Ion Temperatures in a Solar Polar Coronal Hole Observed by Sumer onSOHO

Abstract

The temperatures of some highly charged ions in the southern solar polar coronal hole are determined from the widths of the extreme ultraviolet (EUV) lines measured by the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) instrument on SOHO (Solar and Heliospheric Observatory). Radiation from both light ions, such as Ne6+, Ne7+, Mg7+, Mg9+, Si6+, and Si7+, and heavy ions, such as Fe9+, Fe10+, and Fe11+, are recorded in off-limb observations. We refer here to the limb as the height, h0, where the limb brightening of the N IV 765 Å line maximizes. After correction of the measured line widths for instrument contributions, the intrinsic widths of these emission lines are derived. These are produced by thermal motions of the ions and turbulent wave motions of the background plasma. Since the turbulent line broadening should be the same for all the different ions, its upper limit can be determined from the widths of the iron lines, assuming zero iron temperatures. Thus, the range of kinetic temperatures permitted for the light species can be delimited. For example, the average temperature of Ne7+, at heights above the southern limb relative to h0 from 17'' to 64'', ranges between 1.3 and 5 × 106 K, and the average temperature of Ne6+ ranges between 1 and 4 × 106 K. The interpretation of these measurements does not require knowledge of the ion formation (or electron) temperature. It is also found that the ion thermal speed decreases with increasing mass per charge, while the ion temperature remains roughly constant. In another observation at heights from 167'' to 183'' above h0, the temperature of the ions increases slightly with increasing mass per charge, while the thermal speed reveals no clear trend. The upper limits of the turbulence amplitude, ⟨δv2⟩1/2, derived for these two altitude ranges are 33-37 and 44 km s-1, respectively. The implications of these results for the heating of the solar corona and the acceleration of the solar wind are discussed.