Modeling the compressibility of Saturn's magnetosphere in response to internal and external influences

The location of a planetary magnetopause is principally determined by the balance between solar wind dynamic pressure D P and magnetic and plasma pressures inside the magnetopause boundary. Previous empirical studies assumed that Saturn's magnetopause standoff distance varies asD P − 1 / αand measured a constant compressibility parameter α corresponding to behavior intermediate between a vacuum dipole appropriate for Earth ( α ≈6) and a more easily compressible case appropriate for Jupiter ( α ≈4). In this study we employ a 2‐D force balance model of Saturn's magnetosphere to investigate magnetospheric compressibility in response to changes in D P and global hot plasma content. For hot plasma levels compatible with Saturn observations, we model the magnetosphere at a range of standoff distances and estimate the corresponding D P values by assuming pressure balance across the magnetopause boundary. We find that for “average” hot plasma levels, our estimates of α are not constant with D P but vary from ∼4.8 for high D P conditions, when the magnetosphere is compressed (≤25  R S ), to ∼3.5 for low D P conditions. This corresponds to the magnetosphere becoming more easily compressible as it expands. We find that the global hot plasma content influences magnetospheric compressibility even at fixed D P , with α estimates ranging from ∼5.4 to ∼3.3 across the range of our parameterized hot plasma content. We suggest that this behavior is predominantly driven by reconfiguration of the magnetospheric magnetic field into a more disk‐like structure under such conditions. In a broader context, the compressibility of the magnetopause reveals information about global stress balance in the magnetosphere.