A new look at sunspot formation using theory and observations

KTH, Nordic Institute for Theoretical Physics NORDITA Department of Astronomy, Stockholm University, SE-10691 Stockholm, Sweden; Nordic Optical Telescope, Apartado 474, E-38700 Santa Cruz de La Palma, Spain; Kiepenheuer-Institut für Sonnenphysik, Schöneckstraße 6, D-79104, Freiburg, Germany;Nordita SU Department of Astronomy, Stockholm University, SE-10691 Stockholm, Sweden; ; Department of Mechanical Engineering, Ben-Gurion University of the Negev, POB 653, Beer-Sheva 84105, Israel Department of Mechanical Engineering, Ben-Gurion University of the Negev, POB 653, Beer-Sheva 84105, Israel Cambridge University Press 2016

Sunspots are of basic interest in the study of the Sun. Their relevance ranges from them being an activity indicator of magnetic fields to being the place where coronal mass ejections and flares erupt. They are therefore also an important ingredient of space weather. Their formation, however, is still an unresolved problem in solar physics. Observations utilize just 2D surface information near the spot, but it is debatable how to infer deep structures and properties from local helioseismology. For a long time, it was believed that flux tubes rising from the bottom of the convection zone are the origin of the bipolar sunspot structure seen on the solar surface. However, this theory has been challenged, in particular recently by new surface observation, helioseismic inversions, and numerical models of convective dynamos. In this article we discuss another theoretical approach to the formation of sunspots: the negative effective magnetic pressure instability. This is a large-scale instability, in which the total (kinetic plus magnetic) turbulent pressure can be suppressed in the presence of a weak large-scale magnetic field, leading to a converging downflow, which eventually concentrates the magnetic field within it. Numerical simulations of forced stratified turbulence have been able to produce strong superequipartition flux concentrations, similar to sunspots at the solar surface. In this framework, sunspots would only form close to the surface due to the instability constraints on stratification and rotation. Additionally, we present some ideas from local helioseismology, where we plan to use the Hankel analysis to study the pre-emergence phase of a sunspot and to constrain its deep structure and formation mechanism.
Export Date: 24 October 2017; Conference Paper; Funding details: ERC, European Research Council; Funding text: We thank Emanuel Gafton and Ariane Schad for useful discussions and comments on the manuscript. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Program (FP/2007-2013)/ERC Grant Agreement no. 307117. This work was partly supported by the European Research Council under the AstroDyn Research Project No. 227952, by the Swedish Research Council under the project grants 621-2011-5076 and 2012-5797 (IRL, AB), and the Research Council of Norway under the FRINATEK grant No. 231444 (AB, NK, IR). J.W. acknowledges funding by the Max-Planck/Princeton Center for Plasma Physics and funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7/2007-2013) under REA grant agreement No. 623609. We acknowledge the allocation of computing resources provided by the Swedish National Allocations Committee at the Center for Parallel Computers at the Royal Institute of Technology in Stockholm and the National Supercomputer Centers in Linköping and the High Performance Computing Center North in Umea. This interdisciplinary collaboration would not have been possible without the support of the Solarnet "Mobility of Young Researches" program, awarded to I.R. Losada. QC 20171205

Helioseismology; Sun: Magnetic fields; Sun: Sunspots; Turbulence; Physical Sciences; Fysik; Conference paper; info:eu-repo/semantics/conferenceObject; text

10.1017.S1743921317004306

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UPE oai:DiVA.org:kth-216838
0000-0002-7304-021X
0000-0002-5744-1160
0000-0001-7308-4768
doi:10.1017/S1743921317004306
ISI:000455235900008
Scopus 2-s2.0-85029916254
1234988158

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