HAO Colloquium Series Presents Matthias Rempel, HAO

Numerical simulations of sunspot decay: On the penumbra – Evershed flow – moat flow connection

Large-scale flows are an integral part of sunspot structure. The most prominent flow is the Evershed flow in the sunspot penumbra, which reaches outflow velocities of a few km/s. On larger scales sunspots are surrounded by an outflow of a few 100 m/s that extends up to about 2 spot radii (moat flow). Numerical simulations that capture both flow systems at the same time are challenging: resolving the sunspot penumbra requires high resolution; capturing the moat flow requires large domains and simulated time scales of a few days. In this talk I will present a series of numerical experiments that target the potential connection between penumbra, Evershed flow and moat flow as well as their role in sunspot decay. To this end I compare a simulation of a sunspot (with penumbra) with a simulation of a “naked spot” (without penumbra). While the sunspot has an Evershed outflow of 3-4 km/s, the naked spot is surrounded by an inflow of 1-2 km/s in close proximity. However, both spots are surrounded by an outflow on larger scales with a few 100 m/s flow speed in the photosphere. While the sunspot has almost constant magnetic flux content for the simulated time span of 3-4 days, the naked spot decays steadily at a rate of 1021 Mx/day. A region with reduced downflow filling factor, which is more extended for the sunspot, surrounds both spots. The absence of downflows perturbs the upflow/downflow mass flux balance and leads to a large-scale radially overturning flow system; the photospheric component of this flow is to the observable moat flow. The reduction of the downflow filling factor also inhibits submergence of magnetic field in the proximity of the spots, which stabilizes them against decay. While this effect is present for both spots, it is more pronounced for the sunspot and explains the almost stationary magnetic flux content.

Date and time: 
Wednesday, February 18, 2015 - 1:30pm to 2:30pm
North Auditorium