For the first time three different methods have been used to calculate the global merging rate during the same substorm growth phase. The ionospheric plasma drift was monitored by six of the Northern Hemisphere SuperDARN radars, allowing the convection pattern to be studied over 12 h of magnetic local time. The radars observed reconnection signatures on the dayside simultaneously with substorm signatures on the nightside. The three methods to calculate the global merging rate are: (i) the equatorward expansion of radar backscatter on the nightside, which provides an estimate of the rate of polar cap expansion, while upstream WIND measurements gave an estimate of the reconnection electric fields; (ii) the derivation of the dayside boundary normal plasma flow velocity and an estimate of the extent of the ionospheric merging gap, from radar observation of dayside reconnection; (iii) utilizing the map-potential technique to map the high-latitude plasma flow and cross polar cap potential (Ruohoniemi and Baker, 1998), allowing the global dayside merging rate to be calculated. The three methods support an extensive magnetopause X-line length of between 30 ± 12RE and 35 ± 15 RE (assuming a single X-line and constant merging rate). Such close agreement between the different methods of calculation are unexpected, especially as the length of the magnetopause X-line is not well known. ; CUTLASS is supported by the Particle Physics and Astronomy Research Council (PPARC grant PPA/R/R/1997/00 256). U.K., the Swedish Institute for Space Physics, Uppsala, and the Finnish Meteorological Institute. Helsinki. GP is supported by PPARC grant PPA/G/O/1999/00 181. The authors wish to thank the Canadian Space Agency for providing the CANOPUS data, G. Sofko for providing HF radar data from the Saskatoon radar, P. T. Newell for providing DMSP data and R. A. Greenwald, J. M. Ruohoniemi and R. Barnes for providing the map-potential software. ; Peer-reviewed ; Publisher Version