Lol, that's not how tropospheric ducting works. If a comet was communicating with the VHF band it'd be likely doing it by way of a satellite relay -- or just with a wild amount of power. VHF, unlike HF, doesn't typically bounce off the ionosphere, but goes right through it. That changes thanks to the charge imparted upon it by the aurora, when suddenly things start reflecting. It knocks out a lot of the HF bandwidth which is something of a bummer but it does give us some playful chaos in the VHF frequencies.