The interaction between an impact-produced neutral gas cloud and the solar wind at the lunar surface

On April 15, 1970, the Apollo 13 S-IVB stage impacted the nighttime lunar surface approximately 140 km west of the Apollo 12 Alsep site and 410 km west of the dawn terminator. Beginning 20 s after impact the Suprathermal Ion Detector Experiment and the Solar Wind Spectrometer observed a large flux of positive ions (maximum flux ∼3 × 108 ions/cm² s sr) and electrons. Two separate streams of ions were observed: a horizontal flux that appeared to be deflected solar wind ions and a smaller vertical flux of predominantly heavy ions (>10 amu), which probably were material vaporized from the S-IVB stage. An examination of the data shows that collisions between neutral molecules and hot electrons (50 eV) were probably an important ionization mechanism in the impact-produced neutral gas cloud. These electrons, which were detected by the Solar Wind Spectrometer, are thought to have been energized in a shock front or some form of intense interaction region between the cloud and the solar wind. Thus strong ionization and acceleration are seen under conditions approaching a collisionless state.