Thanks, Nick, for confirmation that this dark lane feature is real - I can see it in the simple log-stretch image of yours at the top.
There are drawings of comets observed visually that sometimes show a similar feature.

To be clear, you see it as the shadow of the dust in the anti-tail projected on the actual tail of the comet and so it would therefore not be a narrow cylinder within the tail that has a lower density of dust particles, which was my hypothesis?

Maybe it could be a combination of both effects?
Something to think about.
Both approaches are amenable to calculation.

Richard

-----Original Message-----
From: baa-comet@simplelists.com <baa-comet@simplelists.com> On Behalf Of Nick James
Sent: 19 October 2024 20:43
To: baa-comet@simplelists.com
Subject: Re: [BAA Comets] C/2023 A3 Anti Tail October 14th 2024

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

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