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Astron. Astrophys. 338, 1089-1101 (1998)

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Infrared lines as probes of solar magnetic features

XIV. Ti I and the cool components of sunspots

I. Rüedi 1, S.K. Solanki 1, C.U. Keller 2 and C. Frutiger 1

1 Institute of Astronomy, ETH-Zentrum, CH-8092 Zürich, Switzerland
2 National Solar Observatory, NOAO, P.O. Box 26732, Tucson, AZ 85726, USA *

Received 21 April 1998 / Accepted 17 July 1998


The first systematic observations of sunspot umbrae using the lines of the Ti [FORMULA] multiplet at 2.2 µm are presented. Their diagnostic capabilities are investigated, developed and used to investigate the magnetic and velocity structure of a sunspot. These lines are most sensitive to cool plasma. In addition, they are extremely Zeeman sensitive. We find that a sunspot is composed of two distinct cool magnetic components. One of them is fairly vertical, has a large magnetic field strength and is associated with the central (umbral) part of the sunspot. The other component is strongest near the outer boundary of the spot (penumbra), is much more inclined, has a very low magnetic field strength and shows the signature of the Evershed effect. In contrast to the smooth transition of field strength from the darkest part of the umbra to the outer penumbral boundary usually visible in observations carried out in other spectral lines, the Ti [FORMULA] lines exhibit a sharp transition between the two magnetic components.

Key words: Sun: activity – Sun: infrared – Sun: magnetic fields – Sun: photosphere – sunspots

* Operated for the National Science Foundation by the Association of Universities for Research in Astronomy.

Send offprint requests to: I. Rüedi

© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998