That's really helpful, Ernesto.
I was hoping that it isn't too aligned with the orbital plane direction - we'll see.

Do you have one or two example images that show this well that I could use too?

Clear skies,
Richard

-----Original Message-----
From: baa-comet@simplelists.com <baa-comet@simplelists.com> On Behalf Of walcom77
Sent: 19 October 2024 20:54
To: baa-comet@simplelists.com
Subject: Re: [BAA Comets] C/2023 A3 Anti Tail October 14th 2024

Hi Richard, Nick

That dark line was visible since at least the end of September in my images. I think it is what they often call in comet literature the "shadow of the nucleus".

Clear Skies,
Ernesto

On Sat, Oct 19, 2024 at 9:44 PM Nick James <ndj@nickdjames.com> wrote:
>
> 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.j
> pg
>
> 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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