The region from 65 km to 95 km altitude is known as the mesosphere and the ionization produced in it by absorption of solar ultraviolet is known as the D region. The ions and electrons initially produced are more likely to associate with molecules than is the case at higher altitudes, on account of the higher ambient air densitites and smaller fractions of molecules broken down to atomic form at those altitudes, as compared to the most dense region of the ionosphere at say 200 km. The chemical reactions occurring are complex and not as well understood as at higher altitudes. The electron densitites are lower than at higher altitudes, and very low at night, so that one does not get a strong return with radar. Nevertheless, the region is important as an absorbing medium for transmission of radio waves, and its ion chemistry and temperature and turbulence structure can be studied at Arecibo, Puerto Rico, and comparisons made with complex models of the chemistry. This award is to perform such measurements and to analyze more fully the results of previous measurements.
