Morridir's page

Maybe I didn't clarify what I meant: I believe the sheet not to refer to the sheet as described within the wall of fire text description, but to the semantic definition, "a portion of something that is thin in comparison to its length and breadth", as given by the Merriam-Webster dictionary.
If the specific rule would have been that it can only be in sheet form as in the wall of fire "sheet", I would assume this to be mentioned after the "wall of fire" part where they added the specific rules, such as "inflicts cold damage", "does not radiate heat".

I believe a sheet doesn't necessarily have to be flat. Just as a sheet of paper can be bent, so can the Aurora Borealis. If you want, you can bend it in a circle. I think it refers to the Aurora Borealis looking like the real-life northern lights, which are 'sheetlike' as in: long but thin.