We used the FUSE ( Far Ultraviolet Spectroscopic Explorer ) satellite to observe O vi emission along two sightlines towards the edge of the interaction zone ( IZ ) between the Loop I superbubble and the Local Bubble .
One sightline was chosen because material in the interaction zone blocks distant X-ray emission , and should thus do the same for non-local O vi emission .
We measured an O vi intensity of I _ { shadowed } = 2750 \pm 550 photons cm ^ { -2 } s ^ { -1 } sr ^ { -1 } along this ‘ Shadowed ’ sightline , and I _ { unshadowed } = 10800 \pm 1200 photons cm ^ { -2 } s ^ { -1 } sr ^ { -1 } along the other sightline .
Given these results , very little ( \lesssim 800 photons cm ^ { -2 } s ^ { -1 } sr ^ { -1 } ) of the emission arises from the near side of the interaction zone , which likely has an H i column density of about 4 \times 10 ^ { 20 } { cm ^ { -2 } } along the ‘ Shadowed ’ sightline .
The O vi emission arising within Loop I ( \sim 10 ^ { 4 } photons cm ^ { -2 } s ^ { -1 } sr ^ { -1 } ) is probably associated with gas of n _ { e } \sim 0.1 cm ^ { -3 } and an emitting pathlength of \sim 1.2 pc , suggesting it arises at interfaces rather than from gas filling Loop I .
In contrast , the C iii emission is similar along both sightlines , indicating that much of the emission likely arises on the near side of the interaction zone .