What's the pale greenish light in FOTR

While Frodo is in the wight's barrow, during Chapter 8 of FOTR, what is the pale green-ish light that appears?

As he lay there, thinking and getting a hold of himself, he noticed all at once that the darkness was slowly giving way: a pale greenish light was growing round him.

It did not at first show him what kind of a place he was in, for the light seemed to be coming out of himself, and from the floor beside him, and had not yet reached the roof or wall.

I'm familiar with most of Tolkien's works that deal with Arda, but I've never understood what the pale-green light was supposed to be, or how it was produced. Was it perhaps, a manifestation of the Ring's power or of Eru Ilúvatar's influence in behalf of the ring-bearer?