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

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

Nov 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004jgra..10911214l&link_type=abstract

Journal of Geophysical Research, Volume 109, Issue A11, CiteID A11214

Physics

Plasma Physics

Space Plasma Physics: Charged Particle Motion And Acceleration, Space Plasma Physics: Shock Waves, Magnetospheric Physics: Magnetotail Boundary Layers, Magnetospheric Physics: Magnetopause, Cusp, And Boundary Layers, Magnetospheric Physics: Solar Wind/Magnetosphere Interactions

Scientific paper

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 $\simeq$ MAVAsin($\theta$2 - $\theta$1)/cos$\theta$1, where MA is the upstream Alfvén Mach number, VA is upstream Alfvén speed, and $\theta$1 ($\theta$2) is upstream (downstream) shock normal angle. After heating, the adiabatic evolution in the tail lobe can transfer a major part of perpendicular thermal energy T$\perp$ to the observed parallel streaming energy W$\parallel$. We have found that weak fast shocks in the polar magnetosphere with MA $\simeq$ 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$\parallel$ ~ 5 keV and perpendicular thermal energy T$\perp$ ~ 220 eV observed at x = 185 RE.

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