Longitudinal variations in thermospheric parameters under summer noontime conditions inferred from ionospheric observations: A comparison with empirical models.

Longitudinal variations in the thermospheric neutral composition ([O] and [N]) and exospheric temperature Tex have been inferred from June monthly median noontime fF and fF observations at mid-latitudes to check for consistency with empirical MSIS models. In general, a similarity in longitudinal variations has been demonstrated, and this is interesting, as similar variations were obtained with very different methods and different data sources. Both inferred and MSISE-00 modelled height-integrated O/N ratios are comparable to TIMED/GUVI observations only under solar minimum conditions but differ substantially under high solar activity. The retrieved height-integrated O/N ratio longitudinal variations are small (∼15%) in comparison to the observed NF variations under high solar activity. The height-integrated O/N ratio cannot be incorporated into the F-layer formation mechanism; therefore, such observations cannot be used for any quantitative interpretation of NF variations.