Cluster observations of a complex high-altitude cusp passage during highly variable IMF

Abstract. On 26 February 2001, the Cluster spacecraft were outbound over the Northern Hemisphere, at approximately 12:00 MLT, approaching the magnetosheath through the high-altitude (and exterior) cusp region. Due to macroscopic motions of the cusp, the spacecraft made multiple entries into the exterior cusp region before exiting into the magnetosheath, presenting an excellent opportunity to utilize the four spacecraft techniques available to the Cluster mission. We present and compare 2 methods of 4-spacecraft boundary analysis, one using PEACE data and one using FGM data. The comparison shows reasonable agreement between the techniques, as well as the expected "single spacecraft" plasma and magnetic signatures when associated with propagated IMF conditions. However, during periods of highly radial IMF (predominantly negative B_X GSM), the 4-spacecraft boundary analysis reveals a dynamic and deformed cusp morphology.