Serts-97 Measurements of Relative Wavelength Shifts in Coronal Emission Lines Across a Solar Active Region

Abstract

We used slit spectra from the 18 November 1997 flight of Goddard Space Flight Center’s Solar EUV Rocket Telescope and Spectrograph (SERTS-97) to measure relative wavelength shifts of coronal emission lines as a function of position across NOAA active region 8108. The shifts are measured relative to reference wavelengths derived from spectra of the region’s nearby quiet surroundings (not necessarily at rest) because laboratory rest wavelengths for the coronal EUV lines have not been measured to sufficient accuracy for this work. An additional benefit to this approach is that any systematic uncertainties in the wavelength measurements are eliminated from the relative shifts by subtraction. We find statistically significant wavelength shifts between the spatially resolved active region slit spectra and the reference spectrum. For He II 303.78 Å the maximum measured relative red shift corresponds to a Doppler velocity ∼ +13 km s⁻¹, and the maximum relative blue shift corresponds to a Doppler velocity ∼ −3 km s⁻¹. For Si X 347.40 Å, Si XI 303.32 Å, Fe XIV 334.17 Å, and Fe xvi 335.40 Å the corresponding maximum relative Doppler velocities are ∼ +19 and ∼ −14, ∼ +23 and ∼ −7, ∼ +10 and ∼ −10, and ∼ +13 and ∼ −5 km s⁻¹, respectively. The active region appears to be divided into two different flow areas; hot coronal lines are predominantly red-shifted in the northern half and either blue-shifted or nearly un-shifted in the southern half. This may be evidence that material flows up from the southern part of the region, and down into the northern part. Qualitatively similar relative wavelength shifts and flow patterns are obtained with SOHO/CDS spectra.