The Si/Ne abundance ratio in polar coronal hole and quiet-Sun coronal regions

Using spectra obtained from the Solar Ultraviolet Measurements of Emitted R adiation (SUMER) spectrometer flown on the Solar and Heliospheric Observato ry (SOHO) spacecraft, we determine the Si/Ne abundance ratio in diffuse, in terplume 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 ef fect. We develop new spectroscopic diagnostics for the determination of the Si/Ne abundance ratio. Assuming ionization equilibrium, we find that the S i/Ne abundance ratio in interplume polar coronal hole regions is about a fa ctor of 2 greater than the photospheric value and is close to or the same a s in coronal quiet-Sun regions. This result pertains to the electron temper ature range 5-8 x 10(5) K. However, the combined atomic physics, instrument al, and statistical uncertainty in this result is about a factor of 2, and therefore this observed enhancement is consistent with no enhancement in th e 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 measur ements in the corona that involve different atomic physics and different in struments. Therefore we feel that our results reflect an actual abundance e nhancement, despite being within an uncertainty level bar that encompasses photospheric abundances. We also examine the Ne/Mg abundance ratio over a 2 4.5 hr observation and find no significant abundance variations. (Mg is a l ow-FIP element.) Thus, no large transient abundance variations appear to oc cur on timescales shorter than about a day, although this result is based o n only one observation. From lines of Mg vn, Mg vm, Mg Ix, and Mg x we find that the electron temperature along the line of sight increases with heigh t above the limb over the polar coronal holes, as has been previously repor ted. We determine the emission measure distribution as a function of height from Mg VII, Mg vm, and Mg x lines. We determine average temperatures alon g the line of sight over the polar holes from Ne VIII/Ne VII, Mg VIII/Mg VI I, and Si VIII/Si VII line ratios. We also discuss the temperature properti es 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 recen t observations that show outflows in coronal holes beginning at about the t emperature of formation of Ne VIII.