Geotail observation of cold ion streams in the medium distance magnetotail lobe in the course of a substorm

We observed cold ion beams in the magneto‐tail lobe at X GSM ∼ −42 Re in the initial operations of the Low Energy Particle (LEP) experiment onboard the GEOTAIL satellite on August 22, 1992, when multiple onsets of substorms took place. These ion beams generally consisted of protons and singly‐charged oxygen ions (O + ), flowing tailward with nearly the same velocities ∼ 100–200 km/s. The H + number density was generally of order 10 −2 cm −3 , while the O + density was of order 10 −3 cm −3 but at times increased sporadically by an order of magnitude. These ions presumably would be transported along magnetic field lines through the polar mantle from the dayside polar ionosphere and convected to the mid‐magnetotail where the observation was made. That different ion species had equal velocities is consistent with the velocity filter effect due to E × B convection. Three‐dimensional determinations of their velocity distributions have revealed off‐ecliptic angles of the flow directions as large as several tens of degrees. We discuss this feature in terms of the enhanced convection velocity that is associated with the plasma‐sheet thinning confined to a limited region in the Y direction. Characteristic changes in the flow velocity are also found in association with a substorm plasmoid passage.