The onset of an isolated magnetospheric substorm occurred at about 0437 UT on 9 February 1995 while the Geotail spacecraft was located in the premidnight sector at a geocentric radial distance of about 30 RE. The onset occurred during a slow northward turning of the interplanetary magnetic field after a period of weakly southward fields as observed by the Wind spacecraft upstream from Earth's bow shock. For several minutes immediately following the onset of the substorm an intense field-aligned current, with maximum current densities of 30 nA/m2, was observed at the Geotail spacecraft. This is the first report of a directly detected current pulse coincident with onset. Unlike most of the magnetotail field-aligned currents which are carried primarily by electrons, this impulsive current at onset is carried by both electrons and protons. It is possible that the onset current pulse is a signature of a trigger mechanism for the subsequent expansion phase of a substorm.¿ 1997 American Geophysical Union