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

Abstract

Observations of the quiet o †-limb corona in most EUV lines and in white light are compatible with a (nearly) hydrostatic stratiÐcation of the density at a temperature of 1.2È1.5 MK. We show that, in contrast , the quiet solar corona in the 171 and 195 passbands of the SOHO/EIT has an e †ective emission A Ž scale height that signiÐcantly exceeds that for hydrostatic stratiÐcation at the observed characteristic temperature. We conclude that this di †erent 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 Ðt to an approximate model suggests that roughly 1/10 to possibly more than half of the detected photons in EIT (and in T RACE) images in the 195 and 171 passbands, respectively, of the A Ž quiet solar corona have been subjected to scattering. This scattering is responsible for much of the background haze that is seen in high-resolution T RACE 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.