Peculiarities of mesosphere/lower thermosphere parameters during solar minimum 23/24

The 2009 solar minimum 23/24 has been characterized by an anomalous strong decrease in thermospheric density. We analyze anomalies of mesosphere/lower thermosphere parameters possibly connected with this effect. Nighttime mean low-frequency reflection heights measured at Collm, Germany, show a very strong decrease after 2005, indicating a density decrease. This decrease is also visible in mean meteor heights measured with a VHF meteor radar at Collm. It is accompanied by an increase/ decrease of gravity wave (GW) amplitudes in the upper mesosphere/lower thermosphere. On the decadal scale, GWs are negatively correlated with the background zonal wind, but this correlation is modulated in the course of the solar cycle, indicating the combined effect of GW filtering and density decrease