Forum der Vereinigung der Sternfreunde

Here's my attempt at one explanation:

The ‘notch’ in the coma may be the missing dust caused when the gas released in the outburst has for the first 10 minutes or so sufficient pressure to inflate the cloud of ejected dust and accelerate the particles in all directions apart from where the nucleus blocks its motion. (This is also a feature of comet 29P which sometimes exhibits a 'Pac-Man' shaped coma after a strong outburst).

The hypothesis is that the nucleus is a very slow rotator (period of say 5 days or more - that underlies its ability to strongly outburst). If you imagine the rotational axis tilted almost side-on to the observer (like Uranus) and the morning terminator is to the south, the outburst happens as the outburst source appears over that limb and is locally heated by the rising sun. As you initially surmised, the escaping dust can imprint a sort of ‘shadow’ in the moving coma cloud owing to the missing material in the sheltered zone created as the dust is accelerated away from the nucleus but is partially blocked in certain directions.

Note the 'notch' in the outburst coma is rather exaggerated in length and seems to comprise two bulges. If the nucleus is bi-lobal in shape (akin to 103P/Hartley), then if the outburst source was towards the end of one lobe, the expanding gas-and-dust cloud would then pass the waist feature (between the two lobes), and a small fraction of dust will start to be accelerated towards the waist for a short while before its motion takes it further outwards and past the end of the nucleus. The result is a rough imprint of the elongated nucleus in the 3-day-old outburst coma.

Lehman, 0.28-m, Weimar (Germany, D15), G, 5.5"
20.83 16.23
24.82 15.38