THEORY AND SIMULATION OF THE EVOLUTION OF THE LARGE-SCALE SOLAR MAGNETIC FIELD

• Main Author: DEVORE, CARL RICHARD
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1986
• Subjects: Astronomy; Astrophysics
• ISBN: 1392513782; 9781392513781

We model the evolution of the Sun's large-scale magnetic field by a transport equation which describes the effects of differential rotation, meridional circulation, and supergranular diffusion on the field at the photosphere. This equation is solved analytically and numerically to investigate three aspects of solar magnetic behavior. First, we calculate axisymmetric solutions to the transport equation which approximate the distribution of flux near the time of sunspot minimum. We find that a poleward meridional flow with a moderate peak speed and a broad profile concentrates the polar fields into very small caps, contrary to observations. A flow which reaches its peak speed at a low latitude and then decreases rapidly to zero at higher latitudes, on the other hand, leads to a field pattern which is consistent with observations. Second, we numerically simulate the evolution of several bipolar magnetic regions over a few solar rotational periods, using digital magnetic-field measurements to initialize the model and to evaluate the results. We find good agreement between our best-fit values for the rates of rotation and diffusion of the field and previous measures of these quantities. Although the best-fit meridional velocities are also consistent in direction and magnitude with recently reported poleward flows, relatively large uncertainties in our velocity determinations render the results for the meridional flow inconclusive. Third, we simulate the evolution of the Sun's mean line-of-sight magnetic field over an eight-year interval during sunspot cycle 21, using nominal values of the transport parameters and estimated properties of the emerging bipolar regions. We find satisfactory agreement between the simulated and observed mean fields. The results suggest that the evolution of the mean field may be regarded as a random-walk process with dissipation, in which the emergence of new flux in sunspot groups causes the random walk while differential rotation, meridional flow, and supergranular diffusion provide the dissipation. Our findings support the hypothesis that the Sun's large-scale magnetic field can be understood as resulting from the transport of flux emerging in sunspot groups.