Depth-dependent global properties of a sunspot

November 04, 2015

A novel inversion technique developed at MPS allowed to investigate the 3-dimensional structure of a sunspot in the photosphere of the Sun. The physical conditions (temperature, flow velocities, magnetic fields) were analyzed in unprecedented resolution using data from the spectropolarimeter onboard the Japanese Hinode spacecraft. A striking similarity between the sunspot umbra and the dark lanes (spines) of the penumbra could be identified.

Maps of physical parameters from the results of inversion (at logτ = 0): a. continuum Intensity, b. magnetic field strength, c. magnetic field inclination, d. line-of-sight velocity. The arrow on continuum intensity map shows the direction of the solar disc center.

Maps of physical parameters from the results of inversion (at logτ = 0): a. continuum Intensity, b. magnetic field strength, c. magnetic field inclination, d. line-of-sight velocity. The arrow on continuum intensity map shows the direction of the solar disc center.

The physical conditions in a sunspot change dramatically when moving from the center of the sunspot towards its outer boundary. The center (umbra) is dominated by 3500 K cool gas embedded in strong, vertical magnetic fields of up to 3500 G. The spine-like structure of the penumbra connects the umbra to the typical granulation pattern outside the sunspot, where the temperature is up to 3000 K higher and the magnetic fields are extremely week.

These extreme differences posed a challenge for the scientists: The retrieval of the atmosperic conditions from spectropolarimetric observations required so far the use of different model atmospheres for the umbra, the penumbra and the surrounding region, introducing discontinuities in the solution. A novel inversion technique, developed at MPS (van Noort, 2012: http://dx.doi.org/10.1051/0004-6361/201220220), overcomes this shortening and produced for the first time continuous, height-dependent maps of the thermal, velocity and magnetic properties of a sunspot from the umbra towards the surrounding quiet Sun.

The results of this analysis were published in Astronomy and Astrophysics: Tiwari, van Noort, Lagg, Solanki (A&A 583, A119, 2015; http://dx.doi.org/10.1051/0004-6361/201526224). The inclination map was selected as a cover page of Astronomy & Astrophysics, (vol. 583).