J. Merka, J. Safrankova, Z. Memecek, J. Simunek

Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771

The polar cusps have traditionally been described as narrow funnel-shaped regions of magnetospheric magnetic field lines directly interconnected with the magnetosheath ones allowing the magnetosheath plasma to precipitate into low altitudes and ionosphere. However, recent middle- to high-altitude observations (i.e., the Interball, Hawkeye, Polar, Image, and Cluster spacecraft) reported the cusps to encompass a broad area near local noon. The present report focuses on a statistical study of the high-altitude cusp and surrounding magnetosheath regions as well as on some peculiarities of the cusp-magnetosheath transition. For a comparison of high- and low-altitude cusp determination, we map two-year Magion-4 (a part of the Interball project) observations of cusp-like plasma along model magnetic field lines (according to the Tsyganenko 96 model) down to the Earth's surface. The footprint positions show a substantial latitudinal dependence on the dipole tilt angle. The dependence can be fitted by a line with a slope of 0.14° MLAT per 1° of tilt. In contrary to previously reported IMF or solar wind influences on the cusp shape or location, we find some differences: (1) a possible IMF B_X dependence of the cusp location, (2) a splitted cusp for B_{Y ¹} 0, and (3) a smaller cusp during periods of higher solar wind dynamic pressure. The conclusions following from the statistical analysis are confirmed by case studies which reveal the physical mechanisms leading to the observed phenomena. Results have shown that (1) reconnection near the cusp not necessarily leads to an observable precipitation, (2) the cusp precipitation in one hemisphere can be supplied from the conjugated hemisphere, and (3) the current cusp geometry depends on the IMF history.

submitted, Surveys in Geophysics, 2002