Tony, Have had a look at your piece on Spaceweather.com. A nice compilation about it - very encouraging for observers, which is what we want. Fingers crossed for a good performance from Comet Pons-Brooks at the time of the April total solar eclipse. BTW: Using the terms ‘explode’ or ‘exploded’ is misleading as this gives the impression that the comet is fragmenting as a result. You would not normally say a volcano explodes, so ‘erupt’ or ‘erupted’ is much better but comet observers usually call these ‘outbursts’ so ‘outbursted’ is an accurate description. The 10-20 outbursting comets that may have cryo-volcanoes or cryo-geysers are definitely of the 'non-fragmenting' variety - that's what makes them special. Also, there are two hypotheses to explain the horns/shadow feature: the unusually-shaped vent, you mention, is the much less likely one. The shielding of the expanding cloud and dust is much the preferred hypothesis. Indeed, there is a subtle detail I shalln’t bore you with here but the effect of the nucleus is to squeeze the hydrodynamic flow rather like the wing of an aeroplane creates a Venturi effect as air speeds up and passes over the top of the wing. In the case of the comet the gas pressure is still appreciable (well within the Knudsen region) and so the Venturi effect acts around the periphery of the nucleus creating a sort of hollow shell of dust and gas, speeding it up slightly in the process – hey presto, the horns, etc. BTW: For this to take place, there has to be a minimum amount of gas produced by the eruption so it is possible to roughly calculate what that minimum quantity is for the effect to be seen. Of course, we also need to know the size of the nucleus as that too comes into the equation. HTH, Richard -----Original Message----- From: rmiles.btee@btinternet.com <rmiles.btee@btinternet.com> Sent: Monday, October 9, 2023 6:40 PM To: 'baa-comet@simplelists.com' <baa-comet@simplelists.com> Subject: RE: [BAA Comets] mismatch between the horns and the ion/dust tails Welcome on board, Tony. Am attaching an image that was shared with the group in a previous note fyi. See some comments below: -----Original Message----- From: baa-comet@simplelists.com <baa-comet@simplelists.com> On Behalf Of Dr. Tony Phillips Sent: Monday, October 9, 2023 6:06 PM To: baa-comet@simplelists.com Subject: [BAA Comets] mismatch between the horns and the ion/dust tails Hi Richard and everyone on this list! I'm the author of Spaceweather.com, and I have been following your discussion to help me report on the latest eruption of 12P. Richard, thank you for inviting me to join. I have a question about something you said in an earlier email: "Just checked on JPL Horizons, comparing the direction of the linear features in the July and October outburst comae with the direction an ion tail or the dust trail would be expected at these two epochs. Turns out that neither fit the observed directions." Approximately how big was the mismatch between the direction of 12P's horns and the expected ion/dust tails? ===> The difference between the July horns and its October ones measures about 55-60° projected on the sky. The mismatch between either of these directions and the ion tail direction and dust tail direction was almost the most it could be! Also, you speculated that "the direction has been imposed by the cryo-eruption ... in the sense of a shadow of the expanding dust shell. " Do you mean that the comet's core is physically blocking material from flowing into the gap? ===> One hypothesis is that for large cometary nuclei the physical body of the nucleus is sufficiently big that the gas-driven inflation of the dust cloud largely finishes around the time that the cloud has expanded to roughly the size of the nucleus, which then creates a significant region of solid angle over which dust was unable to flow - hence the apparent 'shadow'. It is likely to happen with 29P creating the Pacman appearance. 174P has also shown this effect. If the nucleus measures only a few km then it is swamped by the expanding cloud which essentially is isotropic in shape, e.g. 17P/Holmes in 2007. --Tony Phillips > Just checked on JPL Horizons, comparing the direction of the linear features in the July and October outburst comae with the direction an ion tail or the dust trail would be expected at these two epochs. Turns out that neither fit the observed directions. So that means the direction has been imposed by the cryo-eruption but in the sense of a shadow of the expanding dust shell. (A bit like the Pac-Man feature we see sometimes when 29P has an energetic outburst). So that means the eruption is in the OPPOSITE direction to the line of symmetry through the linear features. > > R. To unsubscribe from this list please go to https://archives.simplelists.com/confirm/?u=aR5jVcAm0PpxqWzIIDZTQWdURWgNdCOC
