Then new JWST image of Jupiter shows a small halo on the right side of the planet. What's causing this feature? I figure it could be that the planet is simply overexposed (or moving slightly during the exposure?). Or perhaps this is a phase effect, and the boundary between the light and dark limbs corresponds to the jovian terminator?
This seems to be airglow.
At least Scott Sutherland claimed this in the June 15, 2022 Weather Network article Webb bonus: Jupiter's clouds, moons, and ring shine bright in new images, and Dr James O'Donoghue gave a nice explanation on Twitter. Apparently we only see this on the right side, as it is slightly in the shadow.
click images for full size:
This is incredible. What we might be seeing here is the glow of Jupiter's upper atmosphere, but separated from the planet itself! What a shot, Great Red Spot, rings, moons, moon shadow and airglow like we see on Earth! Quote Tweet
And it fits the detection in UV wavelengths.