Variation of the cold plasma density structure above the polar ionosphere associated with geomagnetic storms

Details Variation of the cold plasma density structure above the polar ionosphere associated with geomagnetic storms Variation of the cold plasma density structure above the polar ionosphere associated with geomagnetic storms

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsm51c0671k&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SM51C-0671

Physics

2481 Topside Ionosphere, 2736 Magnetosphere/Ionosphere Interactions (2431), 2776 Polar Cap Phenomena, 7954 Magnetic Storms (2788)

Scientific paper

Plasma outflow from the polar ionosphere into the magnetosphere is one of the most important processes in the magnetosphere-ionosphere coupling in the polar region. Recent satellite observations have clarified that plasma outflow takes an important role for plasma transport into the magnetosphere, abrupt changes of the ring current ion composition, and the disappearance of the auroral acceleration region during geomagnetic storms. In the present study, we analyzed the electron density data observed by the Akebono satellite in an altitude range from 300 to 10500 km, in order to clarify the formation process of the plasma density enhancement above the polar ionosphere. The electron density along the satellite path was derived using the upper-hybrid resonance (UHR) frequency and maximum frequency of whistler-mode waves observed by the PWS instrument onboard the Akebono satellite with the time resolution of 2 seconds. In the present data analysis, we used the electron density data from March, 1989 to July, 1990 for statistical analyses. First, we investigate the statistically averaged density distributions during the quiet time in summer, equinox, and winter seasons. The data are sorted by day and night in magnetic local time, 5 degrees in invariant latitude and 100 km in altitude. The logarithmically averaged data in each bin are fitted by using the non-linear least square fitting method in altitude direction, using the equation of sum of the exponential and power law functions. Then, the fitted profiles are interpolated in ILAT direction by exponential functions. Finally, we obtain electron density distribution on the meridian plane. From comparing these distributions, it is identified that electron density in summer is 5 to 50 times larger than that in winter below 5000 km altitude in the polar cap and auroral zone. Next, we perform case studies for the geomagnetic storm events which occurred on June 6, June 9, 1989 and March 30, 1990. In these events, enhancements of the plasma density are identified in the entire polar cap being associated with the period of storm main phase. In June 7, the electron density enhanced up to 100 times larger than the quiet-time level. In March 30, the SMS instrument onboard the Akebono satellite observed upward flow of oxygen ions, in the electron density enhancement region. These results indicate that a large amount of the ionospheric plasma flows upward to at least about 10000 km in the polar magnetosphere during geomagnetic storms.

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