Magnetopause Standoff Distance at Mercury from Dipole Pressure Balance Across the Solar-Wind Pressure Range

Mercury's magnetosphere is the smallest of any planet with an internal dynamo, small enough that the magnetopause often stands barely a thousand kilometres above the surface. This paper computes the subsolar standoff distance from pressure balance between the published dipole field and the solar-wind dynamic pressure, evaluated at low, mean and high pressures spanning the published range encountered at Mercury's orbit. The headline results are: subsolar_surface_field_nT = 380; standoff_RM_low_5nPa = 1.502; altitude_km_low_5nPa = 1225. The standoff moves inward from about 1.5 planetary radii at five nanopascals to near 1.15 radii at thirty nanopascals, quantifying how close the solar wind routinely presses to the surface and why direct solar-wind impact on the dayside regolith is a normal condition at Mercury rather than an exceptional one.