[1] A statistical investigation into the horizontal propagation of $8-30 min gravity waves (GWs) in the ionosphere over a 1 year period from June 2010 to May 2011 is presented. The GWs were observed by multipoint continuous Doppler sounding systems installed in the Czech Republic and in the Western Cape, South Africa. Measurements of GW propagation in the ionosphere over South Africa have never been presented before. Simultaneous measurements from nearby ionosondes made it possible to estimate the height of the GW observations and show that the analyzed GWs propagated at altitudes from $150 to $250 km. The analyzed waves were mainly observed after sunrise and around sunset. Our statistical study shows that the analyzed GWs propagated with horizontal velocities from $70 to 250 m/s. The average observed horizontal velocities were $100 m/s in the local summer and 125-150 m/s in the local winter. The waves propagated approximately poleward in the local summer, whereas roughly equatorward propagation was observed in the local winter. Westward propagation was rarely observed in the Czech Republic, and eastward (southeast) propagation was seldom observed in South Africa. A comparison with neutral wind velocities shows that the analyzed GWs propagated approximately against the neutral winds calculated by the HWM07 model. The estimated horizontal wavelengths of the analyzed waves were $100-300 km.

A neural network (NN) model has been developed for the critical frequency of the F2 layer (foF2) at Resolute (74.70°N, 265.10°E) using data obtained from the Space Physics Interactive Data Resource (no longer available) for the period between 1975 and 1995. This model is a first step toward addressing the discrepancies of the International Reference Ionosphere (IRI) foF2 or peak electron density (NmF2) at high latitudes recently presented by Themens et al. (2014). The performance of the NN model has been evaluated using foF2 data obtained from the Canadian Advanced Digital Ionosonde at Resolute (74.75°N, 265.00°E) for the period between 2009 and 2013, in comparison with the IRI predictions. The 2012 version and the International Union of Radio Science option of IRI have been used. The NN nighttime monthly median foF2 variation demonstrates good agreement with observations compared to the IRI. The NN model is able to reproduce the enhancements in foF2 during the equinoxes and also shows an improvement during disturbed days. Root mean square errors were computed between hourly and monthly median model predictions and observations, and on the whole, the NN model seems to perform better during low solar activity and the equinoxes. The NN model shows an improvement in performance on average by 26.638% for hourly foF2 and 32.636% for monthly median foF2, compared to 7.877% obtained for the same station by the most recent NN model that attempted to predict foF2 at a polar cap station (Oyeyemi and Poole, 2005).

This article presents the first results regarding the investigation of the response of the equatorial ionospheric F region in the African sector during geomagnetic storm periods between April 2000 and November 2007 using GPS‐derived vertical total electron content observed at Libreville, Gabon (0.35°N, 9.67°E, dip latitude −8.05°S). We performed a superposed epoch analysis of the storms by defining the start time of the epoch as the storm onset time. During geomagnetic storms, the altered electric fields contribute significantly to the occurrence of negative and positive ionospheric storm effects. Our results showed that the positive storm effects are more prevalent than the negative storm effects and generally last longer irrespective of storm onset times. Also, the positive storm effects are most pronounced in the daytime than in the premidnight and postmidnight periods.