Matthias Rempel

NameMatthias Rempel
AffiliationHigh Altitude Observatory / NCAR, USA
TitleFrom Flux Emergence to Sunspot Structure – A Modeling Perspective
AuthorsMatthias Rempel
AbstractOur understanding of flux emergence has undergone in the past few decades a substantial paradigm change. While models more than 2 decades ago treated flux emergence as an active process in which super-equipartition flux tubes rise towards the surface owing to their magnetic buoyancy, more recent models rely mostly on a passive transport by means of the ambient convection. This change was due to both modeling advances and new observational constraints. Rising flux-tube models that incorporate the advective transport from ambient convection were able to explain active region properties with flux tubes of weaker field strength. Global dynamo simulations demonstrate that flux bundles with a strength of a few 10kG can be formed within the turbulent convection zone and rise towards the surface mostly due to convective transport. Helioseismic flow measurements in the 24 hours prior to the onset of active region formation were not able to discern the flow fields that were predicted by active flux emergence models, such as retrograde flows and persistent upflows in the flux emergence region. Instead, flux-emergence appears mostly as a “stealth” process in the last 20Mm beneath the photosphere, implying that flux is primarily transported by the present convective flows with only minor modification. This is in part due to a significant weakening of the magnetic field strength as consequence of horizontal expansion in the highly stratified convection zone, leading to flux bundles of just a few 100G reaching the photosphere. While the subsequent reamplification to spots of several kG in the photosphere has been found as a robust feature in most radiation MHD simulations, there remains some debate about the detailed physical processes involved. Another critical detail that still evades the grasp of current radiation MHD simulations is the formation of a penumbra as part of the spot formation process. While sunspot fine-structure has been modeled in great detail, the presence of a penumbra remains heavily dependent on details of the numerical setup. While fine-structure models about a decade ago produced penumbra through an artificially enhanced horizontal field component at the top boundary, recent research focuses more on the critical role of the sunspot sub-surface structure. In this talk I review about 3 decades of model development while strongly focusing on recent results and critical observations that have guided the way.