The E-region electron density diurnal asymmetry at Saint-Santin - Observations and role of nitric oxide

Measurements of the E-region electron density were made with the Saint-Santin incoherent scatter radar during consecutive days in June 1978, March 1979 and December 1980. On the basis of a statistical study, the observations show the presence of a diurnal asymmetry of the electron density, with morning values usually exceeding the afternoon densities by 3-20 percent. Two possible causes of the dissymmetry are examined, the asymmetry in the diurnal variation of the neutral composition, and the effect of nitric oxide. The presence of NO partly converts O2(+) into NO(+) ions and increases the effective recombination rate of the electrons in the afternoon. Numerical simulations assessing the relative importance of the two factors are, in general, in good agreement with the measurements.