Magnetosphere on May 11, 1999, the day the solar wind almost disappeared: II. Magnetic pulsations in space and on the ground

Simultaneous observation's by Wind and IMP-8 in the upstream region on May 11, 1999, when the solar wind density was well below its usual values and t he IMF was generally weakly northward, indicate there were upstream waves p resent in the foreshock, but wave power was an order of magnitude weaker th an usual due to an extremely weak bow shock and tenuous solar wind plasma. Magnetic pulsations in the magnetosphere have been observed in the magnetic field data from Polar and at mid-latitude ground stations. By comparing Ma y 11 with a control day under normal solar wind conditions and with a simil ar foreshock geometry, we find that the magnetosphere was much quieter than usual. The Pc 3-4 waves were nearly absent in the dayside magnetosphere bo th at Polar and as seen at midlatitude ground stations even through the for eshock geometry was favorable for the generation of these waves. Since the solar wind speed was not unusual on this day, these observations suggest th at it is the Mach number of the solar wind flow relative to the magnetosphe re that controls the amplitude of Pc 3-4 waves in the magnetosphere.