On the solar wind control of Pc5 ULF pulsation power at mid-latitudes: Implications for MeV electron acceleration in the outer radiation belt

On the basis of observations made at six sites (L shell range 3.75-6.79) belonging to the International Monitor for Auroral Geomagnetic Events (IMAGE) magnetometer array, we provide a statistical study of the relationship of solar wind speed to daily ULF pulsation power for 1995, 1996, and 1997, an interval corresponding to the declining phase of solar cycle 22. We find a very strong positive correlation between solar wind speed and dayside magnetospheric pulsation power across the entire L shell range studied. In addition, we identify a number of geomagnetic storm onset days on the basis of Dst variations and demonstrate that on these days large-amplitude ULF pulsation activity can be driven during only moderate solar wind speed conditions. Removing the onset day population improves the correlation of pulsation power to solar wind speed and demonstrates how an improved solar wind speed proxy based on pulsation power can be determined by considering Dst. We also demonstrate an exponential decay of 1-10 mHz Pc5 wave power with decreasing L shell, the decay rate increasing with solar wind speed, thus indicating a stronger dependence of pulsation power on solar wind speed at higher L shells in the range L=3.75-6.79. Further, the L shell range studied is consistent with that over which storm-time enhancements of MeV electron fluxes have been observed in the outer radiation belt, and we discuss the implications of the dependence of Pc5 ULF wave power on both solar wind speed and L shell for mechanisms which implicate large-amplitude ULF pulsations in the acceleration of storm-time MeV energy electrons.