A possible mechanism for the observed streaming of O+ and H+ ions at nearly equal speeds in the distant magnetotail

Abstract

Several observations have been reported of O+ and H+ ions streaming away from the earth with nearly equal velocities in the distant magnetotail region. In these events, the O+ ions have an energy in the range 320–3200 eV, while the H+ ions are less energetic by a factor which is approximately equal to the O+/H+ mass ratio. A possible explanation for the creation of such streaming events is presented. The mechanism involves the acceleration of O+ and H+ ions through their interaction with electrostatic hydrogen cyclotron (EHC) waves on auroral field lines at altitudes above parallel potential drops. The H+ and O+ ions are accelerated sequentially in the following manner. When an upward flowing H+ ion beam interacts with a cold background H+ ion plasma, EHC waves can be strongly excited. These waves heat the cold background H+ ions to an average energy (W(H+)) in the range of from a few tens to a few hundreds of electron volts. The resulting pitch angle distribution of the energized H+ ions peaks at angles near 110°, which leads to an upward flowing ion cloud in the northern hemisphere. When a fresh H+ ion beam permeates this warm H+ ion cloud, weak EHC waves can be excited if the beam energy is near that needed for marginal instability. If the H+ ion beam is accompanied by an O+ ion beam, part of the O+ beam can interact with the weak EHC waves, producing O+ ions with energies W(O+) ≃ 16 W(H+), and the pitch angle distribution of these O+ ions matches that of the warm H+ ions. When such a synchronism is achieved, the O+ and H+ ions stream together along the field lines away from the source region. In the distant magnetotail, their pitch angle distribution becomes field‐aligned owing to the magnetic mirror force.