Official Comet Hunting Guide
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LASCO C3 Transits
STEREO L4/L5 Campaign
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C2/C3 Comet Tracks
NASA Citizen Science
SOHO Real-Time Images
Comet McNaught in 2007
15-years of SOHO Comets
Why Do Some Months See More Comet Discoveries?If you glance at the list of SOHO comet discoveries, you might notice that certain times of year have a lot more comet discoveries than others. Why is that, you ask? Here's how it works...
Most of SOHO's discoveries -- about 85% -- come from the Kreutz group of comets. If we look at the number of Kreutz comet discoveries each month during the history of the SOHO mission, it is obvious that we get a lot more comet discoveries at certain times of the the year than others*:
It's pretty clear that (approximately) May and June, and October through December are very busy times of year, while January-March and July/August are noticeably quieter for discovering Kreutz-group comets. This was illustrated just recently when we had twenty comet discoveries in seventeen days in early June, and then since June 23 until now, we have only had three! (As an interesting aside, there is a slight peak in the number of non-Kreutz comet SOHO discoveries at these times too. A possible reason is that the comet hunters are paying more attention to the LASCO C2 images at these periods, so happen to notice more of the less-predictable non-Kreutz comets.)
So why is this?The reason we see this 'seasonality' of Kreutz comet discoveries is due primarily to the geometry of the comet orbits. The Kreutz group is a stream of (mostly very small) comets all following the same path (or orbit) through space. A good analogy would be cars on a race track, where the cars are the comets and the race track is their orbit (and it takes each car about 900 years do do one lap!). The Kreutz orbit is fixed in location in the solar system and does not move (appreciably, at least... Jupiter, for example, does actually tweak it somewhat, but on fairly slow timescales). But our location in the solar system -- and hence SOHO's location, since it stays between the Sun and the Earth at all times -- does move: we constantly travel around the Sun, completing one orbit every ~365 days. This means that at different time of the year, the Kreutz orbit is going to look like it is in a different location as we view it from different places in the solar system. Going back to the race car analogy, we can walk around the race track and the cars will look like they are going in a different direction depending on which side of the track we are -- but the track has not changed: just our view of it has. Therefore, at different times of year SOHO has a "better" view of the Kreutz orbit than at other times of year, leading to more SOHO Kreutz discoveries during these "better" times. But that's still not the full explanation...
Instrument sensitivityYou might also notice that during times of high comet discovery rates (e.g. May/June, October-December), the majority of the comets are only seen in the LASCO C2 camera. This is not a coincidence! LASCO C2 is more sensitive to Kreutz comets than LASCO C3 is. It has better resolution (~12 arc-seconds per pixel, compared to C3's ~56arc-seconds per pixel); it is more sensitive to fainter objects (limiting magnitude ~m9, whereas C3 is maybe ~m7.5 - m8); and its filter (Orange) corresponds to a wavelength of light that the Kreutz comets show up particularly well in (C3 has a 'clear' filter). Therefore, LASCO C2 allows us to see much smaller and fainter comets than C3 does, even though those comets must have passed through C3 to reach the C2 camera. Finally, as you can see from the tracks of the comets in C2, at certain times of the year, the comets do not appear in the C2 camera until they are much closer to the Sun, and thus only the larger brighter ones make it this close to the Sun while still remaining visible.
Viewing angleThere is actually one more factor in play, which is the 'phase angle' of the comet. The phase angle can be thought of as the angular distance that we observe between the comet and the Sun. The apparent brightness of the comet depends on the phase angle at which we see it, and varies according to how much of the comet appears to be illuminated by the Sun from our view-point. So at certain angles the comet can appear brighter, or fainter, than it actually is. This is not usually a strong effect for the SOHO Kreutz comets and so I will not go into it any further here, but interested readers can find easily more information about phase angle effects on many excellent web sites.
Those wishing for further reading on this subject might be interested in the following papers about light scattering in comets by Dr. J.Marcus:
* Many thanks to Dr. Matthew Knight for getting the data to me for the plot (so that I didn't have to count the discoveries myself!), and also for allowing the direct link to his paper.