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 sh eet 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 groun d magnetometer records. We provide evidence that large Poynting fluxes occu r 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 to ward the ionosphere, and were associated with Alfven waves. These observati ons demonstrate the importance of Alfven wave power as a means of energy tr ansport from the distant magnetotail to the ionosphere during the most dyna mic phase of substorms.