Small Scale Magnetic Fields and their Role in Magnetic Flux Transport

1. Small Scale Magnetic Fields and their Role in Magnetic Flux Transport One of the goals in our LWS project is to understand how the small scale magnetic field, believed to be generated by a “convective dynamo”, affects the global transport of magnetic flux on the Sun. To address this goal we have first had to study the details of how the small scale field is generated, what determines its properties and distribution in the upper convection zone (see snapshot of the field-strength distribution, below left), and the coronal manifestations of this small-scale field. Shown above left (dashed line) is the unsigned magnetic flux computed with our model as compared with measurements of the Quiet Sun field by Pevtsov et al (2003), showing excellent agreement. Above, we have computed a coronal potential field from the top plane of the convective dynamo simulations, and have then used graduate student Loraine Lundquist’s code for solving the coronal energy equation to compute the temperature and density structure in the resulting coronal loops. The colors indicate coronal temperature, with orange hotter and blue cooler. The flux tube radius in this figure is proportional to B-1/2.

2. The Magnetic Connection Between the Solar Interior & Corona • Another of the goals of this project is a physically self-consistent coupling between the solar corona and the solar interior. A new approach we have recently employed is to develop a single MHD code which can accommodate the very different conditions in the convection zone, photosphere, and corona, rather than coupling 2 different codes together. The new code, AMPS (the adaptive MHD parallel solver) solves the 3D MHD equations with a sophisticated treatment of the energy equation, and can be run in a fully implicit mode. AMPS is currently being used to • Study Quiet Sun magnetic fields from the convection zone to the corona (e.g., images to the right) • The emergence and decay of active region magnetic structures. • Global MHD simulations of the solar interior • are a vital part of understanding active region emergence and evolution, but there are few codes in existence that can easily model this part of the Sun. We have thus recently written the code SANMHD to study the generation and evolution of magnetic fields in the solar interior. SANMHD is currently being debugged and tested. Complementing its local, Cartesian predecessor ANMHD, SANMHD is a 3D global anelastic MHD code that includes an overshoot layer. The code approach: • Spherical harmonic decomposition in the polar and azimuthal directions for • maximum resolution on a spherical shell • non-uniform finite difference grid in the radial direction to allow greater flexibility in complicated boundary layers, and to ease in code parallelization • Tunable diffusion parameters that are functions of depth Log density in a vertical slice from below the Quiet Sun photosphere into the corona. Same as left panel, but different slice Flow field along a horizontal slice in the portion of the domain representing the convection zone. MPI block boundaries are shown. Top: Vertical flows along a horizontal slice Bottom: |B| along a horizontal slice below the photosphere in the convection zone.