Solar cycle dependence of solar wind coupling at the Ice Giants

The dominant form of mass and energy transport between the Sun and the Ice Giant magnetospheres of Uranus and Neptune remains an open question.  Predictions based on theory suggest that a combination of the weaker internal magnetospheric plasma sources and significantly tilted magnetic dipole fields of Uranus and Neptune may enable increased solar wind-magnetospheric coupling. Much of this coupling is dependent on the local solar wind parameters, specifically the Alfvénic Mach number (M_A). Despite predictions of transport driven by solar wind coupling, the Voyager 2 flyby of Uranus observed a large M_A of ~23 and a loop-like plasmoid in the magnetotail, suggestive of more internal planetary plasma driving.  In order to better constrain the possible scenarios of internally-driven vs. externally-driven magnetospheric convection at a given planet, a quantitative assessment of upstream plasma variations is required. The interaction between the solar wind and a planetary magnetosphere is often parameterized in terms of M_A, with lower values enabling enhanced rates of magnetopause reconnection and energy exchange between the interplanetary and planetary environments. Here we perform a comprehensive analysis of upstream M_A throughout the solar system using data spanning from 0.3 AU to 75 AU, collected by the Helios 1 & 2, Voyager 1 & 2, and Pioneer 10 & 11 spacecraft from 1972-2005.  We find that systematic increases in solar wind magnetic pressure during periods of high solar activity lead to lower-than-expected M_A upstream of the giant planets. These lower M_A values combined with the significant tilt of the magnetic dipole axes at Uranus and Neptune likely result in amplified solar-wind-magnetospheric coupling at solar maximum. The results indicate that magnetospheric dynamics at Uranus and Neptune may be strongly dependent on solar cycle.