The solar cycle dependence of the solar wind interaction with the Ice Giants

The dominant form of mass and energy transport between the Sun and the planetary magnetospheres of Uranus and Neptune remains an open question. The interaction between the solar wind and a planetary magnetosphere is often parameterized in terms of the upstream Alfvénic Mach number, M _A , with lower values enabling enhanced rates of energy and mass exchange between the interplanetary and planetary environments. Here we perform a comprehensive analysis of upstream M _A in 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, similar to those measured at Mercury, where magnetic reconnection plays a dominant role in plasma transport. These values combined with the significant tilt of the magnetic dipole axes at Uranus and Neptune and weak internal plasma sources likely result in amplified solar-wind-magnetospheric coupling at solar maximum.