Gravity Waves and Ionisation (MIP-MAG)

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.

The Earth`s lower ionosphere coincides with the mesosphere, i.e. a height region above the stratosphere up to the mesopause at around 85 km, and is characterised by a complex ion chemistry with a very strong temperature dependence. In Summer the temperatures in the mesosphere are generally more stable than in Winter, notably at high latitudes. However, temperature variation and associated pressure perturbation, including gravity waves can be triggered by severe thunderstorms from as far away as 1,400 km. The upward propagating gravity waves can lead to a number of processes, affecting both the neutral and the ionised atmosphere. The temperature variations in mesosphere are known to exert an influence on the size of the water cluster ions (both positive and negative) and on the positive conductivity in the 40 to 60 km region. Scientific Activity The mesosphere was investigated by rocket borne optical diagnostic instruments. Two salvoes of measurements were carried out in a quiet night and an unstable one; the latter was based on real-time indications from the US national weather service. The two main scientific rocket payloads reached apogees just under 100 km and descended on parachutes in order to assure longer measuring time and controlled flow conditions. Before and after each of the main sounding rockets a meteorological rocket flight took place to establish temperature profiles with high vertical resolution. Four UV lamps were switched according to a special pattern and the changes thus induced in the plasma densities were analysed by plasma density measurements using several different methods. Initial results are very promising, i.e. technically all instruments functioned as anticipated. Scientifically there was notably a significant difference in the response of the mesosphere to the illumination by Krypton lamp: In the disturbed night it lead to plasma density enhancements in excess of a factor of ten, whereas in the quiet night the induced enhancements were only of a few percent. Preliminarily one can speculate that the density of neutral nitric oxide may have been significantly larger in the disturbed night, a reasonable assumption since gravity waves are expected to enhance turbulence and in consequence the redistribution of minor species.