In this paper, we investigated the impact of solar flares on the horizontal (H), eastward (Y) and vertical (Z) components of the geomagnetic field during solar cycles 23 and 24 (SC23/24) using data of magnetometer measurements on the sunlit side of the Earth. We examined the relation between sunspot number and solar flare occurrence of various classes during both cycles. During SC23/24, we obtained correlation coefficient of 0.93/0.97, 0.96/0.96 and 0.60/0.56 for C-class, M-class and X-class flare, respectively. The three components of the geomagnetic field reached a peak a few minutes after the solar flare occurrence. Generally, the magnetic crochet of the H component was negative between the mid-latitudes and Low-latitudes in both hemispheres and positive at low latitudes. By contrast, the analysis of the latitudinal variation of the Y and Z components showed that unlike the H component, their patterns of variations were not coherent in latitude. The peak amplitude of solar flare effect (sfe) on the various geomagnetic components depended on many factors including the local time at the observing station, the solar zenith angle, the position of the station with respect to the magnetic equator, the position of solar flare on the sun and the intensity of the flare. Thus, these peaks were stronger for the stations around the magnetic equator and very low when the geomagnetic field components were close to their nighttime values. Both cycles presented similar monthly variations with the highest sfe value (ΔH_sfe = 48.82 nT for cycle 23 and ΔH_sfe = 24.68 nT for cycle 24) registered in September and lowest in June for cycle 23 (ΔH_sfe = 8.69 nT) and July for cycle 24 (ΔH_sfe = 10.69 nT). Furthermore, the sfe was generally higher in cycle 23 than in cycle 24.

In this study we examined the influences of geomagnetic activity on the Earth surface electric field variations at low latitudes. During the International Equatorial Electrojet Year (IEEY) various experiments were performed along 5° W in West Africa from 1992 to 1995. Among other instruments, 10 stations equipped with magnetometers and telluric electric field lines operated along a meridian chain across the geomagnetic dip equator from November 1992 to December 1994. In the present work, the induced effects of space-weather-related geomagnetic disturbances in the equatorial electrojet (EEJ) influence area in West Africa were examined. For that purpose, variations in the north–south (E_x) and east–west (E_y) components of telluric electric field were analyzed, along with that of the three components (H, D and Z) of the geomagnetic field during the geomagnetic storm of 17 February 1993 and the solar flare observed on 4 April 1993. The most important induction effects during these events are associated with brisk impulses like storm sudden commencement (ssc) and solar flare effect (sfe) in the geomagnetic field variations. For the moderate geomagnetic storm that occurred on 17 February 1993, with a minimum Dst index of −110 nT, the geo-electric field responses to the impulse around 11:00 LT at LAM are E_x = 520 mV km⁻¹ and E_y = 400 mV km⁻¹. The geo-electric field responses to the sfe that occurred around 14:30 LT on 4 April 1993 are clearly observed at different stations as well. At LAM the crest-to-crest amplitude of the geo-electric field components associated with the sfe are E_x = 550 mV km⁻¹ and E_y = 340 mV km⁻¹. Note that the sfe impact on the geo-electric field variations decreases with the increasing distance of the stations from the subsolar point, which is located at about 5.13° N on 4 April. This trend does not reflect the sfe increasing amplitude near the dip equator due the high Cowling conductivity in the EEJ belt.

In this study, the longitude dependence of the equatorial electrojet (EEJ) has been reviewed. The most remarkable feature is the “wave-four” structure of the EEJ longitude profile. In order to understand the background physical process of this structure, we have examined the influence of the geomagnetic-field intensity and that of migrating thermospheric tides. The results show a good correlation between the EEJ and the main geomagnetic-field intensity. However, the wave-four structure cannot be explained through this strong correlation. By means of simulation through the National Center for Atmospheric Research (NCAR)'s Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM), we have shown that the combination of the longitudinal inequalities of the main geomagnetic-field intensity and the tidal-wave longitudinal structures is necessary to produce the wave-four structure of the EEJ longitude profile. Finally, we have shown the seasonal variability of the EEJ longitude profiles, which may be controlled by the seasonal behaviors of the wind system in the thermosphere.