The Si/Ne Abundance Ratio in Polar Coronal Hole and Quiet‐Sun Coronal Regions

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Abstract
Using spectra obtained from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer flown on the Solar and Heliospheric Observatory (SOHO) spacecraft, we determine the Si/Ne abundance ratio in diffuse, interplume polar coronal hole regions, as well as the ratio relative to quiet-Sun coronal regions. Ne has the second highest first ionization potential (FIP) of solar abundant elements, and Si is a low-FIP element. Thus the Si/Ne ratio is a sensitive indicator of abundance variations due to the FIP effect. We develop new spectroscopic diagnostics for the determination of the Si/Ne abundance ratio. Assuming ionization equilibrium, we find that the Si/Ne abundance ratio in interplume polar coronal hole regions is about a factor of 2 greater than the photospheric value and is close to or the same as in coronal quiet-Sun regions. This result pertains to the electron temperature range 5-8 × 105 K. However, the combined atomic physics, instrumental, and statistical uncertainty in this result is about a factor of 2, and therefore this observed enhancement is consistent with no enhancement in the polar hole abundances. Nevertheless, our results follow the same trend, i.e., a greater than photospheric abundance ratio of low-FIP elements in the corona relative to high-FIP elements, as found from other abundance measurements in the corona that involve different atomic physics and different instruments. Therefore we feel that our results reflect an actual abundance enhancement, despite being within an uncertainty level bar that encompasses photospheric abundances. We also examine the Ne/Mg abundance ratio over a 24.5 hr observation and find no significant abundance variations. (Mg is a low-FIP element.) Thus, no large transient abundance variations appear to occur on timescales shorter than about a day, although this result is based on only one observation. From lines of Mg VII, Mg VIII, Mg IX, and Mg X we find that the electron temperature along the line of sight increases with height above the limb over the polar coronal holes, as has been previously reported. We determine the emission measure distribution as a function of height from Mg VII, Mg VIII, and Mg X lines. We determine average temperatures along the line of sight over the polar holes from Ne VIII/Ne VII, Mg VIII/Mg VII, and Si VIII/Si VII line ratios. We also discuss the temperature properties of the coronal hole and quiet-Sun regions using forbidden lines of Fe X and Fe XI. We comment on the possibility that ionization equilibrium is not valid in polar coronal hole regions, a possible scenario in light of recent observations that show outflows in coronal holes beginning at about the temperature of formation of Ne VIII.