Effects of geomagnetic activity in the winter thermosphere: 1. Magnetically undisturbed conditions

A two‐part study of the influence of magnetic variability on the structure and dynamics of the winter thermosphere as observed by incoherent scatter radars has been conducted. Here, in paper 1, we present a thermospheric case study for magnetically undisturbed geophysical conditions, which is used as a reference in studying the magnetic storm effects in paper 2. Neutral exospheric temperatures derived from incoherent scatter radar measurements made at Millstone Hill (43°N), Arecibo (18°N), and Sondrestrom (67°N) between January 14 and 17, 1986, serve as the control case. Mean temperatures and diurnal temperature variations resulting from Fourier decomposition of these data are compared with the predictions of the MSIS‐86 model and recent theoretical calculations. These comparisons confirm the largely reliable predictive capabilities of the magnetically undisturbed winter thermosphere models, but point to some inconsistencies in the horizontal structure of diurnal temperature amplitudes between modeled and measured results. In particular, exospheric temperatures determined from Arecibo measurements appear to be suppressed with respect to both middle‐latitude temperature determinations and the predictions of MSIS. A tidal analysis of neutral wind determinations from Millstone Hill measurements, however, reveals that middle‐latitude meridional mean and diurnal flow was consistent with previous analogous results.