Cause and Effect: Stellar Convection Studied Through Flickering Brightness, and the Convectively-Driven Motions of Solar Bright Points
A PhD Dissertation defended at the University of Colorado, Boulder, on May 7, 2021. Included also is a copy of supplemental animations which are also available on the author's website.
Abstract: Magnetic bright points on the solar photosphere, prominent in the G band but also visible in the continuum, mark the footpoints of kilogauss magnetic flux tubes extending toward the corona. Convective buffeting of these tubes is believed to excite MHD waves, which can propagate to the corona and there deposit heat. Measuring wave excitation via bright-point motion can thus constrain coronal and heliospheric models, and this has been done extensively with centroid tracking, which can estimate kink-mode wave excitation. DKIST will be the first telescope to provide well-resolved observations of bright points, allowing shape and size measurements to probe the excitation of other wave modes that have been difficult, if not impossible, to study to date. I develop two complementary ways to take the first step in such an investigation, which I demonstrate on bright points in MURaM-simulated images of DKIST-like resolution, as a proof-of-concept in preparation for future DKIST observations. This demonstration shows that accounting for these additional wave modes may increase the energy budget of this wave-heating model by a factor of two.
I also investigate the convection which drives bright-point motion. I present a simplified model of solar granulation, which I use alongside MURaM to explore how bright-point motion depends on properties of convection, and I show the importance of turbulence to high-frequency motion.
Separately, I investigate high-frequency, stochastic brightness fluctuations (“flicker” or F8) in Kepler light curves, which are the signature of stellar convection (as well as a source of noise for exoplanetary studies). I confront a physical model of this flicker with measured values across the H-R diagram. I revise this model to improve its agreement with observations by including the effect of the Kepler bandpass on measured flicker, by incorporating metallicity in determining convective Mach numbers, and by using scaling relations from a wider set of numerical simulations. I also explore what future lines of research might improve the model further. In doing so, I help to establish this flicker as a source of stellar constraints on simulations of convection, which may support future advances in understanding both stellar and solar convection.
- Has part
- Thesis: arXiv:2108.10987 (arXiv)