We identify a characteristic signature observed in Halley HF radar data during the substorm growth phase and investigate its relationship to magnetotail evolution leading up to substorm onset. The signature is a super-diurnal equator-ward propagation of the equatorward edge of the HF radar backscatter returns. It is found to be characteristic of a particular class of substorms, those which are the first to occur following quiet magnetospheric and ionospheric conditions. By using solar wind data, we estimate the reconnection electric field at the magnetopause for four such events. An empirical relationship is found to exist between the time integral of this electric field and the latitude of the HF radar backscatter. Relating this solely to the addition of open magnetic flux to the polar cap, we would estimate the length of the dayside reconnection X line to be 12 RE. It is likely that additional factors affect this empirical relationship. ¿ 1998 American Geophysical Union