Abstract

Contributed Talk - Splinter Solar

Tuesday, 10 September 2024, 15:20   (S15)

Sunspot simulations with potential field initial conditions

Markus Schmassmann¹, Nazaret Bello González¹, Rolf Schlichenmaier¹, Jan Jurčák², Matthias Rempel³
¹Institut für Sonnenphysik (KIS), Freiburg im Breisgau, Germany. ²Astronomical Institute, Czech Academy of Sciences, Ondřejov, Czech Republic. ³High Altitude Observatory, NCAR, Boulder, CO, USA

There are many observations of sunspots, but few attempts at sunspot simulations. Rempel (2012) presented realistic magneto-hydrodynamic (MHD) sunspot simulations. Jurčák et al. (2020) showed that the magnetic field of such simulations differs from observations; in particular, the Bᵥₑᵣ at the umbral boundary is too low. Using the MURaM MHD code and a potential field top boundary condition, we simulated a set of sunspots with different box widths and potential initial fields. The fields were defined by setting Bᵥₑᵣ at the bottom boundary to a Gaussian with different parameters and subtracting uniform vertical fields. Initial field strengths at the bottom of less than 160kG result in much too narrow penumbrae. Those extreme field strengths at 7.4Mm beneath the photosphere decay quickly, to below 45kG in the time range we studied. In general, the penumbra-to-spot ratios in potential simulations are smaller than those of observed sunspots. Simulations with fluxes >10²²Mx in the Gaussian produce unrealistic strong magnetic fields in the umbra. Widening the box and decreasing the overall flux through the box has minimal effect on the dynamics and magnetic field distribution within the spot, but allows control over the average vertical field outside the spot. The potential simulations do not show a pure Evershed (radially outward) flow. The B₀=160kG and F_{Gauss}=10²²Mx simulations show bi-directional flows: inflows in the inner penumbra and outflows in the outer penumbra, as observed in high-resolution observations of penumbra formation by García-Rivas et al. (2024). The potential-field simulations provide an excellent scenario for the little-investigated processes of penumbra formation, which we will use to compare with the 3D velocity fields in penumbral filaments of simulations of stable spots.