... The most spectacular form of discrete auroras is the substorm related auroral bulge that grows out from a quite ("breakup") arc that is deformed . The bulge is formed close to the midnight sector, and it is characterized by rapid poleward motion. ... a surge (westward termination of the bulge, also known as the Westward travelling surge, WTS) . ... Res., ... Nakamura, R., T. Oguti, T. Yamamoto, and S. Kokubun, Equatorward and poleward expansion of the auroras during auroral substorms, J. Geophys. ...