Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsm21a0445f&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #SM21A-0445
Physics
2730 Magnetosphere: Inner, 2731 Magnetosphere: Outer, 2736 Magnetosphere/Ionosphere Interactions, 2784 Solar Wind/Magnetosphere Interactions, 2788 Storms And Substorms
Scientific paper
Oxygen ions in the Earth's magnetosphere are supplied from two sources, that is, the ionosphere and the solar wind. The charge states of oxygen from the ionosphere are generally low such as O+, while those from the solar wind are highly charged (mainly O6+). Investigating the amount of oxygen with each charge state in various energy ranges in the magnetosphere and its source region gives us information how it is energized during travel from its origin to the magnetosphere. This would be a clue to know the acceleration mechanisms of oxygen in the magnetosphere. However, only a few satellites carried instruments which can measure the ions' charge states. Furthermore, they are not sensitive enough in the high energy range (more than 100 keV) because the number of ions in this energy range is small. It is also difficult to analyze a phenomenon in a short time scale such as ion injection from the near-Earth tail into the inner magnetosphere at substorm onset. In this study, we estimate the charge state of injected oxygen in various energy ranges by using the method of Sibeck et al. [Geophys. Res. Lett., 15, 1287 - 1290, 1988] with the flux data from the EPIC/ICS instrument on Geotail; it can measure mass of ion in the energy range of 50 keV - 3 MeV but not charge state. This method uses the energy dispersion of flux enhancements of injected ions, which is dependent on the charge state of them. This dispersion is responsible for the drift velocity of ions in the inner magnetosphere depending on their energy and charge state. We analyzed the ICS data statistically to estimate charge state of oxygen ions for the period of 1996 and October 2000 to September 2001. We also used data from the Geotail/EPIC STICS sensor, which can measure charge state and ion mass, so that the O+/O6+ ratios were given for a few cases where STICS provided enough ion flux to be analyzed for these events. From the analysis we found that (1)In the 50 to 150 keV energy range, the amount of injected O+ ions was much larger than that of injected O6+; (2)In the 150 to 250 keV energy range, the amount of injected O+ ions was comparable with that of injected O6+; (3)In higher than 250 keV energy range, the amount of injected O6+ ions was much larger than that of injected O+; and (4)The amount of injected O+ during the solar maximum period was larger than those during the solar minimum period. Results 1 - 3 would mean that the energy of solar wind oxygen in the magnetosphere would be generally higher than that of ionospheric oxygen, though solar wind oxygen might be energized more effectively only at substorm injection. Result 4 confirms the well-known fact that the amount of O+ ions of ionospheric origin become larger during the solar maximum period [Young, D.T., H. Balsiger, and J. Geiss, J. Geophys. Res., 87, 9077-9096, 1982].
Christon Stephen P.
Fujimori Toshiaki
Keika Kunihiro
Nose Mikiha
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