Observations of dawn-dusk aligned polar cap aurora during the substorms of January 21, 2005

► This paper reports that a new type of polar cap aurora, dawn-dusk aligned polar cap aurora (DDAPCA), was detected during the exceptionally intense January 21, 2005 substorm (AEmax=3504 nT). ► This transpolar aurora event was developed during the southward IMF. ► It is different from the “theta aurora” and “double oval”. ► It is speculated that the DDAPCA arises from the formation of an X-line in the distant (>80RE) tail due to polar cap magnetic field reconnection under unusually high solar wind compression conditions.