Poynting flux in the dayside polar cap boundary regions from DMSP F15 satellite measurements

Poynting flux, which describes electromagnetic energy flux, is an important energy source for the high-latitude upper atmosphere. After the launch of Defense Meteorological Satellite Program (DMSP) F15 spacecraft with a boom-mounted magnetometer on board, there was a new opportunity to calculate Earth-directed Poynting flux at satellite altitudes (similar to 850 km) in the upper atmosphere. A persistent enhancement of thermospheric density in the dayside polar cap boundary regions has been reported in the CHAMP satellite observations. To understand the significance of different physical mechanisms including Poynting flux and particle precipitation, and the correlation between them, a statistical study of Poytning flux and particle energy flux in the dayside cusp and low-latitude boundary layer (LLBL) regions has been conducted based on DMSP F15 measurements. DMSP satellite observations showed a dominate downward Poynting flux for most cases in the cusp region. Our analysis of DMSP F15 data for five years (2000-2004) reveals that approximately 53% of 660 cusp crossings at 800-850km showed strong downward Poynting flux (S > 10 mW/m(2)), 32% of the crossings had noticeable downward Poynting flux (S > 3 mW/m(2)), and 7% of the crossings did not show clear Poynting flux (S < 1 mW/m(2)). Only 13 out of 660 cusp crossings (similar to 2%) showed noticeable upward Poynting flux. In the LLBL region, 35% of 11,641 LLBL crossings showed significant downward Poynting flux, 34% of the crossings had noticeable downward Poynting flux, and only 13% of the crossings did not show clear Poynting flux. On average, Poynting flux in LLBL is smaller than that in the cusp. The results show a slightly negative correlation between Poynting flux and particle precipitation energy flux in the dayside polar cap boundary regions. Statistically, Poynting flux in the cusp is enhanced during interplanetary magnetic field B-y positive conditions.