Why does my DSLR have a purple halo in two corners after long exposures?

On exposures over 30 seconds I get an odd purple halo in the top two corners of the frame that gets increasingly noticeable with longer exposures. It's not the lens, I changed that out. I've had the camera (Nikon D80) for some time now and I've never seen it do this before.

Added (1).

Added (2). There's no LED that stays on and I covered the viewfinder. Right now I have no lens on it and a cover instead, and it's still there.

Your sensor may be heating up in these long exposures. Have you used 30 second exposures before or is this a new adventure for you? You would not see it in "normal" exposure times.

It can also be caused by internal light leaking - do you have LED on your camera that stays on while exposing the shot? It can also be a light leaking through a view finder - try to cover it.

Its light coming thru the view finder or lens flare

cover the view finder during exposure with black tape or a view finder cap

also use a decent lens hood - they are easy to make

EDIT: its strange, ring the manufacturor, ask in the camera section here, try photonet, what's the age of the camera? Still under warrenty? Might be a sensor issue?

Do a few more sessions of long exposures at different location. Those halos could be anything. You might not get those halos every time. And if you get them all the time then it is either camera body or sensor. And just check that no lens hood is on, the next time. (just a precaution)

There's been reported purple halos in long exposure in some other Nikon bodies - it turned out to be a light leak through the AF assist light window - try putting tape over it.

Other possible cause could be sensor heat up, but try the above first.