journal 'The astrophysical journal' which is contained within this thesis.; Jiong Qiu, Robert Leamon and Dana Longcope were co-authors of the article, 'A flux transport model for computation of open magnetic field' submitted to the journal 'The astrophysical journal' which is contained within this thesis.Coronal holes are regions of the Sun's surface that map the footprints of open magnetic field lines as they extend into the corona and beyond, into the heliosphere. Mapping their footprint 'dance' throughout the solar cycle is crucial for understanding this open field contribution to space weather. Coronal holes provide just this proxy. Using a combination of SOHO:EIT, SDO:AIA, and STEREO:EUVI A/B extreme ultraviolet (EUV) observations from 1996-2014, coronal holes can be automatically detected and characterized throughout this span, enabling long-term solar-cycle-timescale study. I have developed a routine to enable automated computer recognition of coronal hole boundaries from these EUV data. The combination of SDO:AIA and STEREO:EUVI A/B data provides a new viewpoint on understanding coronal holes. As the two STEREO spacecraft drift ahead of and behind the Earth in their orbits, respectively, they are able to peek 'around the corner', providing the ability to image nearly the entire solar atmosphere in EUV wavelengths, using SDO data in conjunction. On the far-side of the Sun, evolving open magnetic field structures impact space weather, despite being unobservable until rotating into view by Earth. By combining our numerical models of solar magnetic field evolution with coronal hole observations, comparison of far-side dynamics becomes possible. Model constraints and boundary conditions are more easily fine-tuned with these global observations. Long-term and transient coronal holes both play an important role as observational signatures of open magnetic field. Understanding the dynamics of boundary changes and distribution throughout the solar cycle yields important insight into connecting models of open magnetic field.