Observation of TM-Mode Induction in a Simulated Solar Wind/Moon Interaction

Abstract

Studies of the interaction of the solar wind with the Moon show that the lunar magnetic field has both steady and transient components. The transient component represents the electrical response of the Moon to electromagnetic action by the solar wind. This response has two modes: a TE mode, driven by interplanetary magnetic field fluctuations and characterized by electrical currents which circulate in the lunar interior; and a TM mode, driven by the solar wind electric field and its variations, and characterized by currents which flow from the Moon and close in the solar wind. The lunar TE mode is observed, but the TM mode has not been detected. The lack of TM mode response is attributed to a cold, resistive lunar crust, and this absence prevents any study of the influence of a surface sheath on the TM mode currents.

A solar wind simulation experiment, VORTEX, has been built to study the plasma aspects of the TM mode response. VORTEX provides a quasi-steady, reproducible, fully ionized argon plasma flow transverse to a magnetic field, by E × B drift around an annular chamber. A preheat discharge provides uniform starting conditions. Typical flow parameters are: density n _e = 6 x 10²⁰ m⁻³, temperature T _e = 5 eV, magnetic field B _z = 0.1 T, flow velocity V _f = 2 × 10⁴ m s⁻¹, magnetosonic mach number M _ms = 1.4, and flow time τ _f = 0.5 ms. Cylindrical obstacles of various electrical conductivities are inserted into the flow, along the magnetic field lines.

Framing photographs of the interaction region show well-defined bow waves and downstream wakes. These features change with mach number and the obstacle conductivity. TM mode currents have been detected in a graphite obstacle by a Rogowski coil buried in the obstacle. The induced current varies with plasma density, velocity and magnetic field, but is insensitive to mach number in the steady state. The induced current decreases as the surface condition of the graphite deteriorates. The current is observed to saturate at a level far below the magnetic back pressure limit of the Sonett-Colburn model. The current saturation is consistent with a sheath limit given by the directed ion flux. These observations suggest that a more complex model, which includes surface effects, is needed to describe the solar wind interaction with a Moon-like object if the TM mode response is present.