[1] The use of neural networks (NNs) has been employed in this work to develop a global model of the ionospheric F 2 region critical frequency, f o F 2 . The main principle behind our approach has been to utilize parameters other than simple geographic coordinates, on which f o F 2 is known to depend, and to exploit the ability of NNs to establish and model this nonlinear relationship for predictive purposes. The f o F 2 data used in the training of the NNs were obtained from 59 ionospheric stations across the globe at various times from 1964 to 1986, on the basis of availability. To test the success of this approach, one NN (NN1) was trained without data from 13 stations, selected for their geographic remoteness, which could then be used to validate the predictions of the NN for those remote coordinates. These stations were subsequently included in our final NN (NN2). The input parameters consisted of day number (day of the year), universal time, solar activity, magnetic activity, geographic latitude, angle of meridian relative to subsolar point, magnetic dip angle, magnetic declination, and solar zenith angle. Comparisons between f o F 2 values determined using NNs and the International Reference Ionosphere (IRI) model (from Union Radio Scientifique Internationale (URSI) and International Radio Consultative Committee (CCIR) coefficients) with observed values are given with their root-mean-square (RMS) error differences for test stations. The results from NN2 are used to produce the global behavior of hourly values of f o F 2 and are compared with the IRI model using URSI and CCIR coefficients. The results obtained (i.e., RMS error differences), which compare favorably with the IRI models, justify this technique for global f o F 2 modeling.

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We present solar quiet (Sq) variation of the horizontal (H) magnetic field intensity deduced from Magnetic Data Acquisition System (MAGDAS) records over Africa during an unusual strong and prolonged 2009 sudden stratospheric warming (SSW) event. A reduction in the SqH magnitude that enveloped the geomagnetic latitudes between 21.13°N (Fayum FYM) in Egypt and 39.51°S (Durban DRB) in South Africa was observed, while the stratospheric polar temperature was increasing and got strengthened when the stratospheric temperature reached its maximum. Another novel feature associated with the hemispheric reduction is the reversal in the north‐south asymmetry of the SqH, which is indicative of higher SqH magnitude in the Northern Hemisphere compared to the Southern Hemisphere during SSW peak phase. The reversal of the equatorial electrojet (EEJ) or the counter electrojet (CEJ) was observed after the polar stratospheric temperature reached its maximum. The effect of additional currents associated with CEJ was observed in the Southern Hemisphere at middle latitude. Similar changes were observed in the EEJ at the South America, Pacific Ocean, and Central Asia sectors. The effect of the SSW is largest in the South American sector and smallest in the Central Asian sector.

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This study presents the longitudinal dependence of responses of the equatorial/low‐latitude ionosphere over the oceanic regions to geomagnetic storms of 28 May and 8 September 2017. We investigated the interplanetary origins of the storms. Total electron content (TEC) data were obtained from Global Navigation Satellite System stations, located around the oceanic areas in the equatorial/low‐latitude regions. The Rate of change of TEC Index (ROTI) was used as a proxy for ionospheric irregularities over the study locations. Further, variations of the horizontal component of the Earth's magnetic fields, obtained from ground‐based magnetometers were studied. We used ionospheric disturbance currents, polar cap and auroral electrojet indices to monitor the storm time electric fields. The May 2017 storm was driven by sheath and magnetic cloud fields, while the September 2017 storm was driven by sheath fields. We observed a comparative dominance of TEC intensities over the Oceans than over the landlocked areas. Empirically, our results validated a theoretical suggestion of the existence of a dynamic ocean‐ionosphere coupling made by Godin et al. (2015, http://10.0.4.162/s40623-015-0212-4). Prompt Penetration Electric Fields (PPEF) was observed to be a key factor that controls TEC responses to storms. PPEFs caused TEC enhancements, mainly over the Pacific Ocean longitudes during the May 2017 storm and enhanced TEC over the Atlantic Ocean and the Pacific Oceans longitudes during the September 2017 storm. These PPEFs triggered irregularities over the Pacific Ocean longitudes, particularly during the main phase of May 2017 storm. Irregularities were generally inhibited by the September 2017 storm.

Total electron content (TEC) has been used to study the response of the African equatorial/low‐latitude ionosphere to the 17 March 2015 intense storm. Daily variations of symmetric H index, z component of interplanetary magnetic field, polar cap index, and H component of the Earth's magnetic field were analyzed along with TEC obtained from stations classified into western (11.53°E to 5.24°W) and eastern (25.00°E to 40.20°E) sectors. Storm time behavior of TEC was compared with the mean TEC of quiet days. TEC was enhanced over both sectors at the beginning of the storm while it reduced after the main phase as a result of prompt penetration of electric field and disturbance dynamo electric field (DDEF), respectively. The magnetic effect of the disturbed ionospheric electric current showed oscillations and minima in response to prompt penetration of electric field, which further enhanced the equatorial ionization anomaly. The effect of DDEF estimated from the ionospheric disturbance dynamo manifested in the form of decrease in the amplitude of the horizontal component of the Earth magnetic field (H) several hours after the beginning of the disturbance and during the recovery phase. Consequently, ionospheric irregularities were suppressed over all stations in both sectors on 17 and 18 March due to westward DDEF. On the 19 March, however, there was a difference in the pattern of irregularities over both sectors.

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