Preface to the Special Issue on Crucial Processes Acting in the Mesosphere/Lower Thermosphere

In the field of Atmospheric Physics, the recent half century was characterized by the development of Dynamical Meteorology, along with its applications in weather forecasting and climate science for the troposphere and stratosphere (the altitude range from about 0 to 50 km). This process was stimulated by the use of comprehensive numerical models combined with new measurement and data assimilation techniques. Exploration of the upper atmosphere (at altitudes above about 120 km) and space was mainly fostered by many dedicated satellites and by novel ground-based instruments. The atmospheric layer between these two heights, nowadays known as the mesosphere/lower thermosphere (MLT, from 50 to 120 km), received comparatively little attention. Although being interesting in its own right because of various fluid dynamical and other physical curiosities, it is only in recent years that the MLT region has become to be recognized as the missing piece in the jigsaw puzzle that connects weather and climate to processes occurring in the upper atmosphere and space. The proper physical description of the MLT region is particularly challenging because it requires the combination of different physical disciplines. The knowledge acquired and the methods used in Meteorology have to be extended by concepts that have been developed for the particular processes operating in the MLT and the upper atmosphere. Amongst those challenges, we mention the proper description of the following