Investigating SUMER coronal hole observations: A robust method of raster reduction

We present a physically constrained, comprehensive, and robust means of reducing long-duration spectroscopic raster observations made in the important 1530-1555 angstrom wavelength range by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on the Solar and Heliospheric Observatory (SOHO). Our method performs corrections in spatial, spectral, and temporal domains following application of the standard SUMER data reduction package to minimize the net Doppler shift of neutral and singly ionized silicon emission lines in the chromosphere. We have applied this method to seven raster observations of the same equatorial coronal hole acquired over a 5 day period, 1999 November 3-8, and to the well-studied observations of 1996 September 21-22. This technique allows us to make physically consistent analyses of multiple SUMER rasters and will aid in future investigations of the solar "rest'' wavelength of the important Ne VIII 770 angstrom emission line and, as a result, of the properties of transition region blueshifts-specifically, their correlation to multithermal radiance structure, supergranular network patterns, and the coronal magnetic environment at the perceived lower boundary of solar wind outflow.