THE DEPENDENCE OF HIGH-LATITUDE PC5 WAVE POWER ON SOLAR-WIND VELOCITYAND ON THE PHASE OF HIGH-SPEED SOLAR-WIND STREAMS

We have calculated the integrated ULF wave power in the Pc5 band at tw o stations, Kevo (part of the International Monitor for Auroral Geomag netic Effects (IMAGE) magnetometer array in Scandinavia, at auroral zo ne latitudes), and Cape Dorset (part of the Magnetometer Array for Cus p and Cleft Studies (MACCS) in Arctic Canada, at cusp latitudes), and compared this power against the solar wind velocity for the last six m onths of 1993, a period characterized by two persistent high-speed sol ar wind streams. We find for both local noon at Cape Dorset, and for l ocal morning at Kevo, the Pc5 band power (0.002 - 0.010 Hz) integrated over a six-hour period exhibits a clear power-law dependence on the s olar wind velocity. At Cape Dorset we found power alpha V-SW(4), with a correlation coefficient r = 0.73, and at Kevo we found power alpha V -SW(6.5), with r = 0.74. Much of the remaining variation in Pc5 power is due to temporal patterns evident at both stations in response to re current high speed streams. Power was strongest at the leading edge of each high speed stream and subsequently decreased more quickly than V -SW. Our observations suggest that it is insufficient to make estimate s of Pc5-range ULF wave power on the basis of V-SW alone: one must con sider other physical factors, either intrinsic to the solar wind or re lated to its interaction with Earth's magnetosphere. The Kelvin-Helmho ltz instability is often considered to play a dominant role in this in teraction, and the level of instability depends on both velocity and d ensity. By means of a simple simulation using typical density and velo city values during the passage of a high speed stream, we were able to obtain good agreement with the temporal variations we observed. Final ly, this study indicates that ground-based pulsation observations can provide reliable proxies of the initial passage of high speed solar wi nd streams past Earth.