The EUV spectrum of the Sun: long-term variations in the SOHO CDS NIS spectral responsivities

¹ DAMTP, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, CB3 0WA Cambridge, UK e-mail: GDelZanna@spd.aas.org
² INAF - Osservatorio Astronomico di Capodimonte, Naples, Italy
³ NASA Goddard Space Flight Center, Solar Physics Laboratory, code 671, MD 20771 Greenbelt, USA
⁴ Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA
⁵ Adnet Systems Inc., NASA Goddard Space Flight Center, code 671, MD 20771 Greenbelt, USA

Received: 16 July 2009
Accepted: 13 April 2010

Abstract

We present SOHO Coronal Diagnostic Spectrometer (CDS) normal incidence, extreme-ultraviolet spectra of the Sun taken from the beginning of the mission in 1996 until now. We use various methods to study the performance of the instrument during such a long time span. Assuming that the basal chromospheric-transition region emission in the quiet parts of the Sun does not vary over the cycle, we find a slow decrease in the instrument sensitivity over time. We applied a correction to the NIS (Normal Incidence Spectrograph) data, using as a starting reference the NIS absolute calibration obtained from a comparison with a rocket flight in May 1997. We then obtained NIS full-Sun spectral irradiances from observations in 2008 and compared them with the EUV irradiances obtained from the rocket that flew on April 14, 2008 a prototype of the Solar Dynamics Observatory EVE instrument. Excellent agreement is found between the EUV irradiances from NIS and from the EVE-prototype, confirming the NIS radiometric calibration. The NIS instrument over 13 years has performed exceptionally well, with only a factor of about 2 decrease in responsivity for most wavelengths.