Richard,

Yes, the dark line is real. It has been there since the 13th but is much
fainter now. It was most obvious on the day of plane crossing (Oct 14)
and it lined up exactly with the anti-tail. I had assumed that it was
dust in the orbital plane attenuating the tail.

My images from the evening of the 14th show it as a very thin line:

https://spaceweather.com/images2024/16oct24/2023a3_20241014_2001_ndj.jpg

The nucleus of this comet is quite small so I wouldn't think that this
was a flow effect but it is an interesting idea.

Nick.

On 19/10/2024 20:36, rmiles.btee at btinternet.com (via baa-comet list)
wrote:
> Fascinating image, Luigi.
>
>
>
> May I draw attention to what appears to be a central dark ‘lane’ extending from the pseudonucleus a long way down the tail of the comet.
>
> Is this real and not an artifact of the processing? I would like to think it is real.
>
>
>
> The reason is that the size of the solid nucleus acting as a block to the flow of dust and gas feeding the tail and the size of the gas-inflated cloud of dust on the sunward side of the nucleus should be of comparable dimensions to create such a dark lane downwind of the nucleus. In particular, the pressure of the gas in the latter cloud should be in the so-called Knudsen region below which the gas can no longer accelerate the dust particles. This sets a limit on the effective size of this ‘close-in’ dust and gas cloud, probably <100 km across for sure.
>
>
>
> I’ll be giving a talk at the BAA on 2025 March 26 in London where this idea will be applied to outbursting comets. The title of that talk is “Eruptive Comets: A Comparison of comets 12P/Pons-Brooks and 29P/Schwassmann-Wachmann”. I would like to use your image for that talk to show the effect can manifest itself in regular comets too.
>
>
>
> Thanks,
>
> Richard