Observations of a current pulse in the near‐Earth plasma sheet associated with a substorm onset

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 R E . 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.