Detection of the oxygen torus in the inner magnetosphere using toroidal Alfvén waves

Details Detection of the oxygen torus in the inner magnetosphere using toroidal Alfvén waves Detection of the oxygen torus in the inner magnetosphere using toroidal Alfvén waves

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm33b1903n&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #SM33B-1903

Physics

[2730] Magnetospheric Physics / Magnetosphere: Inner, [2752] Magnetospheric Physics / Mhd Waves And Instabilities, [2768] Magnetospheric Physics / Plasmasphere, [2788] Magnetospheric Physics / Magnetic Storms And Substorms

Scientific paper

It is reported by previous studies employing the RIMS instrument onboard the DE-1 satellite that a large amount of thermal O+ ions exists in the outer plasmasphere (L=2.0-5.0) [Horwitz et al., 1984; Roberts et al., 1987; Comfort et al., 1988; Fraser et al., 2005], which is called "the oxygen torus". These thermal O+ ions can be frequently observed even in low Kp periods (Kp=0-2+) in the late evening hours. After the termination of the DE-1/RIMS operation in ~20 years ago, there have been no instruments that make a direct measurement of the oxygen torus, because of difficulty in measuring thermal ion flux with mass and charge state information. In this study, we intend to detect the oxygen torus with an indirect method, using the magnetic field data and the plasma wave data obtained by the CRRES satellite. We estimate the magnetospheric local mass density (ρ) from the frequency of the toroidal standing Alfvén waves. The local electron number density (ne) is derived from the plasma wave spectra. Combining these two quantities makes us possible to infer the local average ion mass (M=ρ/ne). We find that M is approximately 3 in the outer magnetosphere (or the plasma trough), while it sometimes shows an enhancement to 5-15 around the plasmapause. This implies an existence of the oxygen torus near the plasmapause. The above method adopting the toroidal wave frequency can be considered as a powerful tool to detect the oxygen torus and to examine the ion mass in the inner magnetosphere. Recent study by Nosé et al. [2010] presumed that the preexisting thermal O+ ions in the oxygen torus are locally and nonadiabatically accelerated by fluctuations associated with dipolarization in the deep inner magnetosphere, resulting in formation of the O+-rich ring current. We will also discuss analysis results of CRRES data in the context of the oxygen torus as an important source of the O+-rich ring current.

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