On May 10-12, 1999, a prolonged interval of extremely low solar wind density was observed by the WIND and ACE spacecraft. This is an important interval to study because it enables us to separate effects such as dynamic pressure on the magnetosphere from effects such as the orientation of the interplanetary magnetic field (IMF). We have reported how the Earth's magnetospheric current systems responded to the drop in solar wind dynamic pressure in an eariler letter [Le et al., 1999]. The drop in solar wind density also changed the solar wind magnetosonic Mach number and hence weakened the bow shock. This too should have noticeable effects on the magnetosphere. Herein we examine how magnetic pulsations in the magnetosphere were altered by the weakening and expansion of the radius of the bow shock using magnetic field data from the Polar spacecraft and from ground-based mid-latitude stations.
Pc 3-4 magnetic pulsations (10-100 mHz) in the dayside magnetosphere are believed to have a energy source from upstream waves in the Earth's foreshock region [Troitskaya et al., 1971,Greenstadt and Olson, 1977,Odera, 1986,Troitskaya, 1994]. The upstream waves are generated by the interaction between the solar wind plasma and backstreaming ion beams in the foreshock region. Then they are carried downstream to the magnetopause along the solar wind streamlines through the magnetosheath. If the IMF cone angle is small (< 45), the upstream waves fill the subsolar upstream region and the pressure fluctuations associated with the upstream waves can be carried to the magnetopause. The magnetopause responds to these pressure fluctuations and ultimately transfers the wave energy into the dayside magnetosphere and generates Pc 3-4 magnetic pulsations. Thus, Pc 3-4 pulsations can be potentially a very useful diagnosis of the state of the solar wind.
In this paper, we compare both Polar and ground-based observations of magnetic pulsations on May 11 to those on a control day, May 14. The interplanetary magnetic field (IMF) and solar wind data for both days have been shown in Figure 1 in our earlier paper [Le et al., 1999]. Here we emphasize the difference in solar wind conditions (mainly the density) and the similarity of IMF conditions for these two days. The IMF cone angles (the angle between the IMF and the sun-Earth line) that determine the foreshock geometry, were very similar ( ~ 40o) on May 11 and 14. This enables us to isolate the effect of solar wind density (as well as properties that depend on the solar wind density) on the generation of upstream waves and their entry into the magnetosphere.