This research team will investigate the fundamental causes of coronal mass ejections (CMEs) using simulations of solar active region formation by flux emergence from the convection zone into the corona. To accomplish their goals, the team members will utilize a new magnetohydrodynamics (MHD) code being developed by the Center for Radiation Shock Hydrodynamics (CRASH) at the University of Michigan.
This CRASH code adds radiation transport and advance treatment of the equation-of-state to the MHD capabilities of existing models developed at Michigan, and will allow the simulation of flux emergence from a turbulent convection zone to the corona in a single computational domain. The Principal Investigator of this project will incorporate this numerical flux emergence component in Michigan's Space Weather Modeling Framework, in order to couple the solar-active-region-scale model to a global-scale coronal model extending to the Earth. The model's predictions will be compared to observations made at different levels in the solar atmosphere, including plasma flow fields in the convection zone determined with helioseismology, flow fields in the corona determined with Dopplergrams, and magnetic fields measured at the photosphere with vector magnetographs.
This new coupled modeling system will address a key National Space Weather Program objective by simulating magnetic flux emerging from the convection zone into the solar corona, erupting and then propagating as a CME into interplanetary space. It thus spans the space weather forecasting domain from Sun to Earth. This research will also support undergraduate and graduate education. Specifically, the requested funding will primarily support a graduate student, while research results will be incorporated into courses in the Department of Atmospheric, Ocean, and Space Science at the University of Michigan.
Sheared magnetic fields are at the epicenter of solar eruptive behavior. Large flares, coronal mass ejections (CMEs), and filament eruptions are found to occur only along the most highly sheared portions of magnetic inversion lines. Shearing and converging flows, flux cancellation, reconnection and sunspot rotation have all been individually found in earlier work to drive to give rise to eruptions. However, our simulations are the first of the coupled system of convection zone, photosphere and corona, which illustrate these multiple processes working simultaneously in the presence of convection to transfer the magnetic flux and energy from the solar interior to convection zone. Our research, for the first time, shows how the complex features of active regions that give rise to solar eruptions are produced and also illustrate the energetic connection between the solar interior and the corona in a realistic way. Major Findings Numerical simulations were performed of magnetic fields buoyantly emerging through the solar atmosphere. The simulations were performed with a realistic numerical model that simultaneously describes the convection zone, photosphere transition region and corona with the use of radiation loss terms, non-ideal equations of state, and empirical corona heating. The model achieved the following results. We simulated the rise of the magnetic flux from a near-surface granular convection zone into the low corona and illustrate the complex interaction of magnetic fields with the turbulent convection and the formation of an ephemeral region. Near-surface convection distorts and deforms the magnetic flux, intense magnetic fields form in the convective downdrafts where flux converges. Photospheric flux concentrations (sunspots and pores) form on the scale convective down flows in the convection zone, which determine the bipolar structure of the emerged solar active region, while near-surface convective collapse intensifies the strength of magnetic fields. An analyses of the buildup of coronal free magnetic energy was performed. It is found that at the polarity inversion line (PIL), shear flows driven by the Lorentz force draw the magnetic fields parallel to the PIL while converging motions near the PIL concentrates the magnetic field and leads to tether-cutting reconnection, which in turn transfers the free magnetic energy higher into the corona. A comparison of a simulation is made with observations of active region (AR) 11158, which produced a fast CME and X-class flare. In both cases, strong magnetic and velocity shear, intensification of horizontal field strength, and concentration of free magnetic energy in the low corona are found to occur. This consistency and overwhelming suggests that shear and rotational flows are driven by the Lorentz force and that tether-cutting reconnection is related to flux cancellation at the photosphere and distribution of coronal free energy. The ubiquitous shear flows and sheared magnetic fields so strongly associated with CMEs are shown to be a result of the Lorentz force that occurs when flux emerges and expands in passing from the convection zone into the corona. This physical process explains and synthesizes many observations of active regions and gives them meaning in a larger context. This shearing mechanism shown in this work explains the following: (1) the coincidence of the magnetic neutral line with the velocity neutral line, (2) the impulsive nature of shearing in newly emerged flux, (3) the magnitude of the shear velocity in different layers of the atmosphere, (4) the large scale pattern of magnetic shear in active regions, (5) the transport of magnetic flux, and energy from the convection zone into the corona, (6) tether cutting reconnection that further transports magnetic energy in the corona. Most important, unlike earlier works these simulation incorporated magnetic convection, which shows the robustness of these physical processes and their applicability to the solar environment. With so much explained, it still remains a numerical challenge to model an active region with sufficient resolution to produce a large scale CME. The rope emergence simulation summarized here only one quarter the size of an active region, which at this scale simply cannot produce an eruption the size of a CME. However, our simulations illustrate the basic process by which magnetic field become energized to produce CMEs, filament eruptions and flares.
