O.S. Bolaji

Magnetic storm effects on the variation of TEC over Ilorin an equatorial station

We have used total electron content (TEC) derived from dual-frequency GPS receivers to study magnetically quiet and storm time variations of the ionosphere at Ilorin(8.47°N, 4.68°E), an equatorial station in the African sector. Four years (2009–2012) data were used for the study. The result on the quiet time variation of the ionosphere showed that the diurnal variation of TEC is not symmetrical about noon. This is a departure from a typical Chapman variation. Daytime maximum occurred after local noon (13–16 LT) for all the seasons and at all solar activity levels considered. A significant effect of solar activity variation was observed on the seasonal trend in 2011. The tendency for magnetic storms to cause increases in TEC is much greater than those of decreases. Daytime maximum TEC usually occurred closer to the noon time during storm periods when compared to those of quiet periods. Maximum percent change in TEC on storm days varied from about 25 to 131%.

On the historical origins of the CEJ, DP2, and Ddyn current systems and their roles in the predictions of ionospheric responses to geomagnetic storms at equatorial latitudes

In this short letter, we recall the differences between the counter electrojet (CEJ), which is a phenomenon observed on the magnetically quiet days and the disturbance dynamo (Ddyn), which can be observed during and after a geomagnetic storm. The CEJ is well known to occur near the geomagnetic dip equator. It can be identified by a reversal in the horizontal component (H) of the geomagnetic field daily regular variations. In contrast to equatorial electrojet (EEJ) that flows eastward in the daytime, the CEJ is considered to flow westward. The magnetic signatures of the reversed solar quiet (Sq) current at the low latitude during magnetic storms are due to the Ddyn. This disturbance (Ddyn) is produced by current systems that are driven by thermospheric storm winds originating from the Joule heating of enhanced high-latitude currents. The DP2 is the magnetic effect of current systems at high latitudes. These currents are associated with the coupling of magnetosphere and ionosphere through geomagnetic field lines. They are associated to the magnetospheric convection. During intense magnetic storms these high-latitude currents are enhanced and their magnetic effects can extend toward the low latitudes. This work shows that the study of magnetic perturbations makes it possible to understand the disturbances of the ionospheric electric currents. The use of an efficient treatment of the magnetic signals makes it possible to separate the magnetic effects of the different perturbations prompt penetration of the magnetospheric convection electric field and disturbance dynamo electric field. This was performed in the paper Nava et al. (2016).

Solar quiet current response in the African sector due to a 2009 sudden stratospheric warming event

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.

Investigation of geomagnetic induced current at high latitude during the storm-time variation

Abstract During the geomagnetic disturbances, the geomagnetically induced current (GIC) are influenced by the geoelectric field flowing in conductive Earth. In this paper, we studied the variability of GICs, the time derivatives of the geomagnetic field (dB/dt), geomagnetic indices: Symmetric disturbance field in H (SYM-H) index, AU (eastward electrojet) and AL (westward electrojet) indices, Interplanetary parameters such as solar wind speed (v), and interplanetary magnetic field (Bz) during the geomagnetic storms on 31 March 2001, 21 October 2001, 6 November 2001, 29 October 2003, 31 October 2003 and 9 November 2004 with high solar wind speed due to a coronal mass ejection. Wavelet spectrum based approach was employed to analyze the GIC time series in a sequence of time scales of one to twenty four hours. It was observed that there are more concentration of power between the 14–24 h on 31 March 2001, 17–24 h on 21 October 2001, 1–7 h on 6 November 2001, two peaks were observed between 5–8 h and 21–24 h on 29 October 2003, 1–3 h on 31 October 2003 and 18–22 h on 9 November 2004. Bootstrap method was used to obtain regression correlations between the time derivative of the geomagnetic field (dB/dt) and the observed values of the geomagnetic induced current on 31 March 2001, 21 October 2001, 6 November 2001, 29 October 2003, 31 October 2003 and 9 November 2004 which shows a distributed cluster of correlation coefficients at around r = −0.567, −0.717, −0.477, −0.419, −0.210 and r = −0.488 respectively. We observed that high energy wavelet coefficient correlated well with bootstrap correlation, while low energy wavelet coefficient gives low bootstrap correlation. It was noticed that the geomagnetic storm has a influence on GIC and geomagnetic field derivatives (dB/dt). This might be ascribed to the coronal mass ejection with solar wind due to particle acceleration processes in the solar atmosphere.

Excursions of Interhemispheric Field‐Aligned Currents in Africa

The interhemispheric field‐aligned currents (IHFACs) are the most evident currents connecting the solar quiet (Sq) current systems in the Northern and Southern Hemispheres. Understanding these coupling currents has practical importance for space weather forecast of the magnetosphere‐ionosphere interactions and their potential impacts on low‐orbiting spacecraft. In this paper, we have examined digital magnetic records from September 2008 to August 2009 at nine Magnetic Data Acquisition System (MAGDAS) stations in order to uniquely determine the variability of IHFACs in terms of their excursions in the African sector. By excursions, we mean the direction in which the IHFACs are moving as a function of local time or latitude. Different from theoretical prediction, we found prominent positive IHFACs around dusk in February and March equinox. A novel feature of this study is the semidiurnal variation control of the coupling between the Northern and Southern Hemispheres. In addition, we found terdiurnal variation in September with their associated strong diurnal variations from October through December. Quite unexpectedly, in January, a significant latitudinal depletion in the intensities of IHFACs was observed from dawn to dusk, compared to any other months under investigation, accompanied with a marked reduction of the prominent diurnal variations at the magnetic equator (Addis Ababa, AAB). The fact that the excursions of IHFACs in January experience an unusual retardation with respect to their in