FYI, I missed this post earlier. That is highly *unlikely* IMO, especially since the surface is discharging to the heliosphere *THROUGH* the neon. What makes you think *any* of the neon will be in the ground state?
1) Who cares? The second ionization energy for Ne is *also* accessible to 171 A photons, so Ne II also contributes to continuum absorption. Do you want be to look up the cross section? From the physics (Ne II is more polarizable than Ne I) I expect the cross section to be similar or perhaps slightly larger.
2) Sol just showed you that your photons are absorbed by the first 3.5 meters of Ne I. If you imagine (absurdly) that 99% of the neon is so highly ionized that it contributes (absurdly) nothing to the opacity, then your photons are absorbed by the first 350 meters instead. Go ahead and plug that into the exponential yourself. 1 km of this stuff is still opaque.
3) Do you have any idea how absurd it is to suppose that some discharge has managed to fully ionize this plasma without heating it up? Do you want anything in this system to obey any of the laws of physics?