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Astron. Astrophys. 346, 285-294 (1999)

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1. Introduction

SOHO - ESA's and NASA's Solar and Heliospheric Observatory - was launched on December 2, 1995 and placed at its observing station near the first Sun-Earth Lagrange point, L1, in February 1996. In constant view of the Sun, many detailed studies of the solar interior and the atmosphere have been conducted since then. Two of the main goals of SOHO's investigations have been defined as understanding the heating of the solar corona and the acceleration of the solar wind.

SUMER - the Solar Ultraviolet Measurements of Emitted Radiation spectrometer - on SOHO was designed for high spectral and spatial resolution measurements in the wavelength range from 500 Å to 1610 Å. The Ne VIII ([FORMULA]770, 780) resonance lines in this range are of particular interest for transition region and coronal studies. Solar observations of the Ne VIII doublet ([FORMULA]) were first reported by Friedman (1960), who gives wavelengths of 770.5 Å and 780.2 Å (misprinted as 880.2 Å). Violett & Rense (1959) do not list the Ne VIII lines in their table of solar emission lines obtained from rocket data, although the less intense O IV line near 780 Å is included. Burton & Ridgeley (1970) measured (770.42 [FORMULA] 0.05) Å (1 [FORMULA]) for the stronger line of the doublet with a rocket spectrograph having a spectral resolution of 0.3 Å. Soon it was recognized by many researchers that the Ne VIII lines with a formation temperature of 630 000 K can provide an ideal link between observations of the transition region and the corona. Emphasis was put on Doppler shift determinations of the Ne VIII ([FORMULA]770) line ([FORMULA]) in an attempt to detect the initial outflow of the solar wind from the lower solar atmosphere. It is then clear that any absolute Doppler shift measurement - and thus the line-of-sight (LOS) ion speed, [FORMULA], deduced - would critically depend on both an accurate measurement of the solar wavelength and a good knowledge of the rest wavelength of this line.

From the Doppler formula, [FORMULA], where [FORMULA] is the vacuum wavelength of the line in question for a source at rest, c is the speed of light and [FORMULA] is the observed wavelength, it can be seen that the detection of bulk motions down to [FORMULA] 1 km s-1, which is the desired velocity sensitivity of SUMER, requires not only a resolving power of the instrument of [FORMULA], which would be adequate to detect relative motions with speeds of this magnitude, but also the knowledge of [FORMULA] with an uncertainty of about [FORMULA] 3 mÅ at [FORMULA] Å for an absolute measurement. We hope to demonstrate below that these demanding requirements can indeed be met, if we use the Sun as a wavelength transfer standard and employ the SUMER spectrometer in sub-pixel mode.

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© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999