Hi Nigel,
My first thought is "the halo is almost perfectly circular" - followed by "I need a 'circular' mask over the halo within which I will use a Histo Transform to reduce intensity"
Now, Juan posted a useful thread on how to form all sorts of clever shapes using PixelMath, so that should help create the circular mask (or even an elliptical mask, if needed), at an identifiable pixel coordinate for 'the centre'. Whether the mask is simply 'binarized' as Black and White, or 'faded' would need to be something that you would have to experiment with.
However, this is heading into the realm of 'painting' a mask - which is something that, currently, PS does better than PI - and popular (but not universal) opinion here is that PI should always be able to create stuff like masks WITHOUT having to resort to 'painting'.
However, it would be valid to resort to 'painting the mask' at first, to allow you to experiment with the HT process (an the likes) to see if the haloes CAN be reduced by this approach. That is how I would tackle the issue.
Then, you would try and create the same (or similar) mask by using the likes of ATWTR and HTto see if the image, itself, can 'provide' the necessary mask.
Remember, half the fun is in 'trying'
