Large Alfven wave power in the plasma sheet boundary layer during the expansion phase of substorms

Observations by the Polar satellite of large Poynting flux in the plasma sheet boundary layer at geocentric distances of 4 to 6 R-E and between 22 and 3 hrs magnetic local time were correlated with H-bay signatures from ground magnetometer records. We provide evidence that large Poynting fluxes occur during the substorm expansion phase. The Poynting fluxes exceeded 1 ergs/cm(2)s (125 ergs/cm(2)s when mapped to 100 km), were dominantly directed toward the ionosphere, and were associated with Alfven waves. These observations demonstrate the importance of Alfven wave power as a means of energy transport from the distant magnetotail to the ionosphere during the most dynamic phase of substorms.