Characterizing the long‐period ULF response to magnetic storms

This study presents an analysis of long‐period ULF wave power observed at 44 ground magnetometer stations in the western hemisphere, at latitudes from the equator to the polar caps, during the magnetic storm intervals of May 1997, March 1998, May 1998, September 1998, and October 1998 identified by the GEM research community. Comparison of the long‐period pulsations (in the Pc 5 frequency range) observed on the ground to the solar wind velocity and solar wind pressure observed by the WIND satellite confirms previous results that high solar wind velocity correlates well with increased Pc 5 power. Although some variability was evident among the five storms, solar wind pressure and variability in velocity or pressure exhibited considerably weaker correlations. Each storm exhibited narrow‐band Pc 5 wave activity during the recovery phase, but only in the dawn‐noon sector at each site. During the main phase, and at all other local times during the recovery phase, wave activity was broadband. The strongest Pc 5 power was observed at frequencies above 2 mHz at stations in the auroral zone and poleward and below 2 mHz at stations at middle and low latitudes. Comparison to published time profiles of energetic electrons at geosynchronous orbit suggests that if ULF wave activity is responsible for the initial energization of these electrons, broadband activity rather than narrowband ( Pc 5) waves must play the dominant role. Our data cannot rule out, however, the importance of narrowband waves in providing additional energization during the recovery phase.