HI Ly- intensities were measured in polar regions during sounding rocket flights in 1979, 1980, and in 1982. The instrument used for the observations was an ultraviolet coronagraph (Kohl, Reeves and Kirkham, 1978). In addition, the Ultraviolet Coronal Spectrometer on Spartan 201 measured HI Ly- intensities and line profiles in the north and south polar coronal holes in April 1993.
Intensities measured in a polar region of the corona observed in 1979 near the time of solar maximum (Withbroe, Kohl and Weiser, 1986) are provided in Table 6. Effects of the Ly- geocorona have been removed from the data. The estimated electron densities are also listed. Outflow velocities were believed to be subsonic out to 4 solar radii.
Measured HI Ly- Intensities in a polar region observed in 1979.
* (ph s cm sr)
** (cm) Intensities for a polar region observed in 1980 are provided in Table 7 (Withbroe, Kohl, Weiser and Munro, 1986). Effects of the Ly- geocorona have been removed from the data. The estimated electron densities are also listed. The outflow velocity appeared to be subsonic out to 4 solar radii.
Measured HI Ly- Intensities in a polar region observed in 1980.
* (ph s cm sr)
Figure 3.5. a--b Total intensities of the lines of the O VI resonance doublet simulated for the equatorial coronal hole at several values of heliocentric distance, considering the contribution of the coronal hole alone (filled squares), or also the contribution of the surrounding quiet corona, either expanding (stars) or static (crosses). c--d As above, but for the polar coronal hole. (From Spadaro and Ventura 1993).
Intensities for the polar coronal hole observed in 1982 are provided in Table 8 (Strachan, Kohl, Weiser, Withbroe and Munro, 1993). Polar maps derived from Mark III K-coronagraph observations indicated that the coronal hole covered a large portion of the northern hemisphere. The effects of the Ly- geocorona have been removed from the data. Electron densities derived from a white light coronagraph, which was a companion to the rocket ultraviolet coronagraph, are also provided. A Doppler dimming analysis suggested that the bulk outflow velocity was about 217 km/s at 2 solar radii. The Doppler dimming and the low densities accounted for the low Ly- intensities.
Measured HI Ly- Intensities in a polar region observed in 1982.
* (ph s cm sr)
** (cm) Intensities for the north and south polar holes observed during the Spartan 201-1 mission in 1993 are given in Table 9. The Spartan white light coronagraph determined that the north polar region had a relatively large number of polar plumes extending out to at least 3.5 solar radii. The plumes were also observed by the Mark III instrument and, in x-rays, by the Yohkoh satellite. The north coronal hole appeared to be relatively small, and its base appeared to be primarily behind the limb. The south polar hole was much larger, and the white light data showed evidence of polar plumes in the south hole. The effects of the geocorona have been removed from the data.
Measured HI Ly- Intensities in north and south polar regions observed in 1993.
* (ph s cm sr) Comparison of the theoretical intensities provided in Tables 3 and 4 to the experimental results given in Tables 6-8 provides an idea of the variation in intensity to be expected from structure to structure. Interestingly, the Spartan observations are similar to the model values in Table 3. In contrast, the 1982 intensities are considerably lower than Table 3. It appears that Doppler dimming has significantly decreased the intensity in the 1982 coronal hole. The polar region observed in 1980 had considerably higher intensities than in Table 3. This appears to be due to the higher electron density. The 1979 polar hole was similar in intensity to both the calculated values and the Spartan 201-1 observations except at 1.5 solar radii.