HR: 1330h
AN: SM12A-1203    [PDF]
TI: The O+ Density Trough at 5000 km Altitude in the Polar Cap
AU: * Zeng, W
EM: zengw@cspar.uah.edu
AF: Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, Huntsville, AL 35899 United States
AU: Horwitz, J L
EM: horwitzj@cspar.uah.edu
AF: Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, Huntsville, AL 35899 United States
AU: Craven, P D
EM: paul.craven@msfc.nasa.gov
AF: Space Sciences Department, Code SD50, NASA/MSFC/NSSTC, Huntsville, AL 35805 United States
AU: Rich, F J
EM: Frederick.Rich@hanscom.af.mil
AF: Air Force Research Laboratory, Hanscom AFB, Hanscom, MA 01731 United States
AU: Moore, T E
EM: thomas.e.moore@nasa.gov
AF: Laboratory for Extraterrestrial Physics, Code 692, NASA/Goddard Space Flight Center, Greenbelt, MD 20771 United States
AB: 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.
DE: 2431 Ionosphere/magnetosphere interactions (2736)
DE: 2467 Plasma temperature and density
DE: 2475 Polar cap ionosphere
DE: 2776 Polar cap phenomena
SC: SPA - Magnetospheric Physics [SM]
MN: 2003 Fall Meeting