The NE BT VIII (lambda 770) resonance line: solar wavelengths determined by SUMER on SOHO

Abstract

The investigation of the source and acceleration of the solar wind was among the prime scientific goals of SOHO - ESA's and NASA's Solar and Heliospheric Observatory. For the coronal regions closest to the Sun, the bright extreme ultraviolet (EUV) resonance lines of the Ne viii spectrum at 770 { Angstroms} and 780 { Angstroms} with a formation temperature of 630 000 K are promising targets for detecting the initial solar wind outflow. The investigations published up to now using data obtained by SUMER - the Solar Ultraviolet Measurements of Emitted Radiation spectrometer - on SOHO gave conflicting results. They accepted a rest wavelength of 770.409 { Angstroms} for the brighter Ne viii line as reported in the literature. We deduce the solar wavelength of this line for several conditions on the Sun and derive a rest wavelength of (770.428 +/- 0.003) { Angstroms} and outflow speeds in coronal hole regions of ~ 9 km s(-1) . This is in agreement with other recent studies using SUMER data. The rest wavelength determination is based on the assumptions of no average Doppler shift above the solar limb and a comparison with laboratory wavelengths of cool chromospheric lines. Quiet Sun areas exhibit an average blue shift of (1 +/- 1) km s(-1) in this line.