Ground Pc3–Pc5 wave power distribution and response to solar wind velocity variations

We examine the magnetospheric wave power in the Pc3–Pc5 range in terms of its growth and decay characteristics and its distribution in L shell in response to the interplanetary plasma bulk velocity, VSW. We use linear and nonlinear (rank-order) correlation and filtering methods to quantify the effective coupling of the wave power to VSW variations. These methods are applied to measurements from 26 ground magnetometers of the IMAGE array and NOAAʹs GOES-10 spacecraft at geosynchronous orbit, taken over 2 years of solar-maximum activity (2002–2003). We find that the ground ULF wave power is structured in the range 3.5<L<6.4 and distributed uniformly in the range 6.4<L<15 (uncertainties in L are estimated to be ±0.5). The response of the wave power to the VSW is characterized by an increase starting 3 days before the VSW peak, intensifying several hours before the peak, and is followed by a fast decrease in the next 2 days. The rapid decay of ULF waves has two stages: one at τ=−6±2 h before the solar wind velocity reaches its peak, and one at the VSW peak, τ=0. We suggest that the first one is brought about by wave–particle interaction with inner-magnetospheric populations while the second one is a dVSW/dt effect. The correlation results are confirmed by calculating the finite-impulse response, which shows clearly the decay of the ULF waves after the VSW peak. The response of the wave power at geosynchronous orbit is remarkably similar to that of the ground wave power at comparable L shells. The above findings characterize the inner-magnetospheric response to interplanetary high-speed streams, as opposed to the more short-lived, higher-amplitude response to CMEs.