A Case for Resonant Scattering in the Quiet Solar Corona in Extreme‐Ultraviolet Lines with High Oscillator Strengths

Abstract

Observations of the quiet off-limb corona in most EUV lines and in white light are compatible with a (nearly) hydrostatic stratification of the density at a temperature of 1.2-1.5 MK. We show that, in contrast, the quiet solar corona in the 171 and 195 A passbands of the SOHO/EIT has an effective emission scale height that significantly exceeds that for hydrostatic stratification at the observed characteristic temperature. We conclude that this different behavior is caused primarily by resonant scattering that occurs in the EUV lines of Fe IX, Fe X, and Fe XII as a result of their exceptionally high oscillator strengths. A fit to an approximate model suggests that roughly 1/10 to possibly more than half of the detected photons in EIT (and in TRACE) images in the 195 and 171 A passbands, respectively, of the quiet solar corona have been subjected to scattering. This scattering is responsible for much of the background haze that is seen in high-resolution TRACE images of the quiet corona; the hotter active-region corona is subject to much less scattering. We discuss these results in view of stellar coronal studies and future solar instrumentation.