Data coverage for D-region modeling

Data from the lower ionosphere are not available from satellites and therefore scant by nature and do not exist on a global basis. A survey of the data is given showing various sources and the geophysical conditions covered by the measurements. From the high latitudes the largest data sets are available, due to the large variability, however, not all situations are adequately covered. The sensitivity of models to the inclusion or omission of single electron density pro-files is demonstrated. DATA SOURCES The D-and E-regions are characterised by a relatively large neutral background which makes many methods that are popular and successful at higher altitudes unreliable below, say 100 km. The undisputed best method to establish elec-tron densities is to use a rocket borne wave propagation experiment, combined with an in-situ measurement by a probe aboard the same payload [1]. Since the advent of sounding rockets 276 such experiments were flown under most diverse geophysical conditions. Many more research rockets carried probes which are inherently uncertain in their absolute values because of payload charging and/or aerodynamic effects. However for comparative studies data obtained from identical instruments, preferably with identical rocket motors, can be used, although a systematic bias can not be ruled out. One such data base is a series of more than 400 electrostatic probe measurements made at Kapustin Yar (Volgograd) in southern Russia with M100B rockets. Incoherent radars have thresholds which make them attractive for measurements in the E- rather than the D-region, except when the latter is disturbed. The resolution is rather poor, but can be assumed to have no bias. The largest such installation is at Arecibo, but due to the variety of scientific objectives pursued with the installation, it has produced fewer electron density profiles than EISCAT since its beginning in 1984.