The response of the ionosphere over Ilorin to some geomagnetic storms B.W. Joshua a,⇑ , J.O. Adeniyi b , B.W. Reinisch c , I.A. Adimula b , A.O. Olawepo b , O.A. Oladipo b , S.J. Adebiyi d a Department of Physics, Kebbi State University of Science and Technology Aliero, Kebbi State, Nigeria b Department of Physics, University of Ilorin, Ilorin, Nigeria c Center for Atmospheric Research, University of Massachusetts Lowell, United States d Department of Industrial Physics, Landmark University, Kwara State, Nigeria Received 25 April 2014; received in revised form 23 August 2014; accepted 25 August 2014 Available online 6 September 2014 Abstract The effects of some geomagnetic storms on the F2 layer peak parameters over Ilorin, Nigeria (Lat. 8:53°N, Long. 4.5°E, dip angle, 2.96°) have been investigated. Our results showed that the highest intensity of the noon bite-out occurred during the March equinox and lowest during the June Solstice on quiet days. Quiet day NmF2 disturbances which appeared as a pre-storm enhancement, but not related to the magnetic storm event that followed were observed at this station. These enhancements were attributed to the modification of the equatorial electric field as a result of injection of the Auroral electric field to the low and equatorial ionosphere. For disturbed conditions, the morphology of the NmF2 on quiet days is altered. Daytime and nighttime NmF2 and hmF2 enhancements were recorded at this station. Decreases in NmF2 were also observed during the recovery periods, most of which appeared during the post-noon period, except the storm event of May 28–29. On the average, enhancements in NmF2 (i.e. Positive phases) are the prominent features of this station. Observations from this study also indicate that Dst, Ap and Kp which have been the most widely used indices in academic research in describing the behavior of geomagnetic storms, are not sufficient for storm time analysis in the equatorial and low latitude ionosphere. Ó 2014 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Geomagnetic storm; Magnetic field; Electric field; Ionosphere; Electron density 1. Introduction The earth’s ionosphere which is the ionized component of the Earth’s upper atmosphere, responds remarkably to varying inputs of solar and magnetospheric energy. The ionosphere is made up of several peaks or crests of ioniza- tion, known as regions or layers. However, most of the ionospheric electron content is formed near the ionospheric F2 peak height (Rishbeth and Edwards, 1989). Production of electrons in the ionosphere is known to be mainly con- trolled by Extreme ultraviolet (EUV) solar radiation, while transport is dominated by equatorial fountain and neutral wind (Adeniyi et al., 2010; Chakraborty and Hajra, 2009; Henderson and Swenson, 2005; Balan and Bailey, 1995; Hanson and Moffett, 1966; Adeniyi, 1986). Geomagnetic storms occur when there is a period of rapid magnetic field variation (Reeves, 2010) caused by coronal mass ejections (CMEs) directed at the Earth from the sun, the arrival of CMEs into the Earth’s magneto- sphere can produce an increase in the ram pressure characterized by sudden increases in solar wind velocity, http://dx.doi.org/10.1016/j.asr.2014.08.027 0273-1177/Ó 2014 COSPAR. Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +234 802 8548128; fax: +234 806 2427276. E-mail addresses: benjaminjoshua7@gmail.com (B.W. Joshua), segun47@yahoo.com, jadeniyi@unilorin.edu.ng (J.O. Adeniyi), bodo. reinisch@digisonde.com (B.W. Reinisch), johndat2003@gmail.com (S.J. Adebiyi). www.elsevier.com/locate/asr Available online at www.sciencedirect.com ScienceDirect Advances in Space Research 54 (2014) 2224–2235