Abstract
AbstractThis paper investigated the behavior of ionospheric irregularities over the African equatorial ionization anomaly (EIA) crests during intense geomagnetic storms that occurred from 2012 to 2015. Irregularities were monitored using the rate of change of TEC index along with variations of the horizontal component of the Earth's magnetic field (H) and ionospheric electric current disturbance (Diono). The predictive capability of the Prompt Penetration Equatorial Electric Field Model (PPEFM) was assessed by comparing prompt penetration electric field (PPEF) inferred from interplanetary electric field and Diono with PPEF derived from the PPEFM, with emphasis on how well the model reproduced enhancement/reduction in the prereversal enhancement (PRE). Eastward PPEF triggered short duration irregularities on 23 April 2012, 17 March 2013, and 20 February 2014 while westward electric field reduced them thereafter. The PPEFM rightly predicted enhancement (reduction) in PRE on 17 March 2013 (19 February 2014) when irregularities were triggered (inhibited). It, however, showed no change in the PRE on 23 April 2012 and 20 February 2014. During the storms recoveries, irregularities were always inhibited/reduced over the trough by westward disturbance dynamo and the inhibition lasted longer during the superstorm of March 2015. Also, there was a hemispheric asymmetry in irregularities over the African EIA crests. On 16–17 July 2012, 15 November 2012, and 19 March 2013, there were differences in irregularities behavior. On these days, the asymmetry of the postsunset crests was pronounced in both hemispheres.
Highlights
Ionospheric irregularities are small to large-scale structures that form in the plasma density (Pekins, 1975)
All the storms were Coronal Mass Ejection (CME) driven except case 1 which is a ‘wake of CME’ [www.spaceweather.com]
On 23 April 2012, irregularities were inhibited over the trough and crests from 16:30 –18:00 UT (Figure 1, green rectangle), and subsequent triggering over the crests from exactly 18:00 UT
Summary
Ionospheric irregularities are small to large-scale structures that form in the plasma density (Pekins, 1975). Their interaction with Global Navigation Satellite System (GNSS) signals could result in rapid fluctuations in the amplitude and/or phase of the signals, giving rise to a phenomenon known as scintillation (Aarons, 1982; Datta-Barua et al, 2015). The outcome could have disastrous effects on the life-critical GNSS applications especially, those utilized in navigation, positioning, search and rescue as well as military operations and surveying (Conker et al, 2003; Sunda et al, 2015) For this reason, adequate information about the actual state of the ionosphere is crucial for the smooth operation of the critical GNSS applications during all- weather conditions. Parameters affecting the generation of irregularities are the: (i) post sunset vertical drift, (ii) components of thermospheric winds, (iii) density gradient at the bottom side of the F-layer, and (iv) initial seed perturbations due to gravity wave from the lower atmosphere (Haerendel, 1973; Ott, 1978; Sultan, 1996; Tsunoda et al, 2013)
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