Solar Coronal Structures and Stray Light in TRACE

Abstract

Using the 2004 Venus transit of the Sun to constrain a semi-empirical point-spread function for the TRACE EUV solar telescope, we have measured the effect of stray light in that telescope. We find that 43% of 171A EUV light that enters TRACE is scattered, either through diffraction off the entrance filter grid or through other nonspecular effects. We carry this result forward, via known-PSF deconvolution of TRACE images, to identify its effect on analysis of TRACE data. Known-PSF deconvolution by this derived PSF greatly reduces the effect of visible haze in the TRACE 171A images, enhances bright features, and reveals that the smooth background component of the corona is considerably less bright (and hence much more rarefied) than commonly supposed. Deconvolution reveals that some prior conlclusions about the Sun appear to have been based on stray light in the images. In particular, the diffuse background "quiet corona" becomes consistent with hydrostatic support of the coronal plasma; feature contrast is greatly increased, possibly affecting derived parameters such as the form of the coronal heating function; and essentially all existing differential emission measure studies of small features appear to be affected by contamination from nearby features. We speculate on further implications of stray light for interpretation of EUV images from TRACE and similar instruments, and advocate deconvolution as a standard tool for image analysis with future instruments such as SDO/AIA.