Acceleration of Oxygen Ions in Dipolarization Events: 1. CPS Distributions

Journal Article · 05 July 2021 · Journal of Geophysical Research. Space Physics

DOI: https://doi.org/10.1029/2021JA029184 · OSTI ID:1805315

Birn, J.  ^[1]; Hesse, M.  ^[2]; Bingham, S. T. ^[3]; Turner, D. L.  ^[3]; Nakamura, R.  ^[4]

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1. Space Science Institute Boulder CO USA, Los Alamos National Laboratory Los Alamos NM USA
2. Department of Physics and Technology University of Bergen Bergen Norway, NASA Ames Research Center Moffett Field CA USA
3. Space Exploration Sector The Johns Hopkins University Applied Physics Laboratory Laurel MD USA
4. Space Research Institute Austrian Academy of Sciences Graz Austria

Abstract Using an MHD simulation of near tail reconnection associated with a flow burst and the collapse (dipolarization) of the inner tail in combination with test particle tracing we study the acceleration and flux increases of energetic oxygen ions (O ⁺ ). The characteristic orbits, distributions, and acceleration mechanisms are governed by the dimensionless parameter σ  =  ω _ci t _n , where ω _ci is the ion gyro frequency and t _n a characteristic Alfvén time of the MHD simulation. For σ  < 1, central plasma sheet (CPS) populations after the passage of the dipolarization front are found to resemble half‐shells in velocity space oriented toward dusk. They originate from within the CPS and are energized typically by a single encounter of the region of enhanced cross‐tail electric field associated with the flow burst. For larger σ values ( σ  > 1) the O ⁺ distributions resemble more closely those of protons, consisting of two counter‐streaming field‐aligned beams and an, albeit more tenuous and irregular, ring population perpendicular to the magnetic field. The existence of the beams, however, depends on suitable earthward moving source populations in the plasma sheet boundary layer or the adjacent lobes. The acceleration to higher energies is found to indicate a charge dependence, consistent with a dominance of more highly charged ions at energies of a few hundred keV. As in earlier simulations, the simulated fluxes show large anisotropies and nongyrotropic effects, phase bunching, and spatially and temporally localized beams.

• Sponsoring Organization: USDOE
• OSTI ID: 1805315
• Journal Information: Journal of Geophysical Research. Space Physics, Journal Name: Journal of Geophysical Research. Space Physics Journal Issue: 7 Vol. 126; ISSN 2169-9380
• Publisher: American Geophysical Union (AGU)Copyright Statement
• Country of Publication: United States
• Language: English