Generation of cold O+ beams observed in the tail lobe by weak fast shocks in the polar magnetosphere

Abstract

Observations of cold O+ beams (COBs) in the lobe/mantle region of Earth's magnetotail showed that O+ ions originating from the ionosphere can stream into the distant tail (40–200 RE). These O+ ions have a high parallel streaming energy (∼1–20 keV) and low perpendicular thermal energy (∼70–210 eV) in the distant tail. In this paper, we propose that the nonadiabatic shock heating of O+ ions in the polar magnetosphere and the subsequent adiabatic evolution of ion velocity can lead to the occurrence of COBs in the tail lobe. The heating and acceleration of heavy O+ ions by fast shocks are studied by a theoretical analysis and hybrid simulations. It is found that after the passing of fast shock, heavy ions gain a gyration speed Vg ≃ MAVAsin(θ2 − θ1)/cosθ1, where MA is the upstream Alfvén Mach number, VA is upstream Alfvén speed, and θ1 (θ2) is upstream (downstream) shock normal angle. After heating, the adiabatic evolution in the tail lobe can transfer a major part of perpendicular thermal energy T⊥ to the observed parallel streaming energy W∥. We have found that weak fast shocks in the polar magnetosphere with MA ≃ 1.05–1.2 can lead to the observed streaming energy associated with COBs. Weak fast shocks in the magnetosphere can be generated by the interaction of interplanetary shocks/discontinuities with Earth's magnetosphere. Escaping ionospheric O+ ions can gain enough energy from shock heating to account for observations of COBs in the tail lobe. For example, the ion heating by a shock with MA ∼ 1.11 at a radial distance of rs = 6.5 RE can lead to COBs with O+ streaming energy W∥ ∼ 5 keV and perpendicular thermal energy T⊥ ∼ 220 eV observed at x = 185 RE.