On variation of outer radiation belt electrons and O+ ions in the inner magnetosphere during large magnetic storms: FAST observations

Details On variation of outer radiation belt electrons and O+ ions in the inner magnetosphere during large magnetic storms: FAST observations On variation of outer radiation belt electrons and O+ ions in the inner magnetosphere during large magnetic storms: FAST observations

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsa12a..01s&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #SA12A-01

Other

2716 Energetic Particles: Precipitating, 2720 Energetic Particles: Trapped, 2730 Magnetosphere: Inner, 2778 Ring Current, 2788 Magnetic Storms And Substorms (7954)

Scientific paper

~~It is observationally known that the O+ ion population becomes an important contributor to the ring current during large magnetic storms. Its importance in terms of energy density increases with increasing geomagnetic activity. On the other hand, the processes responsible for this dramatic composition change of the ring current are still under debate. Enhanced polar outflows during geomagnetically active periods are considered responsible for the composition change. However, the link between the outflows and high-energy ring current is not clear due to the lack of low-energy ion observations in the inner magnetosphere inside GEO (L~6.6 Re). ~~The electrostatic analyzers (ESAs) and the time-of-flight energy angle mass spectrometer (TEAMS) onboard the FAST satellite had been operated in the mid-latitude regions above ~45 degrees for the past 7 years and observed ions below 30 keV. Contaminations due to penetrating radiation-belt electrons into the ESA instrument needs to be corrected before calculating the moments of these low-energy ions. The radiation belt contamination is almost uniform in energy. Utilizing this feature, we developed an automated method to subtract the radiation contamination from ESA data. Comparison between the estimated background and energetic particle data at GEO (L~6.6 Re) orbits indicates that electrons with energies of 0.5-1.5 MeV are the main contributor to the background. The background count variation during several magnetic storms indicates that the outer radiation belt electron fluxes decreased dramatically during the storm main phase, recovered from the inner L shells during the rapid recovery phase, and the peak flux location finally moved to higher latitudes (close to the pre-storm position). This time evolution is consistent with previous observations of the outer radiation belt. ~~The data after the background correction show that O+ becomes the major ion component in the inner magnetosphere during large magnetic storms in the energy range of < 30keV. One of remarkable features is that preceding the development of the O+ ring current, intense O+ ion population comprising multiple peaks at different energies in the distribution function appears in the early main phase. During a storm beginning on April 11, 2001, for example, intense O+ ions with the multiple energy bands below 12 keV were observed at low latitudes (50-60 degrees). The velocity ratio of higher-energy bands to the lowest energy band corresponds to 2, 3, 5, and 7, and it suggests an ionospheric source of these O+ ions. The result of O+ trajectory tracings in empirical magnetic and electric field models of the magnetosphere (Tsyganenko 1996 and Weimer 1996 models) supports the inference: It shows that the velocity ratio of each energy band in the O+ distribution function is explicable with an intense broad-energy O+ outflow at the timing of the interplanetary shock arrival at the beginning of the storm and the subsequent velocity filter effect on closed field lines. The trajectory tracing also suggests that the ionospheric O+ are transported to the inner magnetosphere during the main phase, and contributes, at least partially, to the O+ ring current during the large storm.

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