Solar activity during solar cycle 23 monitored by GOME

The daily solar spectral measurements by the Global
Ozone Monitoring Experiment (GOME) aboard ERS2
are well suited to track solar activity by looking at irra-
diance changes in the center of selected Fraunhofer lines.
The major modulation of the irradiance changes con-
tain periodicity of about 27-28 days, corresponding to
the mean solar rotation period, and eleven years due to
solar activity associated with the 22-year magnetic cycle
of the sun.
The MgII doublet at 280nm and the CaII K line at
around 395nm have been analysed to derive a proxy so-
lar activity indicator time series from GOME starting in
July 1995 near the end of the declining phase of solar
cycle 22. GOME is currently observing the onset of solar
cycle 23. An excellent correlation with solar observations
provided by the UARS instrument SUSIM (Solar Ultravi-
olet Spectral Irradiance Monitor) has been achieved after
removing instrumental artefacts such as UV degradation
and etaloning from the GOME level-1 spectra. An esti-
mate of solar UV irradiance variability with a change in
the GOME MgII index is given.