In situ observations of magnetotail reconnection prior to the onset of a small substorm

On April 15, 1979 at 0645 UT, several minutes prior to the expansion phase onset of a weak substorm, the ISEE 1 and 2 spacecraft were located 16 R E downtail and close to local midnight. The two spacecraft were on opposite sides of the current sheet, allowing us to better constrain the possible interpretations of the available plasma, magnetic field, and energetic particle data. Plasma sheet acceleration occurring earthward of the spacecraft was evident in the energetic particle data 2–3 min before the first ground‐based signatures of substorm onset. The ensuing magnetic field and particle changes are consistent with an X ‐type neutral line moving tailward of ISEE 1 and 2 with an apparent speed of ∼30 km/s, while the plasma sheet thickness decreased to a minimum of less than 0.2 R E . Although the substorm effects at geosynchronous altitude and on the ground were weak, the reconnection rate was locally quite intense (>1 mV/m) and produced earthward bursty bulk flows of peak velocity >600 km/s. Ion velocity‐space distributions confirm that the flows are indeed convective flows of a single‐ion population and not counterstreaming beams, as is often the case at the plasma sheet boundary. The fast tailward flows observed at the initial stage of reconnection were localized to a thin layer surrounding the neutral sheet. The magnitude of the current disruption and the region 1 sense field‐aligned currents associated with reconnection were sufficiently large to explain the intensity of the substorm current wedge, as inferred from ground magnetometers.