SOLAR-WIND INTERACTION WITH SMALL BODIES .1. WHISTLER WING SIGNATURES NEAR GALILEOS CLOSEST APPROACH TO GASPRA AND IDA

Data from Galileo's two asteroid flybys reveal magnetic fluctuations that ive interpret as perturbations of the solar wind magnetic field caused by an interaction with the nearby asteroid. The scale sizes of the bodies (similar to 14 km for Gaspra and similar to 30 km for Ida) are intermediate between the ion and electron gyroradii, which implies that the asteroid-imposed perturbations propagate in the whistler mode. Special properties of the whistler mode include phase velocities that can exceed the solar wind speed and confinement of the disturbance to directions nearly aligned with the magnetic field. These features of the interaction impose a structure on the solar wind disturbance that differs greatly from the forms familiar for either magnetized or unmagnetized bodies of magnetohydrodynamic spatial scales. We examine both data and computer simulations of the interaction with special attention to what can be inferred about the interaction itself from analysis of the data.