On the detection of solar coronal high-velocity fields using the XUV Doppler telescope

The XUV Doppler Telescope (XDT) is a sounding rocket experiment designed to detect flows in the solar corona using filter ratios. The XDT, successfull y launched on 1998 January 31, is a normal incidence telescope composed of narrow-bandpass multilayer mirrors and capable of obtaining images 2 Angstr om above and 2 Angstrom below the Fe XIV 211 Angstrom (T = 1.7 MK) emission line. It has the potential to make a velocity map of the entire solar disk with just a few minutes of observation. The image ratio maps show features that translate to Doppler shifts of 200 km s(-1) or more, including severa l 'redshift' features located near footpoints of coronal loops. However, no corresponding velocity features were seen by the Solar and Heliospheric Ob servatory (SOHO) Coronal Diagnostic Spectrometer (CDS) in the Mg IX 368 Ang strom (T = 1 MK) line, suggesting that the features are not caused by Doppl er shift. Instead, the features seem to be related to contamination of lowe r temperature (T = 1 MK) emission lines and the nearby density-sensitive Fe XIII lines. We conclude that while no flows were positively identified by the XDT, this observing technique is capable of detecting flows of 1000 km s(-1) independently, and 200 km s(-1) when combined with simultaneous plasm a diagnostic observation.