Observations of ultraviolet emission lines in solar coronal holes on the disk with SUMER on SOHO

Abstract

The Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrograph and other instruments on the Solar and Heliospheric Observatory (SOHO) have observed many solar polar coronal holes (PCHs) during the minimum of the sunspot cycle in 1996 and 1997. Based on the results obtained for PCHs, investigations of near-equatorial coronal holes (ECHs) have been planned and carried out at the beginning of the new sunspot cycle, when ECHs regularly occurred on the solar disk. These studies have been performed using ultraviolet emission lines with formation temperatures ranging from 20 000 K to one million kelvin as well as continuum radiation. The spectroscopic evaluation provides information on line shifts and widths, and on the dynamics of the low corona in coronal holes (CHs). The characteristics of ECHs found are very similar to those of PCHs observed during the sunspot minimum. Specifically, we find outflow speeds of Ne ⁷⁺ ions of about 5 km s ^-1 and of Mg ⁸⁺ ions of 15 km s ^-1 with respect to quiet-Sun (QS) regions. The Doppler widths of the Mg IX (λ706) line inside ECHs and in the QS are not significantly different and correspond to v_{1/ e} ≈ 60 km s ^-1.