Evidence That Solar Wind Fluctuations Substantially Affect the Strength of Dayside Ionospheric Convection

Abstract

Ionospheric convection is occasionally observed to be substantially enhanced even when the interplanetary magnetic field (IMF) is not strongly southward and the IMF B_y is not large. Such enhanced convection flows tend to exhibit large oscillations with ∼10–30 min periodicity. We have considered the solar wind characteristics that lead to these oscillatory convection enhancements. We have used an extensive set of Sondrestrom radar observations of ionospheric convection within the dayside polar cap. We find that IMF ULF power is closely associated with the strength of dayside convection. Convection flows during periods of large north–south IMF fluctuations are observed to be as strong as for steady and large southward IMF periods. Enhanced convection is also observed during northward IMF intervals when the interplanetary magnetic field exhibits high ULF power. We find that ULF power enhances the convection strength, independent of an observed direct effect from the solar wind speed. These observations thus suggest that IMF ULF fluctuations can significantly influence ionospheric convection. Therefore, in addition to the well-established contributions from the direction and magnitude of the IMF and the solar wind dynamic pressure, ULF fluctuations may also be an important contributor to coupling of the solar wind to the magnetosphere-ionosphere system. We speculate that resonance between IMF fluctuations and natural magnetospheric oscillation frequencies or magnetopause boundary oscillations might be responsible for the connection between ionospheric convection and IMF ULF power. We have also found evidence for a connection between the ULF power in the solar wind dynamic pressure and the strength of convection.