Mercury's magnetopause and bow shock from MESSENGER observations

We establish the time-averaged shape and location of Mercury's magnetopause and bow shock from orbital observations by the MESSENGER Magnetometer. We fit empirical models to the midpoints of boundary crossings as well as to probability density maps of the magnetopause and bow shock positions. The magnetopause is fit by two different surfaces: (1) a paraboloid, and (2) a surface for which the position R from the planetary dipole varies as [1+cos(θ)]^-α, where θ is the angle between R and the dipole-Sun line, and α is a flaring parameter that governs whether the magnetotail is closed (α < 0.5) or open (α ≥ 0.5). The paraboloid magnetopause model is not able to fit simultaneously both the dayside and nightside magnetopause crossings, but the second surface gives the best-fit overall shape to the observations with a subsolar stand-off distance, R_ss, of 1.45 R_M (where R_M is Mercury's radius), and a flaring parameter α = 0.5. The average magnetopause shape and location were determined under a mean solar wind ram pressure, P_Ram, of 14.3 nPa. The best-fit bow shock shape established under an average Alfvén Mach number (M_A ) of 6.6 is described by a hyperboloid having R_ss = 1.96 R_M and an eccentricity of 1.02. These boundaries move as P_Ram and M_A vary, but their shape remains unchanged. The magnetopause R_ss varies from 1.55 R_M to 1.35 R_M for P_Ram in the range 8.8 to 21.6 nPa. The bow shock R_ss varies from 2.29 R_M to 1.89 R_M for M_A in the range 4.12 to 11.8. To first order, the boundaries are well approximated by figures of revolution. Additional effects of the interplanetary magnetic field are masked by the large dynamic variability of these boundaries. Despite the moderate average magnetic shear conditions at Mercury, the magnetotail surface is nearly cylindrical, with a radius of ~2.7 R_M at a distance 3 R_M downstream of Mercury. By comparison, Earth's magnetotail flaring continues until a downstream distance of ~10 R_ss. This result may indicate that reconnection in Mercury's magnetotail does not take place beyond ~3 R_M downstream of the planet.