Alfvénic Mach Number Variations in the Outer Solar System: Implications for Giant Planet Magnetospheres

The interaction between the solar wind and a planetary magnetosphere is often parameterized in terms of the upstream Alfvénic Mach number (M_A). Interplanetary M_A throughout the solar system scales as the ratio of solar wind dynamic pressure to magnetic pressure. The lower upstream M_A in the inner heliosphere leads to increased plasma depletion and magnetic flux pileup at the magnetopause boundary, enabling enhanced rates of magnetic reconnection. Consequently, in the outer heliosphere, higher upstream M_A is theorized to limit the rate of magnetopause reconnection in giant planet magnetospheres. Here we analyze data from Helios 1 & 2, Voyagers 1 & 2, and Pioneers 10 & 11 to quantify the variation in M_A throughout the solar system from 1972-2007. We find that solar cycle variations in solar wind magnetic pressure lead to wide ranges of upstream M_A at Jupiter, Saturn, Uranus, and Neptune. Voyager 2's flyby of Uranus during solar minimum in 1986 resulted in a very high upstream M_A of ~20. However, during strong solar maximum conditions where the overall solar wind magnetic pressure is increased, the M_A in the outer heliosphere can become almost Mercury-like (i.e., ~4). This variation suggests that that magnetopause reconnection may play an increasingly important role in all giant planet magnetospheres during solar maximum, in particular in the sparse magnetospheres of Uranus and Neptune.