The Washington DC Ionosphere: A Six Decade Overview

Ionogram films from the Washington, DC ionosonde WA938 located at Ft. Belvoir in Fairfax, VA (38.7° N, -77.1° E) dating back to 1949 are archived at NOAA's National Geophysical Data Center (NGDC). The Wallops Island ionosonde WP937 (37.9° N, -75.5° E, about 165 km southeast of WA938) became the primary sounder for the area when WA938 was shut down in 1968, providing film ionograms until about 1980 and digital ionograms after 1987. In recent years, NGDC has been able to digitize several months from selected years of the archived films. These digitized ionogram images may then be analyzed by Space Environment Corporation's Expert System for Ionogram Reduction (ESIR) to extract scaled parameters and electron density profiles (EDPs). The SEC ESIR analysis and calibration methodology for WA938 ionograms was described in Rice et al (2012). Similar procedures may be used for the WP937 film ionograms. The vast majority of film ionograms from these and other sites were only scaled for hourly F-region parameters, even though ionograms were made at 15 minute intervals, and occasionally at higher cadences. Thus, apart from special campaigns and studies, less than a quarter of the available ionograms have ever been examined in any detail, and very few were inverted to provide EDPs. This study presents examples of film ionogram analysis from the 1950s through the 1970s, together with present-day WP937 data. EDPs are obtained from the film ionograms and reduced to obtain true height hmF2 values. This parameter is particularly important in long-term studies of ionospheric change since it is very sensitive to the predicted changes in thermospheric temperature. The hmF2 values are compared with recent hmF2 values from the Wallops Island ionosonde and a discussion of uncertainties in this parameter are discussed. Historical hmF2 records are extremely sparse and were based on graphical approximation techniques, since the numerical inversion of ionograms was very difficult prior to the introduction of digital sounders in the 1980s. The NGDC film ionogram collection combined with modern computer analysis techniques provides a rich source for data mining and long-term variation studies. Reference: Rice, D. D., J. J. Sojka, J. V. Eccles, and R. D. Hunsucker (2012), Pre-IGY Ionosphere over Washington DC, Abstract SA31B-2148, presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3-7 Dec.