Atmospheric Gravity Waves and Medium Scale Traveling Ionospheric Disturbances at Auroral Latitudes

To investigate physical links between the Earth atmosphere and ionosphere, we present data of the medium-scale atmospheric gravity waves (AGWs, periods 25-100 min) observed at auroral latitudes. The AGWs at 80-100 km altitude were inferred from the wind data of the Nordic meteor radar Cluster with spatial/height/time resolution 90 km/5 km/10 min respectively. At the same time, medium-scale traveling ionospheric disturbances (MSTIDs) were detected as variations of the electron density (critical frequency foF2) at the height of F2 maximum (hmF2, 250-350 km) in the data of the ionosonde at Sodankyl & auml; Geophysical Observatory (67 degrees N, 27 degrees E, Finland) operating with 1-min time resolution. We found that, except a "fall anomaly" in mid-September-mid-December, the season-local time distributions of AGW at 90 km and MSTID at hmF2 are similar. Namely, larger amplitudes are observed in the dark-sky conditions, such that the separation between smaller and larger amplitudes occurs at solar terminator. However, during the fall anomaly, amplitudes of MSTID at hmF2 are the same as in spring- and wintertime, whereas AGWs at 90 km are practically suppressed. This anomaly starts with the fall transition in the atmospheric circulation and is associated with a sharp change of the phase of semi-diurnal tides. The results are consistent with the idea that the AGWs observed near the mesopause may be generated due to turbulence in the lower atmosphere (below) or due to electrodynamical forces and auroral activity in the ionospheric E-layer. The latter plays a major role in the auroral region and may be more important in dark-sky conditions.