The central ball is white, while the ejected balls that are observed are always green in colour. This is ascribed to radiation pressure produced by the interaction between very low frequency electromagnetic waves (VLF) and atmospheric ions (present in the central white-coloured ball) through ion-acoustic waves. O 2 ions (electronic transition b Σ g → a Π u), with green emission lines, are probably the only ones transported by these waves. Electronic bands of O 2 ions occur in auroral spectra.