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.

Statistical properties and possible supply mechanisms of tailward cold O+ beams in the lobe/mantle regions

“…Candidate mechanisms such as the resonant acceleration by electrostatic hydrogen cyclotron (EHC) waves [e.g., Singh and Schunk, 1985] and the ion-ion two-stream instability [e.g., Winglee et al, 1989] have been proposed.…”

Recent advances in studies of magnetosphere-ionosphere coupling.

“…It was found that the oblique double layer does increase both the perpendicular and the parallel energy of ions, but the amount of perpendicular energization is greater for O+ than for H+, in agreement with observations. A possible mechanism has just been reported by SINGH and SCHUNK (1985) to explain the observed streaming of O+ and H+ ions at nearly equal speeds in the tail. This mechanism involves the acceleration of the ions through their interaction with electrostatic hydrogen cyclotron waves, what they call EHC, on auroral field lines.…”