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Thursday, March 24, 2011

Jan Stenflo

Swiss Federal Institute of Technology (ETH)
and University of Zurich

Collapsed, uncollapsed, and hidden magnetic flux on the quiet Sun

Abstract: Since the first applications of the Stokes V line ratio in the early 1970s and the Hanle depolarization effect in the early 1980s we have had a dualistic view of quiet-sun magnetism: intermittent kG flux tubes surrounded by an ocean of turbulent fields with strengths of order 10-100 G. There has been the concern that this dualism could be an artifact of using two mutually almost exclusive diagnostic tools, the Zeeman and Hanle effect. We find however that the Hinode line-ratio
data alone, without any reference to the Hanle effect, reveal the existence of two distinct flux populations, representing strong (collapsed) and weak uncollapsed) flux.

The collapsed population is preferentially located in the ntergranular lanes, while the uncollapsed population is most visible in the bright cell interiors. From a comparison between the intrinsic field strengths, as derived from the line ratio for the collapsed population, and the corresponding flux densities, we can deduce the size distribution of the flux tubes. The majority of them are found to have sizes in the range 10-70 km. The intrinsic flux tube field strength decreases with diminishing size to become substantially smaller than kG for sizes below about 60 km.

Comparison between the average of the unsigned flux density in the Hinode quiet- sun data set and earlier constraints from the Hanle depolarization effect shows that most of the flux remains invisible at the Hinode resolution scale due to cancellation of the opposite magnetic polarities within the spatial resolution element. We have derived the cancellation function that describes how the visibility of the hidden flux improves with increased spatial resolution. It needs to be extrapolated to extremely
small scales before the constraints imposed by the Hanle effect get satified, which suggests that the bulk of the hidden flux resides at scales near the end of the magnetic scale spectrum (of order 10 m).

Bio: Professor of astronomy at the Swiss Federal Institute of Technology (ETH) and University
of Zurich 1 April 1980 - 30 November 2007. Director of the Institute of Astronomy, ETH Zurich, 1 April 1980 - 30 September 2006. Special position as Scientist in Astrophysics of the Swedish Natural Science Research Council, 1 July 1975 - 31 March 1980. Docent (Lecturer) in astronomy at Lund University, 1 January 1969 - 30 June 1975.

Time: 4:00pm – 5:30pm

Location: Physics and Astrophysics Conference Room 102/103

Light refreshments available 4:00pm; Presentation begiins 4:15pm

Open to All