The ionised part of the mesosphere (the ionosphere's D-region) is highly temperature dependent. In Summer gravity waves originating in the troposphere can propagate up to the mesosphere where they produce a number of effects. These effects can be due to large temperature variations or vertical winds. The purpose of the proposed research project is to investigate in more detail than previously the different conceivable mechanisms leading to the gravity-wave induced modifications such as clustering processes, detachment of large aerosols or the possible existence of a species with a hitherto unknow low ionsation threshold. Two salvos of instrumented sounding rockets are to be flown well after sundown, each accompanied by two meteorological rockets to establish the background conditions. The salvoes intentionally should be launched at times of large and small gravity wave activity, respectively. Beyond this primary scientific aim, the payload configuration will provide new insights into the composition of the lower mesosphere even in absence of gravity waves.