Seasonal and Annual Variations of Equatorial Electrojet and Counter Elecrojet Strength during the Ascending Phase of Solar Cycle 24

Abstract

The seasonal and annual variation of the equatorial electrojet strength at Addis Ababa and Mbour during the ascending phase of solar cycle 24 has been investigated. We used magnetic field measurements obtained from International Real-time Magnetic Observatory Network (INTERMAGNET) stations in Ethiopia, Addis Ababa (AAE) and in Senegal, Mbour (MBO) for the duration from January 2009 to December 2014. The results showed that both seasonal and annual peaks for Equatorial Electrojet (EEJ) at AAE occurred at around local noon and the amplitudes were greater than those at MBO, which peaked much later. The late peaks at MBO were attributed to the effect of tidal winds and the variations in the geomagnetic field. However, the Counter Electrojet (CEJ) at MBO peaked in the late morning and presented larger amplitudes than AAE which peaked much earlier. This implied that EEJ occurring around noon had greater amplitudes which were attributed to higher ionization rates during this period; hence EEJ and CEJ were to found to be local time dependent phenomena. The Addis Ababa station displayed large peaks of EEJ with very small peaks of CEJ while Mbour station displayed large CEJ peaks and small EEJ peaks.  The EEJ strength annual variations showed a longitudinal dependence with the peak value at Addis Ababa being higher at 40nT, than the value at Mbour which was about 20nT in the same year 2012. This was attributed to the fact that AAE was much nearer to the dip equator than MBO, hence a greater enhancement of the EEJ current at AAE. For the seasonal variation, the CEJ amplitude at MBO was larger than the amplitudes at AAE, with the largest peak at MBO being -40nT observed in the year 2011 during the September E-season while the largest for AAE was -20nT in the year 2013 for both March and September E-seasons. EEJ exhibited a longitudinal variation.