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

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

Corresponding author: A. Pauluhn, anuschka.pauluhn@issi.unibe.ch

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  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  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.