Longitudinal structure of oxygen torus near the plasmapause deduced from simultaneous observations by Arase and Van Allen Probes
Abstract
In the deep inner magnetosphere, O+ ion density sometimes enhances in a limited range of L shell, which is known as "the oxygen torus". This is a term that was originally used by Chappell [1982] and Horwitz et al. [1984] to imply a longitudinal spread of the O+ density enhancement. However, there has been only one study about MLT distribution of the oxygen torus by Roberts et al. [1987] who analyzed the DE-1/RIMS data and statistically found broad peaks of the occurrence distribution in the premidnight and morning sectors. No simultaneous observations of the oxygen torus at different MLT by multiple satellites have been performed. Now is a good time to investigate the MLT distribution of the oxygen torus, because the Exploration of energization and Radiation in Geospace (ERG) "Arase" satellite and the Van Allen Probes continue a survey in the deep inner magnetosphere at a radial distance less than 6.0 RE.
Using the magnetic field obtained by the MGF instrument and plasma wave data obtained by the PWE/HFA and OFA instruments on board the Arase satellite, we search for enhancements of O+ ion density in the deep inner magnetosphere. We estimate the local plasma mass density (ρL) and the local electron number density (neL) from the resonant frequencies of standing Alfvén waves and the upper hybrid resonance band, respectively. The average ion mass (M) can be calculated by M ρL/neL under the assumption of quasi-neutrality of plasma. During 0400-0710 UT on April 24, 2017, Arase was flying along an inbound path from (L, MLT)=(6.2, 3.4 hr) to (2.0, 9.0 hr). Arase detected an enhancement of M up to 3.5 amu around L=5 and MLT=5.0 h, implying that the plasma consists of 83% H+ and 17% O+. This enhancement was found just outside of the region where neL has a steeper radially-inward gradient. The same analysis was applied to the magnetic field and plasma wave data obtained by the EMFISIS instrumentation suite onboard the Van Allen Probes. Probe-A was outbound from (L, MLT)=(2.0, 12.0 hr) to (6.2, 17.7 hr) at 0410-0740 UT on the same day. We found that M shows no clear enhancements and rather stays between 1-2 amu in the afternoon sector. These results suggest that the longitudinal extent of the O+ density enhancement in the dep inner magnetosphere is not described by a torus but a crescent-shaped torus or a pinched torus.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMSM12A..05N
- Keywords:
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- 2730 Magnetosphere: inner;
- MAGNETOSPHERIC PHYSICSDE: 2774 Radiation belts;
- MAGNETOSPHERIC PHYSICSDE: 7845 Particle acceleration;
- SPACE PLASMA PHYSICSDE: 7867 Wave/particle interactions;
- SPACE PLASMA PHYSICS