On April 21, 2001 an interplanetary CME started to interact with the magnetosphere when CLUSTER spacecraft were travelling in the cusp region. This interaction produced a strong magnetic storm (Dst = 103 nT) at Earth. Cusp proton aurora was caused by the leading phase of the CME. Cusp proton aurora generally appear during northward IMF, this paper demonstrates that they can also be triggered during southward IMF when the reverse convection has been generated by an IMF dominated Y component. The location of the cusp proton aurora shifted about 30° from dawnside to duskside when IMF By changed from −10 to 5 nT. The requirement to produce proton cusp aurora seems to be a reverse convection in the cusp region instead of the northward IMF (not only northward IMF, but IMF By is also an important parameter!).