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Issue A&A Volume 411, Number 2, November IV 2003 Page(s) 249 - 256 Section Diffuse matter in space DOI 10.1051/0004-6361:20031119

A&A 411, 249-256 (2003)
DOI: 10.1051/0004-6361:20031119

Spatial and temporal fluctuations in sunspots derived from MDI data

S. K. Solanki¹ and I. Rüedi<sup>2

1</sup>  Max-Planck-Institut für Aeronomie, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
²  Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center, Dorfstr. 33, 7260 Davos-Dorf, Switzerland

(Received 23 April 2003 / Accepted 15 July 2003)

Abstract
The penumbra radiates an energy flux that is 0.75-0.8 times the photospheric value. One mechanism proposed to bring this flux to the surface is interchange convection according to which hot flux tubes rise to the surface, lie horizontally there while they cool and finally sink down again. We search for possible signatures of such a process using time series of magnetograms and continuum images recorded by the Michelson Doppler Imager (MDI) in its high resolution mode (0.6´´pixels). The data reveal that at the spatial scales accessible to MDI, magnetic structures are on average smaller in the azimuthal direction than brightness features. The small-scale magnetic pattern resolvable by MDI lives for well over two hours, i.e. longer than the brightness pattern. As shown in a parallel paper (Schlichenmaier & Solanki 2003) this result, taken together with theoretical predictions, suggests that interchange convection is unable to account for the observed penumbral radiative flux.

The need for higher resolution data obtained under stable conditions is pointed out.

Key words: Sun: activity -- Sun: magnetic fields -- Sun: photosphere -- sunspots

Offprint request: S. K. Solanki, solanki@linmpi.mpg.de

© ESO 2003

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