Magnetic Field and Electric Current Measurements of Critical Phenomena in Solar Wind Interactions

Magnetic fields play a crucial role in every case of solar wind/planetary interaction observed to date. In upstream waves, in bow shock phenomenology, in magnetosheath turbulence characteristics, and in the stability of the magnetopause, the magnetic field mediates critical processes. It is through the magnetotail magnetic field that solar wind energy is stored and released in terrestrial substorms. In the case of non-magnetic bodies, the solar wind magnetic field is central to the ion pickup process, and at Venus (and perhaps Mars) the solar wind magnetic field influences much of the planetary ionospheric behavior. Here we discuss new magnetometer techniques needed to answer some outstanding questions on the stability and structure of planetary magnetopauses and magnetotail dynamics. In particular we focus on using the four spacecraft Cluster configuration as a single instrument to make measurements of currents in flux transfer events and for sounding near-tail magnetic structure. Outstanding issues in the solar wind interaction with unmagnetized bodies include the emplacement and evolution of solar wind magnetic field in a planetary (or cometary) ionosphere and the formation mechanism of magnetic flux ropes. Finally, we note that a mission to study the lunar interaction would not only provide crucial insight on how atmosphereless, unmagnetized bodies such as asteroids (and the Martian moons Phobos and Deimos) affect the surrounding medium, but would also serve double duty as a monitor of the solar wind and terrestrial magnetotail at 60 RE.