Ionosphere and atmosphere of the Moon in the geomagnetic tail

During the 4-day period when the moon is in the geomagnetic tail, the principal constituents of the lunar atmosphere are neon and argon. The surface concentrations of neon and argon are calculated from a theoretical model to be 3.9×103 and 1.7×103, respectively. The lunar atmosphere is ionized by solar ultraviolet radiation, resulting in electrons at a temperature of about 1.5×105 °K and ions at about 370°K. We investigated dynamic properties of the lunar ionosphere in the high-latitude tail lobes during quiescent times when plasma energy density from external sources is below the sensitivity threshold of the suprathermal ion detector at the lunar surface. We found that a hydrostatic model of the ionospheric plasma is inadequate because the gravitational potential energy of the plasma is considerably smaller than its thermal energy. A hydrodynamic model, comparable to that used to describe the solar wind, is developed to obtain plasma densities and flow velocities as functions of altitude. The hydrodynamic flow of the ionospheric particles is away from the sunlit hemisphere, in a direction parallel to the magnetic field, and forms a cylinder whose base is the lunar diameter. At 100-km altitude the calculated ionospheric density is 1.2×10−2 cm−3, with a flow velocity of 4–7 km/s. The corresponding energy density is 2.5×10−13 erg/cm3. Flow under these quiescent conditions exists approximately one third of the time in the geotail. During other times when cross-tail electric fields are present, the steady flow away from the moon is disrupted by drift velocity components perpendicular to the geomagnetic field lines; also, sporadic occurrences of plasma sheet or lobe plasma temporarily dominate the plasma environment during nonquiescent times. The electromagnetic properties of the quiescent ionosphere are investigated, and it is concluded that plasma effects on lunar induction studies can be neglected for quiescent conditions in the geomagnetic tail lobes.