A comparative study of in-situ and remote intermediate layer measurements against wind model predictions of vertical ion drift

Measurements from a sequence of rockets launched from Wallops Island (37.95°N, 75.47°W) on a single night in 1968 still provide the only in-situ data on the midlatitude intermediate layer phenomenon. TMA vapor trail observations made during this sequence of rocket flights were the first to experimentally examine the wind shear theory for the formation and slow downward motion of the layers. Using these rocket data from solar maximum, as well as several intermediate layer data sets from the Arecibo Observatory (18.35°N, 66.75°W) at solar minimum, we compare the location and motion of the layer observations to the predictions of two widely used thermospheric wind models: the HWM-93 thermospheric wind model and the TIEGCM. HWM-93 is an empirical model derived largely from measurements that had limited capabilities for determining night-time winds at lower thermospheric altitudes, and TIEGCM is a self consistent first principles model. Like an earlier study by Szuszczewicz et al., 1995, we find that some of our intermediate layer data agree quite well with the location of a convergent null in the meridional wind predicted by TIEGCM, but the correlation between layer altitude and the null in the vertical ion drift is in general poor. This finding contradicts theoretical expectations based on the two fluid plasma equations. We also demonstrate that uncertainties in the ion neutral collision frequency do not improve the agreement between the model vertical drifts and the observed layer behavior. Furthermore, we find some evidence that the HWM-93 model may overemphasize diurnal tidal effects in the winter, post-midnight period. The discrepancies found in this study demonstrate that our present understanding of midlatitude lower thermospheric wind systems is inadequate for predicting intermediate layer dynamics in the post-sunset period.