Timescales for the penetration of IMF By into the Earth's magnetotail

Cluster data were obtained from the Cluster Active Archive (http://caa.estec.esa.int/caa/). The OMNI data were obtained from the SPDF OMNIWeb interface at http://omniweb.gsfc.nasa.gov. S.D.B. was supported by Science and Technology Facilities Council (STFC) studentship ST/M503794/1 R.C.F. was supported by STFC Ernest Rutherford Fellowship ST/K004298/2. A.G. was supported by STFC grant ST/M001059/1. S.E.M. was supported by STFC grant ST/N000749/1. The work at the Birkeland Centre for Space Centre, University of Bergen, Norway, was supported by the Research Council of Norway/CoE under contract 223252/F50.