I'm lucky to live in an area where seeing the Northern Lights is quite common - actually I think in my bones tonight maybe good, if the fog stays low down. The best time seems to be 3 or 4 hours after sunset, and generally the LHR to SFO is chasing sunset over Greenland (somewhere I've visited a few times and haven't seen the Northern Lights!). So you're a bit early I think. I guess in mid December you may have a chance, particularly if the service is running late. And there's no harm booking a seat on the starboard side just in case. The return is theoretically better.