User effects may be: (a) Alteration of high-latitude electric fields and currents that interfere with communication systems by altering ionospheric morphology and in particular the formation of equatorial Spread F. (U,H) (b) Alteration of location and magnitude of auroral current systems that adversely affect power transmission networks and longline telephone systems. (U,H) (c) Alteration of global atmospheric electricity circuit modifies tropospheric weather and climate. (U/L,U) 2.11 ATMOSPHERIC EXPERIMENTS 2.11.1 Water Vapor in the Mesosphere (Sundararaman) We need to know the variability and trend profiles for water vapor. At present there is essentially no information above 70 km; what little is available (see Figure 2) has errors of about ± 50% between 30 and 70 km. Some satellite measurements (e.g., Nimbus VII) exist, and more are planned, which raises the question of how good these measurements are. We clearly need a few good profiles in the short term. In the long term, satellite measurements will be available, and they must be verified against ground truth observations. Thus in the short term we make the following recommendations: 1. Evaluate how good ground-based systems (e.g., microwave spectrometry) are in obtaining profile information in the mesosphere, noting that at present the microwave measurements are about a factor of 2 or 3 greater than available rocket measurements. 2. Develop new instruments or adapt existing ones (such as the NOAA Lyman-a instrument) for rocket platforms so that a systematic program of measurements can be undertaken to determine the water vapor content above 20 km altitude, and also that a "ground truth" verification of forthcoming satellite measurements can be undertaken. (This is a midterm recommendation, to be carried out over the next 2-5 years). 2.11.2 Noctilucent Clouds (Sundararaman, Turco) In the context of natural noctilucent clouds, the following studies should be made: 1. Examine archived pictures of NLCs to determine if possible their spatial extent, latitudinal/longitudinal distribution, day-to-day and seasonal variabilities and the seasonal duration of occurrences.
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