The O+ Density Trough at 5000 km Altitude in the Polar Cap

Details The O+ Density Trough at 5000 km Altitude in the Polar Cap The O+ Density Trough at 5000 km Altitude in the Polar Cap

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsm12a1203z&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #SM12A-1203

Physics

2431 Ionosphere/Magnetosphere Interactions (2736), 2467 Plasma Temperature And Density, 2475 Polar Cap Ionosphere, 2776 Polar Cap Phenomena

Scientific paper

At altitudes near 5000 km over the Southern polar cap region of the terrestrial magnetosphere/ionosphere, the Thermal Ion Dynamics Experiment (TIDE) onboard the Polar satellite has observed O+ ion density trough regions, in which the O+ densities were at least one order of magnitude lower than the surrounding O+ densities. In the O+ density trough regions, the estimated O+ densities were generally lower than 0.01 per cc. From December 1, 1997 to November 30, 1998, polar cap O+ troughs in Polar/TIDE observations occurred at a frequency of about 48%. Statistical examination of the Polar perigee observations from December 1, 1997 to November 30, 1998 shows that the Polar perigee passes evenly covered the southern polar cap region, while the O+ density trough was always located on the nightside portion of the polar cap magnetosphere/ionosphere, and that invariant latitude spans of such troughs could be as large as 23o in extent. The trough occurrence displayed strong seasonal dependence; in the winter season (e.g. for July in the southern hemisphere) the O+ ion density trough occurrence frequency ranged up to 92%, while in the summer season (e.g. for January in the southern hemisphere) it decreased to as infrequent as 15%. The O+ ion density trough occurrence appeared relatively independent of the geomagnetic Kp index, and IMF Bz, By conditions, although it generally anti-correlated with the solar wind dynamic pressures in the solar wind dynamic pressure range 0.8 - 2.6 nanopascal. However, as suggested by the seasonal dependence, the O+ ion density trough occurrence was strongly related to the solar zenith angle (SZA). In the SZA range 50o to 125o, the trough occurrence increased monotonically with SZA. Also, case-by-case examinations of near-simultaneous O+ densities and vertical velocities observed by the DMSP satellite group orbiting at 840 km altitude indicate that the O+ density troughs observed at 5000 km altitude exhibit moderate correlation or anti-correlation with topside ionosphere density or velocity features at 840 km altitude for approximately half of the near-conjunction cases examined.

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