Planetary Mach cones: Theory and observation

This study uses observations by a number of spacecraft to investigate the asymptotic behavior of planetary bow shocks. Toward this end a single standard method has been used to model distant bow shock position and shape. Mach cone angles of 13.9±2°, 11.4±3°, and 8.1±4° at Venus, Earth, and Mars, respectively, were determined from the observational shock models. These cone angles and their decrease with growing distance from the sun are consistent with downstream bow shock position being limited by the MHD fast mode Mach number. Gas dynamic solutions for solar wind flow about Venus, Earth, and Mars were computed up to 50 R OB (i.e., obstacle radii) behind each planet and compared with observed bow shock location. In each case the position of the shock was well predicted up to a certain distance downstream: −4 R OB at Venus, −6 R OB at Earth, and −10 R OB at Mars. Beyond this point the observed shock position lies farther from the aberrated sun‐planet line than the gas dynamic model with the discrepancy greatest at Venus and least at Mars. The better agreement between gas dynamic theory and observation with growing distance from the sun is attributed to an increase in the accuracy of the gas dynamic approximation with decreasing IMF strength.