The Elephant's Trunk: Spectroscopic diagnostics applied to SOHO/CDS observations of the August 1996 equatorial coronal hole

Abstract

A large equatorial coronal hole, named the “Elephant's Trunk,” was observed by the coronal diagnostic spectrometer (CDS) on board the Solar and Heliospheric Observatory (SOHO) during the last week of August 1996. Spatially averaged spectra within the hole and in neighboring quiet sun regions are analyzed and calibrated in intensity. Spectroscopic diagnostic techniques that make use of the CHIANTI atomic database are applied in an attempt to characterize such large‐scale features. Line intensity ratios are used to obtain information on the electron density, temperature, and element abundances inside and outside the coronal hole. The transequatorial coronal hole showed, on average, coronal electron densities Ne ≃ 2 − 3×l08 cm−3, a factor of 2 higher than the quiet sun areas near the hole. A differential emission measure analysis shows that emission measures in the coronal hole were much lower at temperatures above a million degrees than in the quiet sun regions. In the Elephant's Trunk, the temperature of peak emission is found to be ≃ 8×105 K, compared to about 9.7×105 K in the quiet sun. Indications of a first ionization potential (FIP) effect differentiating between cell centers and the network are found, both in coronal hole and quiet sun regions. A small plume was detected inside the Elephant's Trunk coronal hole at low latitude, on the disc.