Coherent magnetic oscillations precede edge localized modes (ELMs) in
TCV. The precursor has been detected prior to ELMs considered to be of
type III and others previously referred to as TCV large: ELMs. This p
ermits the identification of both as type III ELMs according to the us
ual classification scheme. The strong localization of these precursors
on the bad curvature side of the plasma and their medium toroidal mod
e numbers indicate their ballooning character. Unlike conventional MHD
modes, these modes start toroidally localized and grow in amplitude a
nd toroidal extent. When the precursor encompasses the whole toroidal
circumference, the increased transport phase, as indicated by the char
acteristic D-alpha spike, begins.