Optical emission lines have now been detected from about 20 high velocity clouds .
These emission lines – primarily H \alpha , secondarily [ N ii ] and [ S ii ] – are very faint and diffuse , spread over the surfaces of the clouds .
We compile emission line measurements and present a model in which the H \alpha is recombination caused by photoionizing radiation escaping the Milky Way .
In such a model , we infer HVC distances of 5–30 kpc .
The photoionization model fails to explain the relatively strong H \alpha emission from the Magellanic Stream , and the O VI absorption seen by FUSE in HVCs and the MS , which require a second source of ionization ( likely collisional ) .
Regardless of mechanism , the fact that HVCs are detectable in H \alpha indicates they are not far away enough to be Local Group objects .
Adopting the HVC distances from the model , there appear to be two classes of HVCs : H \alpha -bright clouds with low velocity deviations from Galactic rotation , and often strong [ N ii ] , which are presumably affiliated with the Galactic disk ; and H \alpha -faint clouds with high velocity deviations , which are likely to be infalling gas .