Two things: 1st: The medieval Church modes are not the same thing as the Greek modes. Calling Gregorian Chants by "Dorian", "Aeolian" etc is tempting but they are different things and therefore the Greek names are an incorrect term to be applied to Gregorian Chants. The Dies Irae is neither Dorian nor Aeolian. It is in Mode 1 or "authentic Protus". 2nd: The Dies Irae begins on FA but the sequence is FA-MI-FA-Re-MI-DO-RE-RE.

Great to see you hear Jacob! Thanks for joining! :)