Experimental evidence for solar cycle and long-term change in the low-latitude MLT region

There is a great deal of interest in long-term change in the upper atmosphere and its possible relationship with human activities, but it is clear that we do not yet have a consistent picture of solar cycle and long-term trends in the MLT region. The long series of rocket measurements made from the early 1960s through to the mid-1990s, over a wide range of latitudes, have been interpreted by some as showing strong long-term trends, but there are considerable doubts as to the reliability of these measurements for heights above about 50 km. Measurements made by various other techniques, such as airglow rotational temperature, VLF reflection height, and satellite-borne radiometers, provide conflicting results. At low latitudes, published results from satellite and sodium lidar measurements indicate negligible trends, but new results from OH rotational temperature measurements, made a 23∘S, show consistent solar cycle variations in very close synchronization with the F10.7 flux, although with a linear trend much larger than expected.