Timing analysis of the relationship between solar wind parameters and geosynchronous Pc5 amplitude

Abstract

Solar wind‐driven magnetospheric ULF waves in the Pc5 band (1.7–6.7 mHz) have been suggested to mediate the radial transport of electrons in the outer radiation belt. To identify the wave generation mechanism and the impact of the waves on the electrons, we performed correlation and superposed epoch analyses of the solar wind control parameters measured by the ACE spacecraft and the amplitude of Pc5 waves and the flux of radiation belt electrons measured at the GOES 12 geosynchronous satellite. The analyses were carried out for solar minimum year 2006, when the solar wind exhibited recurrent co‐rotating interaction regions. We used hourly averages of solar wind velocity Vsw, the root‐mean‐square amplitudes of the solar wind dynamic pressure variations in the Pc5 band σPsw, the azimuthal (σBy) and compressional (σBz) magnetic field components of geosynchronous Pc5 waves, as well as relativistic (>2 MeV) electron flux Je at geosynchronous orbit. We found that σPsw has a higher correlation with σBy and σBz than Vsw and that the correlation between σPsw and the wave amplitudes peaks at or very near zero time shift. We conclude that the major driver of geosynchronous Pc5 waves is solar wind pressure variations rather than the Kelvin–Helmholtz instability on the magnetopause. Solar wind pressure pulses propagate into the inner magnetosphere at the MHD wave speed and it takes less than 1 hour for the signal to reach GEOS 12 from the bow shock nose. The pressure pulses also excite standing Alfvén waves that contribute to the σPsw–σBy correlation. There is a positive correlation between Vsw and Pc5 amplitude, but the correlation peaks at a time shift of about −1 day (the Pc5 peak occurs earlier than the Vsw peak). We attribute this shift to an intrinsic time shift between σPsw and Vsw. Je exhibits a minimum at the Dst minimum, coincident with the maximum of σPsw, σBy, and σBz, and increases by 2 to 3 orders of magnitude over a period of about one day during which σBy and σBz decrease by one order of magnitude and Vsw reaches a peak value. However there are storms that do not exhibit electron flux enhancement despite the presence of strong Pc5 activity. Therefore the correlation between Pc5 amplitude and electron flux at geosynchronous orbit is not unique. The difference in electron response implies either the existence of other competing acceleration/loss mechanisms or that electron response to the Pc5 driver is nonlinear: similar driving conditions produce different response.