Historical comparisons of IRI and early ionograms

• The IRI2012 model has been extended to provide specification of the ionosphere from 1950–1957.
• This period includes early ionospheric soundings and a particularly quiet solar minimum.
• Ionograms from Ft. Belvoir (Washington DC) in 1951 and 1954 are compared to this new capability.

The IRI2012 provides ionospheric modeling from 1 January 1958 through the present and near future. However, archives of ionogram films exist dating back to the late 1940s, and are potentially valuable for studying long-term trends and change. IRI is very useful for the analysis and interpretation of the films, so Space Environment Corporation (SEC) has modified IRI2012 to extend its operations back to 1 January 1950.This paper describes results from IRI2012 and observations from the Washington DC ionosonde WA938 (38.7°N, −77.1°E) for 1951 (active post-solar maximum) and 1954 (quiet solar minimum).The comparison shows general agreement between the extended IRI2012 and the ionosonde observations. A nighttime enhancement found in IRI results is observed in some ionograms, with modification by atmospheric waves. A significant discrepancy between IRI and observations was found in nighttime 1954 solar minimum results.