Observations of ion streaming during substorms

On the basis of three‐dimensional measurements of energetic particles and plasma from the ISEE 1 spacecraft, the occurrence of ion beams at the plasma sheet boundary in the magnetotail previously reported by Williams (1981) is examined in relation to substorm activity. In all seven cases studied it is found that ion beams streaming earthward and/or tailward are always present at the edge of the plasma sheet adjacent to the tail lobe. Ion beams penetrating into the plasma sheet region with no detectable density gradient are also observed. The observed ion beams may occur at high energies of tens of keV, or at medium energies of a few keV, or even at energies as low as tens of electron volts. Their intensities and energies do not have any obvious relationship to the substorm onset and expansion phases or the magnitude of the substorm disturbance as indicated by the AE index. However, ion beams tend to be more intense and occur at higher energies when the plasma sheet is energized during the substorm recovery phase. The result suggests that the activation of the source for the streaming ions is not directly associated with any particular phase of a substorm.