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Astron. Astrophys. 354, L71-L74 (2000)

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1. Introduction

The solar irradiance in the ultraviolet and its variability is of fundamental importance due to its effect on the upper atmosphere of the Earth. In several spectral lines in the UV and EUV, the irradiance has been measured by the SUMER spectrograph (Solar Measurements of Emitted Radiation) on the Solar and Heliospheric Observatory (SOHO) during 1996 and 1997 (Wilhelm et al. 1998). From the spatially resolved images of the Sun, the contributions of active and coronal hole regions could be obtained, that contribute to the short term variation of the irradiance of spectral lines due to solar rotation. These results provide important input parameters for models of the solar irradiance spectrum (Fontenla et al. 1999) and the VUV irradiance variability based on the contrast ratio of different regions on the solar disk. It was also possible to measure the detailed centre-to-limb variations in these emission lines.

The long-term variability on the other hand, which goes along with the solar activity cycle, is known to be very strong (Hall et al. 1969), but has not been measured for many spectral lines (Tobiska et al. 1998, Woods et al. 1998) and is not understood in detail (Lean 1991). In particular, it is unclear whether the variability of the irradiance of the Sun can be modelled by active region and plage area contributions or is also due to a global change of the quiet-Sun radiance. The latter can be a global brightening of the emission or may be caused by changes of fine scale structures in the chromospheric and transition region network. Even a change in the statistics of transient brightenings may be possible.

The SUMER spectrograph on SOHO has been calibrated in the laboratory before the launch of SOHO. The careful tracking of the instrument responsivity during the mission has been accomplished by repeated observations of quiet-Sun areas on a regular basis. These measurements have shown that the responsivity had not degraded during two and a half years of the mission. Contrary, an increasing trend of the radiance of some lines was found. The measurements can thus be used to derive long term variations of the radiance of the observed spectral lines. The spatially resolved images give us a first clue about the variability of different spectral lines from the chromosphere, transition region and the lower corona and its relation to the network structure.

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© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000