Elsevier

Advances in Space Research

Volume 66, Issue 6, 15 September 2020, Pages 1441-1459
Advances in Space Research

Quiet-time and storm-time variations of the African equatorial and low latitude ionosphere during 2009–2015

https://doi.org/10.1016/j.asr.2020.05.038Get rights and content

Abstract

This study investigates the quiet-time and storm-time variations of the African equatorial and low latitude ionosphere during 2009–2015. We obtained TEC observation data from four Global Navigation Satellite System (GNSS) stations over the African equatorial and low latitude, namely: Asab (ASAB, geographic coordinates 13.0°N, 42.65°E; geomagnetic coordinates 4.85°N, 114.34°E), Arba Minch University, (ARMI, 6.03°N, 37.56°E; 3.06°S, 109.29°E), Yamoussoukro (YKRO, 6.87°N, 5.24oW; 2.84°S, 67.41°E) and Mbarara (MBAR, 0.60°S, 30.74°E; 10.27°S, 102.36°E). Seven selected major Geomagnetic Storms (GSs) over solar cycle 24 were investigated and the responses of TEC and Rate of change of TEC Index (ROTI) as proxy for ionospheric irregularities to the storms were also studied. We recorded weaker ionospheric plasma density distributions around the magnetic equator (trough) than at the inner flank of the Equatorial Ionization Anomaly (EIA) crests: A clear evidence of EIA formation. Furthermore, hemispherical asymmetry was observed in the latitudinal distribution of TEC, with consistently higher distribution in the Northern Hemisphere (NH) than Southern Hemisphere (SH). TEC and ionospheric irregularities showed clear dependence on solar activity. Negative responses dominated GSs that occurred at night-time and positive effects for those that occurred during the daytime. The enhancement or inhibition of irregularities during GS is strongly dependent on the time of GS unfolding. As previously established, the daily maximum distribution of TEC at 1400–1600 LT and minimum at 0400–0600 LT were validated by this study, using long time (7 years) data. Seasonally, TEC showed semi-annual pattern of variations, with highest values in equinoxes, and lowest in solstices.

Introduction

The ionosphere over the equatorial and low latitude regions is highly dynamic and complex. In our context, equatorial region spans around the magnetic equator and magnetic latitude of ±5°, while low latitude region spans from the equatorial region boundary to the northern or southern crests of the Equatorial Ionization Anomaly (EIA) (magnetic latitude range: ±5°–±15° to ±20°) (Depueva, 2012). On a global scale, a lot of studies on equatorial and low latitude ionosphere have been carried out in the South American and Asian longitudinal sectors. Paradoxically, documented studies on the African equatorial and low latitude ionosphere are quite paltry, despite the fact that many evidences abound in the literature on the comparatively high dynamics and complexities of the electrodynamics of the ionosphere over the African equatorial and low latitude region (Burke et al., 2004, Hei et al., 2005, Gentile et al., 2011, Akala et al., 2013a, Akala et al., 2013b).

Rapidly varying ionosphere poses threat to trans-ionospheric radio system with attendant enhancements in range errors (Amaechi et al., 2018a, Amaechi et al., 2018b). Therefore, ionospheric variability and the factors influencing such variability are a matter of concern to trans-ionospheric radio systems’ users and service providers. Generally, the ionosphere responds significantly to GSs (Akala et al., 2013a), causing remarkable changes in its variables, e.g., Total Electron Content (TEC), electron density, critical frequency among others. These variations have the capabilities of impacting negatively on space- and ground-based systems (National Research Council (NRC) Report, 2008, Akala et al., 2013a, Oughton et al., 2017). Variations in TEC can cause range errors in navigation signals to the order of 1 TECU to 0.163 m range error at GNSS L1 frequency (1.5754 GHz) (Klobuchar, 1996, Akala et al., 2013a, Akala et al., 2013b). TEC is the number of electrons in a column of a cross-sectional area of one square meter between the GNSS satellite and the receiver (Hofmann-Wellenhof et al., 1994). Sharp and rapid variations in TEC are manifestations of occurrences of ionospheric plasma-density irregularities (Valladares et al., 1996) which are capable of causing scintillations of trans-ionospheric radio waves (Wernik et al., 2003, Akala et al., 2012, Akala et al., 2014, Akala et al., 2016a), and their effects can lead to significant signal degradation (Akala et al., 2016b, Akala et al., 2017, Akala et al., 2019).

GS is a major component of space weather events. Space weather is a combination of impacts of time varying conditions of physical processes from the Sun, all the way down to the Earth’s space environment via the interplanetary medium (Poppe and Jorden, 2006). Occurrences of extreme space weather events are potential threats to the performance and reliability of modern space- and ground-based technological systems, with attendant huge socio-economic consequences (American Meteorological Society (AMS), 2007, National Research Council (NRC) Report, 2008, Cannon et al., 2013, Schrijver et al., 2015, Eastwood et al., 2017). GS occurs when ejections from the Sun via the interplanetary medium impinge on the Earth’s magnetosphere, distorting its shape to cause enhancements in the magnetospheric ram pressure. This process creates ring currents around the Earth’s equatorial region (Chen and Hasegawa, 1988). The night-side of the magnetosphere is stretched out with characteristic magnetic reconnections to form large field aligned currents over the high latitude regions. Under extreme conditions, these currents extend to low latitudes. The severity of GS is quantified by the disturbance storm time (Dst) index (Sugiura, 1964, Hamilton et al., 1988; Gonzalez et al., 1994), the symmetric disturbance field in H (SYM-H) index (Iyemori, 1990) and K planetary index (Kp) (Bartels, 1938, Rostoker, 1972). The Dst and SYM-H indices give information about the strength of the ring currents in the Van Allen belt (Dessler and Parker, 1959). Kp is a global GS index based on 3-hour measurements with a range of 0–9 (Bartels, 1938).

Previously, Adewale et al. (2011) investigated the responses of the African low latitude ionosphere to GSs during 2000–2007 at Libreville, Gabon and reported negative and positive responses of the ionosphere over Libreville to the GSs. Similarly, Akala et al. (2013a) investigated the responses of African equatorial/low-latitude TEC to the intense geomagnetic storms that occurred during the ascending phase of solar cycle 24. The authors reported that the ionosphere over the study locations responded positively to the GSs. Akala et al. (2013b) investigated TEC variability over the South American and African longitudinal sectors and observed higher TEC dynamics over the African sector. Amaechi et al. (2018b) studied the response of the African EIA to the 2015 St. Patrick’s Day GS and reported enhancements in the ionospheric data investigated during the GS. Furthermore, Habarulema et al. (2013) observed TEC responses to November 2004 GS at African equatorial and mid-latitude regions and reported positive phases to the storms at equatorial latitudes. Xu et al. (2007) studied effects of major storms on GPS-TEC and scintillations in the Asian longitudinal sector and reported enhancements in scintillations intensity as well as TEC depletions during the November 2004 GS. De Abreu et al. (2010) investigated the response of the ionosphere over the Brazilian sector to April 2000 GS. The authors explained the observed depletions of TEC at the equator and enhancements at the low-latitude based on the theories of Prompt Penetration Electric Field (PPEF). In most of the previous studies, few geomagnetic storms, few locations, and few years of data were covered. To this end, the specific objectives of this study are to investigate: (i) variations of TEC at the four study locations over the different phases of solar cycle 24: minimum (2009), ascending (2010–2011), maximum (2012–2014) and descending phase (2015); (ii) the responses of the ionosphere (TEC and ionospheric irregularities) at the four locations to the GSs.

Section snippets

Data and method of analysis

GNSS observation data were obtained from the University NAVSTAR Consortium (UNAVCO) website (www.unavco.org/data/gps-gnss/data-access-methods/dai2/app/dai2.html#) for four African GNSS stations: Asab (ASAB, geographic coordinates 13.0°N, 42.65°E; geomagnetic coordinates 4.85°N, 114.34°E) [Eritrea] (equatorial stations), Arba Minch University, (ARMI, 6.03°N, 37.56°E; 3.06°S, 109.29°E) [Ethiopia] (equatorial stations), Yamoussoukro (YKRO, 6.87°N, 5.24oW; 2.84°S, 67.41°E) [Cote d’Ivoire]

Results

Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8(a–d) show the diurnal and seasonal variations of TEC for years 2009–2015 at Asab (ASAB), Armi (ARMI), Yamoussoukro (YKRO) and Mbarara (MBAR), respectively. The Local Time (LT) at ASAB, ARMI and MBAR are: LT = UT + 3 Hrs and at YKRO, LT = UT. Generally, TEC values recorded maximum at 1300–1500 LT and minimum at 0300–0500 LT. TEC diurnal morphologies occasionally showed two crests, during the noon-time and during the sunset hours. On a

Discussion

Generally, the morphologies of TEC on a diurnal basis showed characteristic, pre-dawn minimum (0400–0600 LT), early morning increase (0700 LT), afternoon maximum (1400–1600 LT) and gradual post-sunset decrease (1800–2000 LT). This result is in agreements with the results presented by Tariku (2015).

The onset and turn-off of solar ionization, orientation of the zonal component of wind disturbance electric fields and plasma diffusion effects are major factors responsible for these morphologies (

Conclusions

We investigated the quiet-time and storm-time variations of the African equatorial and low latitude ionosphere during years 2009–2015. Overall, over the African equatorial and low latitude ionosphere, we concluded that:

  • 1.

    TEC possessed characteristic features of maximum values around 1400–1600 LT and minimum around 0400–0600 LT, with occasional two peaks, specifically during the noon-time and the sunset hours. Seasonally, highest TEC values were recorded during equinoxes (mainly in the months of

Acknowledgements

The CME catalogue and solar wind data were obtained online at https://cdaw.gsfc.nasa.gov/CME_list/halo/halo.html and https://omniweb.gsfc.nasa.gov/hw/hmtl, respectively. We obtained the Dst data from www.wdc.kugi.kyoto-u.ac.jp. GNSS observation data were obtained at www.unavco.org/data/gpsgnss/dataccessmethods/dai2/app/dai2/hmtl and www.garner.ucsd.edu/pub/rinex/2013/117. Gopi Seemala provided the TEC processing software (GPS GOPI_V2.9.5) used in this study. Our diagnosis of the IP causes of

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      While there exists, numerous studies addressing the effects of different geomagnetic storms on the ionosphere, it is very much necessary to compare the ionospheric variations during quiet and disturbed geomagnetic conditions using long term observations for a better understanding of space weather effects on the morphological features of ionospheric parameters. There are limited studies using long term observations on the variations of ionospheric TEC during geomagnetically quiet and disturbed conditions (Fuller-Rowell et al., 1997; Bhattacharya et al., 2008; Sanjay Kumar et al., 2012; Singh et al., 2019; Akala and Adewusi, 2020). In this study, we try to understand the impact of geomagnetic storms on the short- and long-term variations of ionospheric TEC in the declining phase of 24th solar cycle.

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