Ion Upflows in the Polar Magnetosphere During Geomagnetic Storms

Details Ion Upflows in the Polar Magnetosphere During Geomagnetic Storms Ion Upflows in the Polar Magnetosphere During Geomagnetic Storms

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsm12a..03k&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #SM12A-03

Physics

2467 Plasma Temperature And Density, 2481 Topside Ionosphere, 2736 Magnetosphere/Ionosphere Interactions (2431), 2776 Polar Cap Phenomena, 2788 Magnetic Storms And Substorms (7954)

Scientific paper

We performed a case study of ion upflows enhanced during a geomagnetic storm using data obtained by the Akebono satellite. Based on the location of intense upflows and their velocities, trajectories of the upflowing oxygen ions were traced using the single particle approach. Using these results, the importance of thermal oxygen ions with a large upward flux was discussed. We used electron density data observed by the PWS, and an ion composition ratio and field-aligned velocities observed by the SMS onboard Akebono. A numerical code developed by Ebihara et al. [2006] was used for calculations of oxygen ion trajectories. In the code, the Tsyganenko-89 and Weimer-2K models were used as magnetic and electric field models, under conditions of Kp = 7, nsw = 10 /cc, Vsw = 500 km/s, IMF By = 0 and Bz = -20 nT. We performed a case study for a geomagnetic storm which occurred on March 30, 1990, using data obtained by Akebono in an altitude range of 6000-10000 km in the dayside polar region. During the main phase of the storm, the electron density enhanced 3-30 times larger than the quiet-time level in the auroral zone and polar cap. The SMS instrument measured intense ion upflows in the entire polar cap along the satellite path. Eighty percents of the upflowing ions were composed of oxygen and the upward velocities of oxygen along the field lines ranged 4-10 km/s, which was comparable to the escape velocity. The upflow flux of the oxygen ion mapped to 1000 km altitude corresponded to 1-4×109 /cm2/s. Based on the observations, we calculated trajectories of the upflowing oxygen ions released at 9000 km altitude, which was near the altitude of the ion upflow observed by Akebono. Initial velocities of the oxygen ions were given in a range of 1-12 km/s, directed to the upward field-aligned direction in the electric field (E×B) drifting coordinate. The initial positions were 8, 10, 12, 14, and 16 MLT at 75° ILAT, and 70°, 75°, and 80° ILAT at 12 MLT. At all of the initial positions, the oxygen ions which had initial velocities of 3 km/s or larger, did not fall down to the Earth, but escaped into the magnetosphere. This result indicates that a large portion of the upflowing oxygen ions observed by Akebono in the dayside polar cap during geomagnetic storms flows into the magnetosphere. The ions released from 12 MLT firstly flew into the premidnight magnetotail, and transported to the duskside at L = 3-5, where the storm-time asymmetric ring current developed. In the region, some ions were energized to more than 50 keV. These results indicate that thermal oxygen ions with a large upward flux, which cause the density enhancement in the polar cap during geomagnetic storms, can reach the plasmasheet and contribute to the ring current formation.

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