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 degrees N, 75.47 degrees W) on a single night in 1968 still provide the on ly in-situ data on the midlatitude intermediate layer phenomenon. TMA vapor trail observations made during this sequence of rocket flights were the fi rst to experimentally examine the wind shear theory for the formation and s low downward motion of the layers. Using these rocket data from solar maxim um, as well as several intermediate layer data sets from the Arecibo Observ atory (18.35 degrees N, 66.75 degrees W) at solar minimum, we compare the l ocation and motion of the layer observations to the predictions of two wide ly 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 the rmospheric altitudes, and TIEGCM is a self consistent first principles mode l. Like an earlier study by Szuszczewicz et al. (1995), we find that some o f our intermediate layer data agree quite well with the location of a conve rgent 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 f luid plasma equations. We also demonstrate that uncertainties in the ion ne utral collision frequency do not improve the agreement between the model ve rtical drifts and the observed layer behavior. Furthermore, we find some ev idence that the HWM-93 model may overemphasize diurnal tidal effects in the winter, post-midnight period. The discrepancies found in this study demons trate that our present understanding of midlatitude lower thermospheric win d systems is inadequate for predicting intermediate layer dynamics in the p ostsunset period. (C) 1998 Elsevier Science Ltd. All rights reserved.