The Relationship of Solar Abundance Measurements to the Electron Temperature in a Polar Coronal Hole

Abstract

We discuss the behavior of the intensity of the Mg VI λ1191.64 spectral line relative to the intensity of the Ne VI λ1005.78 spectral line as a function of height above the limb in the solar north polar coronal hole. The intensities of Mg VI lines relative to Ne VI lines have been shown to be excellent indicators of element abundance variations due to the first ionization potential (FIP) effect. We find that the Mg VI/Ne VI intensity ratio increases with height above the limb by factors ranging from 1.7 to 4 over a height range extending from about 6" above the limb to 28" above the limb. We conclude that this intensity ratio increase is primarily due to an increase of electron temperature with height, rather than the result of an FIP effect, and therefore caution must be exercised in using any Mg VI/Ne VI line ratio as an abundance diagnostic above the limb in the polar holes. At 6" above the limb, the Mg VI/Ne VI line ratio indicates that the solar Mg/Ne abundance ratio is probably within a factor of 2 of the photospheric abundance ratio. The spectra we use were recorded by the Solar Ultraviolet Measurements of Emitted Radiation spectrometer on the Solar and Heliospheric Observatory spacecraft.