NASA T SOLAR WIND INTERACTION WITH MERCURY

With the exception of its higher density, the planet Mercury is almost equivalent to the moon in many of its physical properties with respect to the solar wind interaction with it. At Mercury's heliocentric distance (0.31 to 0.47 A,U,,) , the solar wind properties are characterized by a speed ratio S = 7-9, B = 0.3 -0.5 1 c^ = 1550 - 1650 and T 11 /`fl = 1.0 - 1.5. A consideration of the motionally induced magnetic field indicates that Mercury, like the Moon, may reveal information about the conductivity of its interior depending upon whether or not a bow shock exits. In the absence of such a phenomenon, the plasma and magnetic field in the wake region can be studied using the guiding-center approach since the solar wind ion Larmor radius is very small compared to the planetary radius. The prediction of a plasma cavity downwind from the obstacle and associated perturba-tion of the interplanetary magnetic field in the wake then follows. Decreases of the magnitude of the field occur inside a Mach cone tangential to the planet but outside of the umbra) region where a field increase occurs. The axis of the Mercurian Mach cone is aberrated some 6 0 - l 1 0 from the west of the sun and the Mach angle itself is about 140 - 190 . These are about twice those of the lunar wake. Possi bly penumbral field increases and stimulated waves upstream and downstream from the wake will also occur.