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Issue A&A
Volume 407, Number 1, August III 2003
Page(s) 359 - 367
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20030877



A&A 407, 359-367 (2003)
DOI: 10.1051/0004-6361:20030877

Dependence of UV radiance of the quiet Sun on the solar cycle: Surface magnetic fields as the cause

A. Pauluhn1 and S. K. Solanki2

1  International Space Science Institute, 3012 Bern, Switzerland
2  Max-Planck-Institut für Aeronomie, 37191 Katlenburg-Lindau, Germany

(Received 25 March 2003 / Accepted 28 May 2003 )

Abstract
The quiet-Sun UV radiance depends on the solar cycle, as shown by data collected by the SUMER spectrograph on the Solar and Heliospheric Observatory (SOHO). The cause of this dependence is still unclear. Here the hypothesis is tested for the He $\scriptstyle {\rm I}$ 584 Å line that these variations are due to changes in the magnetic network. The quiet-Sun variability is investigated with the two EUV instruments CDS (Coronal Diagnostic Spectrometer) and SUMER (Solar Ultraviolet Measurements of Emitted Radiation) and the MDI (Michelson Doppler Imager) magnetograph on SOHO. Using a monthly data set of co-spatial and co-temporal observations of quiet Sun areas near disk centre we follow the evolution of the quiet Sun over four years from solar cycle minimum to maximum conditions and find that the magnetic flux of the quiet network increases during this period. Furthermore, its variation is well correlated with the radiance change in the He $\scriptstyle {\rm I}$ 584 Å line. Also, we find that the largest fractional change is in the flux of the strong network elements (largest average field strengths), while the weaker elements do not exhibit a significant change.


Key words: Sun: UV radiation -- Sun: magnetic field

Offprint request: A. Pauluhn, anuschka.pauluhn@issi.unibe.ch




© ESO 2003


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