Inhomogeneous magnetic field in penumbra
Final Report Abstract
The Sun, being the closest star to earth, serves as a laboratory to study the nature of cosmic magnetic fields and its interactions with convective plasma and radiation. The magnetic activity of the Sun is most visible in sunspots. They are characterized by strong magnetic fields, and a vigorous interaction of magnetism, radiation and plasma motion. Sunspots consist of an ’umbral’ core, surrounded by a ’penumbral’ ring. The penumbrae of sunspots with their radially oriented filamentary structure and interlaced magnetic field are ideal objects to study and understand magneto-hydrodynamical processes in a hot astrophysical plasma. In penumbrae, motion of magnetized gas with a speed of several kilometers per second, and strong magnetic fields with different orientations coexist and interact on small spatial scales (less than 200 km on the Sun). High-resolution spectro-polarimetric observations of these magneto-convective processes obtained with the Japanese-US-American HINODE satellite allow us to gain new insights into the mechanisms of energy transport in penumbrae. The data of this mission is publicly available. In this project we calibrate, analyze and interprete such spectro-polarimetric measurements. Our investigation lead to a new understanding of those magneto-convective processes, and hence are crucial for modeling the physics of sunspots: • We characterized the small-scale flow pattern with unprecedented detail and accuracy, and • found no evidence of overturning convective motions as predicted by certain penumbral models. • We demonstrated convincingly that the Evershed flow is magnetic, and that • penumbral down-flows, which are ubiquitous in a sunspot penumbra, harbor a magnetic field of opposite polarity. • We found a systematic relation between net circular polarization N = V (λ) dλ, 3-lobe Stokes-V profiles, and line wing Doppler shifts of Stokes I. This is observational relation is to be reproduced by theoretical models. • We could show that the varying gradients of the magnetic field strength are the origin of alternating positive and negative N in the center side penumbra. This project contributed significantly in the field of sunspot physics and produced a significant impact within the Solar physics community. This is indicated by the fact that, two year after its publication, our paper on ”The velocity field of sunspot penumbrae. I. A global view” in Astronomy and Astrophysics (2009) is already cited more than 20 times.
Publications
- Spectral Analysis of Sunspot Penumbrae Observed with Hinode. The Second Hinode Science Meeting: Beyond Discovery-Toward Understanding. ASP Conference Series, Vol. 415, p.369 (2009)
M. Franz & R. Schlichenmaier
- The velocity field of sunspot penumbrae. I A global view. Astronomy & Astrophysics 508, p. 1453 (2009)
M. Franz & R. Schlichenmaier
- Center to limb variations of penumbral Stokes V profiles. Astronomische Nachrichten 331, p. 570 (2010)
M. Franz & R. Schlichenmaier
- The formation of a sunspot penumbra. Astronomy & Astrophysics 512, L1 (2010)
R. Schlichenmaier, R. Rezaei, N. Bello Gonzalez, T. Waldmann
- Spectropolarimetry of Sunspot Penumbrae: A comprehensive Study of the Evershed Effect Using High Resolution Data from the Space-Bourne Solar Observatory HINODE,Cuvillier Verlag Göttingen 2011, ISBN 978-3-86955-870-7
Morten Franz
