Okay, breaking my rule about not posting when sleep-deprived...

1.) The comet tail is ionized via its interaction with the solar wind.

2.) Both the solar wind (protons) and the light (photons) travelling from the sun, are perfectly parallel to each other.

3.) Anything which can block the light can certainly block the solar wind.

4.) Something which can block the solar wind may or may not be able to block light.

5.) Some parts of the comet's tail will be in the light-shadow of the comet's nucleus, either fully (the penumbra, which won't be much) or partially. These areas will also be in a the solar wind-shadow.

6.) Unless you are proposing that the ions will continue to flouresce when they're no longer being struck by the solar wind, then you must concede that some part of the comet tail will be in both shaded and non-flourescing.

7.) A better analogy than casting a shadow on a lightbulb would be casting a shadow on a cathode-ray tube. The sun is the electron-beam gun and the ion tale is the phosphor elements on the screen. If you put an object between the "sun" and the "tail", on the inside of the CRT, then you will absolutely cast a shadow on the screen. Since the comet nucleus is between the sun and the tail, and parts of the tail will be in both its photon-shadow and proton-shadow, those parts will neither reflect light nor flouresce.

8.) Finally, it all boils down to this: there is a feature in the picture which appears to be a shadow, and for which no other reasonable explanation has yet been advanced. If we deny the evidence of our own eyes, choosing instead to quote doctrine that explains why we just can't be seeing what we're seeing, then we will be guilty of scientism. This degrades the true spirit of science, and is how the Lindys and Dan Apteds of the world gain purchase (eg by pointing out that up until the 1880's, scientists refused to believe in meteorites, because they "just weren't possible").

But anyhow, it's a really silly issue to spend this much verbiage on... :-) So I hope that I haven't offended you by holding my ground.

Cheers,

Nathan.