We have used the Wisconsin H \alpha Mapper ( WHAM ) facility to measure the interstellar H \alpha emission toward the high Galactic latitude O star HD 93521 ( \ell~ { } = ~ { } 183 \fdg 1 , b~ { } = ~ { } +62 \fdg 2 ) .
Three emission components were detected along this line of sight .
These components have radial velocities of –10 km s ^ { -1 } , –51 km s ^ { -1 } , and –90 km s ^ { -1 } with respect to the local standard of rest ( LSR ) and have H \alpha intensities of 0.20 R , 0.15 R , and 0.023 R , respectively , corresponding to emission measures of 0.55 cm ^ { -6 } pc , 0.42 cm ^ { -6 } pc , and 0.06 cm ^ { -6 } pc .
We have also measured the H \alpha emission toward the direction \ell~ { } = ~ { } 148 \fdg 5 , b~ { } = ~ { } +53 \fdg 0 , which lies in the region of exceptionally low H i column density known as the Lockman Window .
An emission component is detected in this direction at –1 km s ^ { -1 } ( LSR ) with an intensity of 0.20 R ( 0.55 cm ^ { -6 } pc ) .
In addition , we studied the direction \ell~ { } = ~ { } 163 \fdg 5 , b~ { } = ~ { } +53 \fdg 5 .
No Galactic emission was detected along this line of sight , and upper limits on the possible intensity of Galactic emission toward this direction are 0.11 R at the LSR and 0.06 R at –50 km s ^ { -1 } .
As part of the process of separating the interstellar from the terrestrial emission , we also detected and characterized twelve faint ( \sim 0.03-0.15 R ) , unidentified atmospheric lines present in WHAM H \alpha spectra .
Lastly , we have used WHAM to obtain [ O i ] \lambda 6300 spectra along the line of sight toward HD 93521 .
We do not conclusively detect interstellar [ O i ] emission toward the star , but place an upper limit of 0.060 R on the [ O i ] intensity of the intermediate velocity ( –51 km s ^ { -1 } ) component .
If the temperature of the gas is 10,000 K , this limit implies that within the H \alpha emitting region , the hydrogen ionization fraction n ( \mathrm { H } ^ { + } ) / n ( \mathrm { H } _ { \mathrm { total } } ) > 0.6 .