Compressibility of Mercury's dayside magnetosphere

Abstract The Mercury is experiencing significant variations of solar wind forcing along its large eccentric orbit. With 12 Mercury years of data from Mercury Surface, Space ENvironment, GEochemistry, and Ranging, we demonstrate that Mercury's distance from the Sun has a great effect on the size of the dayside magnetosphere that is much larger than the temporal variations. The mean solar wind standoff distance was found to be about 0.27 Mercury radii ( R M ) closer to the Mercury at perihelion than at aphelion. At perihelion the subsolar magnetopause can be compressed below 1.2 R M of ~2.5% of the time. The relationship between the average magnetopause standoff distance and heliocentric distance suggests that on average the effects of the erosion process appears to counter balance those of induction in Mercury's interior at perihelion. However, at aphelion, where solar wind pressure is lower and Alfvénic Mach number is higher, the effects of induction appear dominant.