for review only...for review only 1 1 variability of the african equatorial ionization anomaly (eia)...

For Review Only

Variability of the African Equatorial Ionization Anomaly

(EIA) crests during year 2013

Journal: Canadian Journal of Physics

Manuscript ID cjp-2017-0985.R2

Manuscript Type: Article

Date Submitted by the Author: 19-Apr-2018

Complete List of Authors: Amaechi, P.O.; Chrisland University, Department of Physical Science; University of Lagos, Department of Physics Oyeyemi, E.O.; Univeristy of Lagos, Department of Physics Akala, A.O.; Univeristy of Lagos, Department of Physics

Keyword: Equatorial Ionization Anomaly, low latitude ionosphere, Total Electron Contrent, Position of the crest, Magnitude of the crest

Is the invited manuscript for consideration in a Special

Issue? : Not applicable (regular submission)

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Canadian Journal of Physics

For Review Only

1

Variability of the African Equatorial Ionization Anomaly (EIA) crests during year 2013 1

P. O. Amaechiab,٭

, E. O. Oyeyemia, A. O. Akala

a 2

aDepartment of Physics, University of Lagos, Yaba, Lagos, Nigeria 3

bDepartment of Physical Sciences, Chrisland University, Owode, Abeokuta 4

5

Abstract 6

This paper discusses the variability of the position and magnitude of the crests of African 7

Equatorial Ionization Anomal (EIA) during noon and post sunset periods. Total Electron Content 8

(TEC) data covered year 2013, and were obtained from a chain of Global Positioning System 9

(GPS) receivers in both hemispheres around 37oE longitude. Local magnetometer data were used 10

to infer the direction and magnitude of the ExB drift, while the solar EUV proxy index (PI) was 11

used as a measure of solar activity. It was found that the time of formation of both crests varied 12

from 1400-1700 LT. Additionally, the position of the crests was found to be asymmetric with 13

respect to the magnetic equator. During noon period, the position of the northern and southern 14

crests varied from 4.91o to 7.36

o and -9.17

o to -12.62

o respectively. During post sunset period, it 15

varied from 8o

to 11.7o and -9

o to -16

o. Seasonally, with reference to the magnetic equator, both 16

crests moved poleward during equinoxes and collapsed towards the equator during 17

winter/summer. Equinoxes recorded the greatest crest magnitude followed by winter then 18

summer over both hemispheres during noon period. However, this trend persisted over the 19

northern crest only during post sunset period. Overall, during noon period, we recorded 20

correlation coefficient of 0.67 and 0.68 between crests magnitude and ΔH, a proxy for equatorial 21

electrojet current, and 0.88 and 0.81 between crests position and ∆H, for the northern and 22

southern crests respectively. During the Halloween day storm of 30th

October 2013, a westward 23

electric field inhibited the development of the post sunset crests. 24

Key words: Equatorial ionization anomaly, low latitude ionosphere, total electron content., 25

position of the crest, magnitude of the crest. 26

1. Introduction 27

The equatorial/low latitude ionosphere is a vast portion that defines the Equatorial Ionization 28

Anomal (EIA) region [1]. During disturbed geophysical conditions, the electrodynamics of the 29

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ionosphere over this region can vary drastically, with significant implications on the operational 30

capability of Global Navigation Satellite System (GNSS) which now have extensive applications 31

in our day-to-day activities. 32

During the day, eastward electric field resulting from tidal and zonal winds interact with the 33

north-south terrestrial magnetic field lifting ionization upward. Under the effect of gravity and 34

pressure gradient, the lifted ionization diffuses downward along the geomagnetic field lines 35

forming two peaks of enhanced ionization on both sides of the magnetic equator at about ±150 36

dip latitude, with a trough in ionization at the magnetic equator. This is the Equatorial Ionization 37

Anomaly (EIA) [2] and the two regions of enhanced ionization are known as the crests of the 38

EIA [3]. 39

The persistence of the EIA into the night is due to the pre-reversal enhancement (PRE) in 40

eastward electric field. The PRE is generated through the interaction of zonal neutral wind in the 41

F-region and conductivity gradient caused by the solar terminator at sunset [4]. It plays a 42

significant role in destabilizing the post-sunset ionosphere, hence, in the generation of 43

ionospheric irregularities through the Rayleigh-Taylor Instability (RTI) mechanism and the 44

associated gravity wave seed perturbation [5, 6]. Irregularities are responsible for scintillations of 45

trans-ionospheric Global Positioning System (GPS) signals which in turn affect adversely the 46

operational capability of critical space based technologies widely used in various endeavours 47

nowadays. As such, the development and decay of the EIA is associated with two significant 48

effects on trans-ionspheric GPS signlas over the equatorial/low latitude region: (i) high total 49

electron content (TEC) which are source of additional range errors [7] and (ii) enhanced 50

scintillations effects on GPS signals [8]. 51

For this reason, variations of the EIA have been extensively studied over various sectors [9-17]. 52

The previous studies have revealed that the EIA main parameters such as the magnitude, 53

occurrence time, locations of the crest as well as crest to trough ratio exhibit significant 54

variations with local time, season, solar and geomagnetic activities. This extreme variability is 55

driven to a large extent by change in composition [18], thermospheric wind and wave, and tidal 56

forces of lower atmospheric origin [19], changes in the equatorial electrojet (EEJ) [20] together 57

with the associated variations in ExB drift velocity [21]; as well as changes in noon solar zenith 58

angle in relation with the geometry of magnetic field lines [22, 23]. 59

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However, most of such studies have been done over the Asian and American sectors with few 60

studies over the African sector. The paucity of such studies is due to the sparse distribution of 61

GPS receivers over Africa. Furthermore, the few studies from Africa on EIA were done with 62

sparsely spaced GPS receivers, in fact, some of these previous studies used data from two GPS 63

stations, one located at the trough while the other located away from the magnetic equator, and 64

mostly in the African southern hemisphere. Zhao et al. [13] argued that because the longitudinal 65

and latitudinal variation of the EIA is large, its temporal and spatial behaviour trends cannot be 66

adequately monitored with just a pair of GPS receivers. Consequently, a better understanding of 67

EIA variability can only be achieved using a chain of receivers well clustered around a specific 68

longitude and reasonably spaced in latitude. 69

Traditionally, the crests of the EIA have been located within about ±15 to ±20 degrees dip 70

latitude [1]. But parameters controlling the variability of the EIA vary significantly from one 71

longitudinal sector to another. For example the drift have been found to be weaker over Africa, 72

where winds are thought to dictate the dynamics of ionospheric processes to a large extent [24]. 73

The implication is that the crests will vary significantly both in magnitude and position. As such, 74

using a fixed crest position is liable to errors. It thus, becomes imperative to characterize the 75

variation of the magnitude and position of the crests of the EIA. 76

This is all the more significant given the fact that there is a dearth of studies on the variation of 77

the magnitude and position of the crests of the EIA over Africa using chain of GPS receivers in 78

both hemispheres simultaneously. This, to a large extent constitutes an impediment to global 79

understanding of ionospheric electrodynamics and a challenge to the specification and forecast of 80

ionospheric effects on the emerging new critical technologies. The aim of this work is to 81

characterize monthly and seasonal variations of the position and magnitude of the crests of the 82

African EIA during noon and post-noon periods. 83

2. Data and method of analysis 84

The following parameters were used to study variations of the EIA over east Africa: Daily solar 85

flux (F10.7) provided by the Dominion Radio Astrophysical Observatory (DRAO), Penticon and 86

obtained at the website htpp://www.spaceweather.gc.ca/solarflux/sx-5-en.php. This was used to 87

derive the improved weighted EUV-proxy index (PI) [25]. The Total Electron Content (TEC) 88

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data were obtained from the University NAVSTAR Consortium (UNAVCO) website 89

htpp://www.unavco.org/data/ data.html. The GPS and magnetometer stations are shown in Fig. 90

1. The magnetometer at the Adigrat station is owned by the African Meridian B-Field Education 91

and Research (AMBER), while the one at Addis Ababa is managed by the Institut de Physique 92

du Globe de Paris (IPGP), France and data are distributed via Intermagnet. We computed the 93

horizontal component of the Earth magnetic field and estimated ∆H as a measure of the strength 94

of the equatorial electrojet (EEJ). Other parameters such as the disturbance storm time index 95

(Dst), the symmetric disturbance index (SYM-H), the x component of the solar wind speed (Vx) 96

and the x-component of interplanetary magnetic field (IMF Bz) were used to monitor 97

interplanetary and magnetic conditions during the Halloween storm of 30 October 2013. Dst and 98

SYM-H are made available by the World Data Centre for geomagnetism (WDC), Kyoto at the 99

website htpp://wdc.kugi.kyoto-u.ac.jp while Vx and IMF Bz were obtained at the website 100

http://www.srl.caltech.edu/ACE. 101

The improved weighted EUV-proxy (PI) defined by Chen et al. [25] was used as a proxy for 102

solar activity. 103

PI = 0.4F10.7 + 0.6F10.7A (1) 104

where F 10.7A is the 81 day mean of daily F10.7 105

The horizontal component of the Earth’s magnetic field (H) was derived from the north 106

component (X) and east component (Y) of the geomagnetic field measured by magnetometers at 107

Addis Ababa and Adigrat stations. 108

H = 𝑋2 + 𝑌2 (2) 109

Baseline values were computed and removed from the hourly values of H which were further 110

corrected for non-cyclic variation following the method of Rabiu et al [26]. However, in the 111

analysis of magnetic data, care was taken to consider only quietest days. As such, five quietest 112

internationally days (IQDs) in each month of year 2013 were selected from the GFZ German 113

Research Centre for Geosciences website ftp://ftp.gfz-potsdam.de. During these very quiet days, 114

the magnitude and direction of ExB drift was estimated using ∆H, a proxy for EEJ current [27, 115

28]. ∆H was computed as the difference between H-components of the field recorded at two 116

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magnetometer stations one located at the equator (Addis Ababa, dip 0.160) and the other off the 117

equator (Adigrat, dip 6.010N). However, for analysis pertaining to the Halloween storm of 30 118

October 2013, there was no data for the magnetometer in Adigrat thus, ∆H was computed using 119

equation 3 [29]. 120

∆H = [Hobs – H2-3 am] + C x [SYM-H – SYM-H2-3 am] (3) 121

where Hobs and SYM-H are the observed magnetic field and the corresponding SYM-H value 122

at a given time, C is a constant determined from least square method [30]. As such positive 123

(negative) ∆H is thus, proportional to eastward (westward) electric field. 124

We processed GPS TEC data following the approach of Seemala and Delay [31]. This entailed 125

leveling the carrier phase with the pseudorange measurements [32], detecting and correcting 126

cycle slips in phase data [33], calibrating slant TEC (STEC) by removing satellite and receiver 127

biases [34] and converting STEC to vertical TEC (VTEC) using the thin shell ionospheric model 128

whose height is assumed at 350 km [35]. Data quality check was done using the Translating 129

Editing and Quality Checking (TEQC) software [36]. We used an elevation cut-off of 30o [37-130

38]. Higher elevation as suggested by Rama Rao et al. [39] over the Indian sector will obviously 131

restrict the range of observations, and thus, obfuscate the full extent of the EIA. Aggarwal et al. 132

[14] used a cut-off elevation of 20o and showed that errors due to low cut off elevation are 133

covered by statistical variations in TEC. 134

In reconstructing the EIA we defined a mean longitude (L) as in equation 4 and estimated time 135

difference, ∆t (hours) between each receiver and that of the mean longitude as in equation 5 [15]. 136

L = 𝜃𝑖

𝑁

𝑁𝑖=1 (4) 137

where 𝜃𝑖 is the longitude of station i and N are the number of stations. 138

∆t = 𝐿−𝐿𝑅𝑋

𝑐 (5) 139

where 𝐿𝑅𝑋 is the longitude of each receiver and c =15 deg/hour, is the hour to longitude 140

conversion factor. 141

Then, time correction was performed by adding ∆t to the time data of each receiver so that the 142

measurement time was corrected to the time of the mean longitude. The African EIA was 143

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reconstructed using monthly mean diurnal values of VTEC with a time resolution of 1 hour and 144

latitudinal resolution of 50. 145

3. Results 146

Fig. 2 shows contour maps of the monthly mean diurnal variation of TEC for the year 2013 over 147

the African EIA. The time of occurrence of maximum TEC as well as the latitudinal position of 148

occurrence of such peak were used to define the time duration and position of the EIA crests 149

respectively. From this figure, the time of occurrence of the crests was found to vary from 1400 150

LT to 1700 LT. In March and August, both crests however, formed at the same time (1700 LT) 151

as well as in April and October (1600 LT) and December (1500 LT). In January, February and 152

November the northern crest formed earlier than the southern crest while the reverse is the case 153

in May, June and September. Data from November to December was unavailable likely as a 154

result of power failure. The position of the southern crest varied from -12.62o

to -9.17o dip 155

latitude while that of the northern crest varied from 4.91o to 7.36

o dip latitude. The magnitude of 156

both crests is shown in Fig. 3 for the noon period only. A maximum in both crests value occurred 157

in March and October-November while a minimum occurred in July. For the northern crest, the 158

October peak is higher than the March peak whereas for the southern crest the November peak is 159

higher. The evolution of the EIA with drift data will be presented in Figure 9. 160

Fig. 4 shows the TEC profile during post sunset period at about 2100 LT from January to 161

December, 2013. Clear variations of the position and magnitude of both crests are observed in 162

this figure. The position of the northern crest is seen at about 8o

dip latitude in January, March, 163

August and October while it shifted to 11.7o dip latitude in April, September, November and 164

December (see broken horizontal line in Fig. 4). In February, May June and July however, it is at 165

about 70

dip latitude. Generally the southern crest is located at about -11.5o dip latitude during 166

most months of the year. It however, shifted to -9o dip latitude in October and -16

o dip latitude in 167

December. As for the magnitude of the post sunset crests, highest (smallest) magnitudes were 168

recorded in November (June) for the northern crest and April (January) for the southern crest. On 169

the other hand, the Northern crest was higher in magnitude than the southern crest in January, 170

February, November and December while the reverse was the case in March, April, May, June, 171

July, August, September and October. 172

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Fig. 5 shows the seasonal variations of the EIA for the year 2013. The main seasons considered 173

were represented by winter (November, December, January, February); summer (May, June, 174

July, August) and equinoxes (March, April and September, October). During winter, the 175

southern crest formed at -10.22o at about 1700 LT with a magnitude of 55.115 TECu (1 TECu = 176

1016

electrons per m2) while the northern crest formed at 4.91

o at about 1500 LT with a 177

magnitude of 55.84 TECu. During equinoxes, the southern crest expanded to -12.42o at about 178

1600 LT with a magnitude of 58.61 TECu while the northern crest expanded to 7.360 at about 179

1600 LT corresponding magnitude of 60.00 TECu. Finally during summer, the crests collapsed 180

at -10.22o

at about 16:00 LT with magnitude of 43.39 TECu for the southern crest and at 4.910 at 181

about 16:00 LT with magnitude of 43.94 TECu for the northern crest. 182

Fig. 6 shows the seasonal variations of the position and magnitude of the EIA during the post 183

sunset period. From this figure, the southern crest formed at -11o

with a magnitude of 27.43 184

TECu during winter while the northern crest formed at 8o with corresponding magnitude of 185

31.94 TECu. During equinoxes, both crests remained at -11o

and 8o with corresponding 186

magnitude of 41.76 TECu and 35.30 TECu for the southern and northern respectively. The 187

southern crest still formed at -11o with a magnitude of 32.64 TECu while the northern crest 188

collapsed slightly to 7o with corresponding magnitude of 23.79 TECu during summer. 189

In Fig. 7, the monthly mean diurnal variations of ∆H from January to September 2013 are 190

presented. There is no available data from October to December 2013. Generally, the profile for 191

∆H is characterized by a negative excursion in the morning below -5 nT followed by a rise to a 192

peak value close to noon, then a decrease in the afternoon. This is known as the counter 193

electrojet (CEJ). The peaks in ∆H were reached at about 1200 LT in the months of January, 194

April, May, August and September. The corresponding values were 43.79 nT, 43.63 nT, 22.95 195

nT, 41.46 nT and 42.15 nT respectively. For the other months the time of occurrence and peak 196

values were: February (11:00LT, 26.32 nT), March (14:00 LT, 45.40 nT); June (14:00 LT, 11.95 197

nT) and July (14:00 LT, 17.84 nT). Overall, March had the highest noon peak while June had the 198

lowest. The seasonal variations of ∆H are presented in Fig. 8. Because of the unavailability of 199

magnetic data from October to December, we took mean values of ∆H for January and February 200

only to represent winter while the mean values of delta H from March, April and September was 201

used for the equinoxes. For summer all for months were represented. From this figure, the 202

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minimum in ΔH occurred at about 0800 LT while a peak value is reached at about 1200 LT 203

during all seasons under consideration. Equinoxes recorded the highest noon peak (43.37 nT) 204

followed by winter (31.32 nT) and summer (19.17 nT). 205

Fig. 9 presents a comparison of the evolution of the EIA and ΔH from 0800 to 1400 LT on a 206

seasonal basis. The EEJ started developing from a negative value of ∆H at about 0800 LT to a 207

peak value at about 1200 LT. The strength of the EEJ is shown in the bottom of the figure. 208

During the three seasons under consideration, right from the time of negative ∆H till when a 209

maximum in ΔH is reached and after about 3 hours, there is only one peak in the EIA profile. 210

The double peak in ionization however, made its appearance at about 1100 LT (3 hours after 211

current reversal). 212

In Fig. 10, variations of monthly PI index are presented from January to December 2013. The 213

months of May, October, November and December recorded the highest PI values of 124.4, 214

133.8, 147.4 and 147.1 sfu respectively. February and September had the lowest PI value of 215

111.1, 195.0 and 106.7 sfu respectively. Correlations between (a) crests magnitude and ∆H, and 216

(b) crests position and ∆H during the noon period are presented in Fig. 11. Correlation 217

coefficients of 0.67 and 0.68 were obtained for crest magnitude versus ΔH as well as 0.88 and 218

0.81 for crest position versus ΔH for the northern and southern crests respectively. 219

Fig. 12 shows variations of interplanetary and magnetic parameters during the Halloween storm 220

of 30 October 2013. Unlike the Halloween storm of 2003, this event is a minor geomagnetic 221

storm with gradual storm commencement and a long duration main phase characterized by a two 222

step response in Dst with minima of 35 nT at 12:00 UT and 56 nT at 2400 UT (panel 3). There is 223

no significant change in the magnitude and direction of ∆H (panel 4) in the local noon period 224

except for the slight reduction in magnitude on the 31st of October. In response to the 225

disturbance, ∆H was negative from about 1200 UT on the 30th

October till about 0500 UT the 226

following day. During this time interval, IMF Bz (panel 1) went on several southward journeys. 227

It nevertheless, remained south from about 1900 to 2200 UT with the associated IEFy (panel 2) 228

westward. The response of the EIA is presented from 1700 to 2000 UT (2000 to 2300 LT) in Fig. 229

13. On 29 and 31 October, both peaks of the EIA are observed from 1700 to 2000 UT. On 30 230

October, they can still be seen at 1700 UT while from 1800 to 2000 UT only a single peak can be 231

seen. 232

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4. Discussion 233

The formation of the crests of the EIA is due to the fountain effect which itself depends on a 234

combination of processes mainly, solar ionization and the ExB drift among other. The time of 235

occurrence of maximum ∆H, a good proxy for ExB, varied on a monthly and seasonal basis 236

(Figs. 7 and 8) and was an important mechanism responsible for the observed variation in the 237

time of occurrence of the noon crests (Figs. 2 and 5). The estimated difference in the time of 238

maximum EEJ as inferred from ∆H and the time of occurrence of the crests of the EIA was 239

found to be about 4 hours during equinoxes and 2-3 hours during solstices. This time delay 240

represents the response time of the EIA to changes in zonal electric field. Aggarwal [14] found 241

that such time varies between 3-4.5 hours over India. The response time is thus, a function of 242

mechanisms such as vertical drift velocity, height of the populated flux tube as well as intensity 243

of meridional winds [14]. Fejer et al [40] showed that the magnitude and time of occurrence of 244

peak drift velocity vary with local time, season and longitude. Zhang et al. [41] observed that the 245

noon peak of the EIA around 1200E longitude formed at about 1300-1400 LT from 1998-2004. 246

The crests were well developed in magnitude and formed farther away from the magnetic 247

equator during equinoxes than winter, then summer especially for the noon period (Figs. 5 and 248

6). Delta H similarly reached peak value during equinoxes, then winter and finally summer 249

(Figure 8). This annual variation in crest magnitude was thus, modulated by similar variation in 250

ExB drift. Sherliess and Fejer [42] similarly explained these seasonal variations in terms of 251

larger daytime ExB drift velocities during equinoctial and winter months as oppose to weaker 252

values during summer months. In addition, the fact that the crests formed farther from the 253

magnetic equator (poleward) during equinoxes and closer (equatorward) during summer and 254

winter is an indication of the contribution of thermospheric neutral winds. According to 255

Dickinson et al [43] and Roble et al [44], solar heating drives a circulation from the equatorial 256

region toward both poles during equinoxes. The poleward winds in conjunction with the stronger 257

equinoxes drift were hence, responsible for the movement of the crests. The significance of 258

winds nonetheless still requires investigation especially over the African sector that has been 259

void of ionospheric observational tool for a long period. 260

On the other hand, during winter the northern crest had higher magnitude than the southern crest 261

while the reverse is the case during summer. This was due to change in composition resulting 262

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from summer to winter winds driven by interhemispherical circulation at solstice. Dickinson et 263

al. [45] had shown that during solstice, solar heating generates a circulation with rising motion 264

over the summer pole and meridional wind in the direction of the winter hemisphere. It is well 265

known that such winds exert a great control on the formation of the crests of the EIA. Since they 266

flow from the summer hemisphere towards the winter hemisphere [44] they can drag ionization 267

along the magnetic field lines thus, increasing or decreasing the rate of diffusion of ionization 268

towards the winter crest or summer crest. As a result, the formation of the summer crest is 269

retarded while the winter crest is enhanced. Huang and Cheng [10] studied cycle variations of the 270

northern EIA crest using TEC from 1985–1994 at Lunping and found that the winter crest 271

appears larger and earlier than the summer crest. Tsai et al. [11] found that both crests of the EIA 272

fully develop around midday in winter, post noon during equinox and late in the afternoon during 273

summer in the Asian sector. 274

The hour by hour analysis of the formation of the crests of the EIA and the EEJ strength revealed 275

that it took 3 hours for both crests to develop after the reversal of current from westward to 276

eastward (Fig. 9). This time delay which often corresponds to the response time of the EIA to 277

transition from counter electrojet (CEJ) to EEJ is consistent with results of Stolle et al [46] 278

obtained over the South America sector. The good correlation between crests magnitude and 279

delta H (Fig. 11a) further highlights the contribution of electric field to the variability of the 280

magnitude of the crests of the African EIA. Stolle et al [45] obtained good correlation between 281

vertical drift velocities, equatorial electrojet strength and EIA parameters over Jicamarca, Peru. 282

In the same vein, Rastogi and Klobuchar [47] found a linear dependence between TEC crest and 283

trough ratio at 14:00 LT and EEJ strength at 11:00 LT from 1975 to 1976 over India. 284

The position of the northern and southern crests is asymmetric with respect to the magnetic 285

equator (Fig. 5). The equatorial fountain effect is not symmetry with respect to the magnetic 286

equator due to trans-equatorial wind blowing from the northern to southern hemisphere and also 287

an asymmetry in ExB drift. As mentioned earlier, this wind can bring more ionization to the 288

southern hemisphere but also push the crest farther from the magnetic equator. Another 289

contributing factor could be the location of the magnetic equator in the northern hemisphere over 290

Africa. Kassa et al. [15] had previously reported the asymmetry in the position of the crests of 291

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the EIA over East Africa during 2012. Le Huy et al. [16] found an asymmetry between the 292

amplitude and the position of the two crests of the EIA over the Asian sector. 293

The post sunset peaks in ionization (Figs. 4 and 6) are brought about by the pre-reversal 294

enhancement (PRE) in eastward electric field. It is important to recall that equatorial electric 295

field during quiet time period is eastward (upward drift) before reversing to westward at about 296

2100 LT. Before the reversal at sunset an increase in electric field occurs. This is the so-called 297

PRE [48]. The increased electric field redistributes ionization near the crests. This additional 298

ionization in conjunction with neutral wind counters the normal decay of ionization thus, 299

producing a second peak or ledge in ionization [49]. The position of the post sunset crests did not 300

vary significantly on a seasonal basis (Fig. 6). However, it was found to be asymmetric with 301

respect to the magnetic equator. This variability is controlled by the variability of the PRE as 302

well as transequatorial wind. 303

During the Halloween storm day, westward electric field associated with the southward turning 304

of IMF Bz as well as increase in convection electric field [50] in the local post sunset to midnight 305

period manifested as negative perturbation in ΔH. Such westward prompt penetration electric 306

field (PPEF) acted to suppress the regular PRE, thereby preventing ionization to be lifted up. As 307

such, both peaks of the EIA could not form in the post sunset period. Generally, during similar 308

electrodynamic conditions (westward electric field), the formation of ionospheric irregularities is 309

usually inhibited during a geomagnetic storm [29]. 310

5. Conclusions 311

In this paper, the variations of the position and magnitude of the crests of the Equatorial 312

Ionization Anomaly over East Africa in the noon and post sunset sector have been investigated 313

using GPS TEC data obtained from a chain of 12 GPS receivers extending from the southern to 314

northern hemisphere during year 2013. 315

(i) The drift as estimated by delta H was found to be a crucial parameter driving the dynamics of 316

the position and magnitude of the crests of the EIA on a monthly and seasonal basis. As such, the 317

northern and southern crests formed within 4.910 to 7.36

0 and -9.17

0 to -12.6

0 dip latitude during 318

noon period; 80

to 11.70 and -9

0 to -16

0 dip latitude during post noon period respectively. 319

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Particularly, variations of the magnitude of the EIA crests were more obvious during post sunset 320

period with the southern crest developing more than the northern crest. 321

(ii) The time response of the formation of the noon crests of the EIA was about 3 hours after the 322

time of occurrence of minimum ΔH. Additionally, the development of the post sunset crests was 323

inhibited during the Halloween day storm of 30th

October 2013 as a result of storm time 324

westward electric field. 325

(iii) The significance of the contribution of plasma drift velocities during noon period was further 326

highlighted with the existence of good correlation between crests position and the EEJ strength. 327

Nevertheless, moderate correlation between crests magnitude and EEJ implied the existence of 328

additional mechanisms mainly transport as well as rate of recombination over both hemispheres. 329

(iv) The poleward and equatorward movement of the crests during equinoxes and winter/summer 330

respectively has not been reported before and as such, underscores the significance of 331

thermospheric meridional wind in modulating the morphology of the African EIA. However, 332

wind pattern as well as transport processes are mechanisms that still require investigation over 333

Africa. 334

Acknowledgements 335

The authors wish to express their gratitude to the following organizations: UNAVCO for the 336

GPS TEC data, the World Data Center for Geomagnetism (WDC) for Dst and SYM-H data, and 337

INTERMAGNET for the magnetometer data. We also acknowledge the Dominion Radio 338

Astrophysical Observatory (DRAO), Penticon for the solar flux data, the ACE SWEPAM team 339

for the ACE data and the GFZ German Research Centre for Geosciences for the international 340

quietest days. The authors thank E. Yizengaw, E. Zesta, M. B. Moldwin and the rest of the 341

AMBER and SAMBA team for their magnetometer data. AMBER is operated by Boston College 342

and funded by NASA and AFOSR. SAMBA is also operated by UCLA and funded by NSF. 343

344

345

346

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Effects on Precise GPS Positioning. Proceeding of the CRCM 93, Kobe. December 1993. 349

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3. W. B. Hanson and R. J. Moffett. J. Geophys. Res. 71, 5559-5572 (1966). 352

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(1986). 354

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8. S. Basu, J.M. Retterer, O. de La Beauhardiere, C.Valladares, and E. Kudeki. Geophys. 358

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10. Y.N. Huang and K.J. Cheng. J. Geophys. Res. 101, 24513 (1996). 361

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106(A12), 30363–30369 (2001). 363

12. C.C. Wu, K. Liu, S.J. Shan, and C.L. Tseng. Adv. Space Res., 41, 611-616 (2008). 364

13. B. Zhao, W. Wan, L. Liu, and Z. Ren. Ann. Geophys. 27, 3861-3873 (2009). 365

14. M. Aggarwal, H.P. Joshi, K.N. Iyer, Y.S. Kwak, J.J. Lee, H. Chandra, and K.S. Cho. 366

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15. T. Kassa, B. Damtie, A. Bires, E. Yizengaw, and P. Cilliers. Adv. Space Res. 55, 184-198 368

(2012). 369

16. M. Le Huy, C. Amory-Mazaudier, R. Fleury, A. Bourdillon, P. Lassudrie-Duchesne, L. 370

Tran Thi, T. Nguyen Chien, T. Nguyen Ha, and P. Vila. Adv. Space Res. 54, 355-368 371

(2014). 372

17. P. Opio, F.M. D’ujanga, and T. Ssenyonga. Adv. Space. Phys. 55, 1640-1650 (2014). 373

18. H. Rishbeth and C.S.G.K. Setty. The layer at sunrise. J. Atmos. Terr. Phys. 20, 263- 374

276 (1961). 375

19. L. Zou, H. Rishbeth, I.C.F. Muller-Wodarg, A.D. Aylward, G.H. Millward, D.W. 376

Idenden, and R.J. Moffett. Ann. Geophys. 18, 927-944 (2000). 377

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20. W.J.J. Ross. Geophys. Res.71, 3671-3676 (1966). 378

21. L. Scherliess, and B.G. Fejer. J. Geophys. Res. 104, 6829–6842 (1999). 379

22. H. Liu, C. Stolle, M. Forster, and S.J. Watanabe. J. Geophys. Res.112, A11311 (2007). 380

23. C.C. Wu, C.D. Fry, J.Y. Liu, K. Liou, and C.L. Tseng. J. Atmos. and Solar. Terr. Phys., 381

66, 199-207 (2004). 382

24. E. Yizengaw, M.B. Moldwin, E. Zesta, C.M. Bioele, B. Damtie, A. Mebrahtu, B. Rabiu, 383

C.F. Valladares, and R. Stoneback. Ann. Geophys. 32, 231-238 (2014). 384

25. Y. Chen, L. Liu, W. Wan, and Z. Ren. J. Geophys. Res., 117, A03313 (2012). 385

26. A.B. Rabiu, A.I, Mamukuyomi, and E.O. Joshua. B. Astron. Soc. India, 35, 607-618 386

(2007). 387

27. D. Anderson, A. Anghel, J. Chau, and O. Veliz. Space Weather, 2, S11001 (2004). 388

28. E. Yizengaw, E.A. Essex, and R. Birsa. Ann. Geophys. 22 (8), 2765–2773 (2004). 389

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32. Hansen, A., J. Blanch, and T. Walter. Ionospheric correction analysis for WAAS 396

Quiet and stormy. ION GPS, Salt Lake City, Utah, 19-22 September 2000. pp. 634-642. 397

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earth’s ionosphere total electron content using the GPS global network. Proceedings of 401

ION GPS-93, Institute of Navigation, 1993. pp. 1323–1332. 402

36. L.H. Estey and C.M. Meertens. GPS Solutions. 3 (1), 42-49 (1999). 403

37. A.O. Adewale, E.O. Oyeyemi, A. Adeloye, C.N. Mitchell, J.A.R. Rose, and P.J. Cilliers. 404

Radio Sci. 47(2), RS004846 (2012). 405

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40. B. G. Fejer, J. W. Jensen, and S.-Y. Su. J. Geophys. Res., 113, A05304 (2008). 409

41. Zhang, M.L., Wan, W., Liu, L., and B. Ning. Adv. Space Res. 43, 1762-1769 (2009). 410

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43. R. E Dickinson, E. C. Ridley, and R. G. Robie. J. atmos. Sci. 32, 1737-1754 (1975). 412

44. R.G. Robel, R.E. Dickinson, and E. C. Ridley. J. Geophys. Res. 82(35), 5493-5504 413

(1977). 414

45. R. E Dickinson, E. C. Ridley, and R. G. Robie. J. atmos. Sci. 34, 178-192 (1976). 415

46. Stolle, C. Manoj, H. Lühr, S. Maus, and P. Alken. J. Geophys. Res. 113, A09310 (2008). 416

47. R.G. Rastogi and J. A. Klobuchar. J. Geophys. Res. 95, 19,045-19,052 (1990). 417

48. B.G. Fejer. J. Atmos. and Solar. Terr. Phys. 53, 677-693 (1991). 418

49. D. Anderson and J. Klobuchar. J. Geophys. Res. 88(A10), 8020-8024 (1983). 419

50. L. Scherliess and B.G. Fejer. J. Geophys. Res. 102 (A11), 24,037 (1997). 420

421

422

Figure caption 423

Fig.1. Location of the stations: the brown solid curve indicates the magnetic equator (dip = 0) 424

while broken curves represents ± 15 degree dip latitudes. 425

Fig.2. Monthly variations of the position and magnitude of the African EIA crests for the year 426

2013. 427

Fig.3. Monthly variations of the magnitude of the north crest (panel 1) and south crest (panel 2) 428

during noon period. 429

Fig.4. Variations of the position of the African EIA crests for 2013 during the post sunset period 430

(≈ 21:00 LT). The double arrow shows the latitudinal extent of variation during this year. 431

Fig.5. Seasonal variations of the EIA for the year 2013. 432

Fig.6. Seasonal variations during the post sunset period (≈ 21:00 LT). The error bars represents 433

the standard deviation used to demarcate the limit of day-to-day variability. 434

Page 15 of 24



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16

Fig.7. Monthly variations of ∆H from January to September 2013 (No data from October to 435

December). The error bars represents the standard deviation used to represent the day-to-day 436

variability. 437

Fig.8. Seasonal variations of ∆H for year 2013. The error bars represents the standard deviation 438

used to demarcate the limit of day-to-day variability. 439

Fig.9. Comparison of EIA and delta H during winter, equinoxes and summer. 440

Fig. 10. Monthly PI index for the year 2013. 441

Fig.11. Correlation between (a) crests magnitude and Delta H, and (b) crests position and Delta 442

H during noon period. 443

Fig.12. Variations of interplanetary and magnetic parameters during from 29 to 31 October 2013. 444

Fig.13. Response of the EIA to the Halloween storm of 30 October 2013. 445

446

447

448

449

450

451

452

453

454

455

456

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17

Figures 457

458

Fig. 1. Location of the stations: the brown solid curve indicates the magnetic equator (dip = 0) 459

while broken curves represents ± 15 degree dip latitudes. 460

461

0

30 E

30 S

0

30 N alwj

namashebasab

adisadig

nege

moiumal2

tucktanz

mbar

nurk

GPS Station

Magnetometer station

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18

462

Fig. 2. Monthly variations of the position and magnitude of the African EIA crests for year 2013. 463

464

Fig. 3. Monthly variations of the magnitude of the north crest (panel 1) and south crest (panel 2) 465

during noon period. 466

January 2013

-20 -15 -10 -5 0 5 10 15 200

4

8

12

16

2024

February 2013

-20 -15 -10 -5 0 5 10 15 200

4

8

12

16

2024

March 2013

-20 -15 -10 -5 0 5 10 15 200

4

8

12

16

2024

April 2013

-20 -15 -10 -5 0 5 10 15 200

4

8

12

16

2024

Lo

ca

l tim

e (

Ho

ur)

July 2013

-20 -15 -10 -5 0 5 10 15 200

4

8

12

16

2024

August 2013

-20 -15 -10 -5 0 5 10 15 200

4

8

12

16

2024

September 2013

-20 -15 -10 -5 0 5 10 15 200

4

8

12

16

2024

October 2013

-20 -15 -10 -5 0 5 10 15 200

4

8

12

16

2024

Dip latitude (degree)

November 2013

-20 -15 -10 -5 0 5 10 15 200

4

8

12

16

2024

December 2013

-20 -15 -10 -5 0 5 10 15 200

4

8

12

16

2024

0

10

20

30

40

50

60

70

May 2013

-20 -15 -10 -5 0 5 10 15 200

4

8

12

16

2024

June 2013

-20 -15 -10 -5 0 5 10 15 200

4

8

12

16

2024

TECu

30

40

50

60

70

802013

No

rth

ern

cre

st

ma

gn

itu

de

(T

EC

u)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec30

40

50

60

70

80

So

uth

ern

cre

st

ma

gn

itu

de

(T

EC

u)

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19

467

Fig. 4. Variations of the position of the African EIA crests for 2013 during the post sunset period 468

(≈ 21:00 LT). The double arrow shows the latitudinal extent of variation during this year. 469

470

471

Fig. 5. Seasonal variations of the EIA for the year 2013. 472

-20 -15 -10 -5 0 5 10 15 205

10

15

20

25

30

35

40

45

50


VT

EC

(T

EC

u)

2013

January

February

March

April

May

June

July

August

September

October

November

December

Lo

ca

l tim

e (

Ho

ur)

Winter

-20 -15 -10 -5 0 5 10 15 200

4

8

12

16

20

24

Dip laitude (degree)

Equinoxes

-20 -15 -10 -5 0 5 10 15 20

Summer

-20 -15 -10 -5 0 5 10 15 200

10

20

30

40

50

60TECu

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20

473

474

Fig. 6. Seasonal variations during the post sunset period (≈ 21:00 LT). The error bars represents 475

the standard deviation used to demarcate the limit of day-to-day variability. 476

477

478

Fig. 7. Monthly variations of ∆H from January to September 2013 (No data from October to 479

December). The error bars represents the standard deviation used to demarcate the limit of day-480

to-day variability. 481

-20 -15 -10 -5 0 5 10 15 200

10

20

30

40

50Winter

TE

C (

TE

Cu

)

-20 -15 -10 5 0 5 10 15 20

Equinoxes

Dip latitude (degree)-20 -15 -10 -5 0 5 10 15 20

Summer

-50

-25

0

25

50

75January 2013

De

lta

H (

nT

)

-50

-25

0

25

50

75February 2013

-50

-25

0

25

50

75March 2013

-50

-25

0

25

50

75April 2013

De

lta

H (

nT

)

-50

-25

0

25

50

75May 2013

-50

-25

0

25

50

75June 2013

0 4 8 12 16 20 24-50

-25

0

25

50

75July 2013

De

lta

H (

nT

)

0 4 8 12 16 20 24-50

-25

0

25

50

75August 2013

Local time (Hour)0 4 8 12 16 20 24

-50

-25

0

25

50

75September 2013

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21

482

483

Fig. 8. Seasonal variations of ∆H for year 2013. The error bars represents the standard deviation 484

used to demarcate the limit of day-to-day variability. 485

486

487

488

489

490

491

492

493

494

495

496

497

498

499

0 5 10 15 20 25-30

-20

-10

0

10

20

30

40

50

Local time (Hour)

De

lta

H (

nT

)

Winter

0 5 10 15 20 25-30

-20

-10

0

10

20

30

40

50

Local time (Hour)

Equinoxes

0 5 10 15 20 25-30

-20

-10

0

10

20

30

40

50

Local time (Hour)

Summer

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22

500

Fig. 9. Comparison of EIA and delta H during winter, equinoxes and summer. 501

502

Fig. 10. Monthly PI index for the year 2013. 503

5

10

15

20Winter

5

10

15

20Equinoxes

5

10

15

20Summer

10

15

20

25

10

15

20

25

10

15

20

25

10

20

30

10

20

30

10

20

30

10

20

30

40

TE

C (

TE

Cu

)

10

20

30

40

10

20

30

40

20

30

40

50

20

30

40

50

20

30

40

50

20304050

20304050

20304050

-20 -15 -10 -5 0 5 10 15 2020304050


-20 -15 -10 -5 0 5 10 15 2020304050


-20 -15 -10 -5 0 5 10 15 2020304050


4 6 8 10 12 14 16 18 20-30

0

30

60

LT (Hours)

De

lta

(H

)

4 6 8 10 12 14 16 18 20-30

0

3060

LT (Hours)

4 6 8 10 12 14 16 18 20-30

03060

LT (Hours)

Hour 9

Hour 8

Hour 10

Hour 11

Hour 12

Hour 13

Hour 14

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec75

100

125

150

175

PI In

de

x

2013

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23

504

505

Fig. 11. Correlation between (a) crests magnitude and Delta H, and (b) crests position and Delta 506

H during noon period. 507

508

35

40

45

50

55

60

65C

rest m

ag

nitu

de

(T

EC

u)

Northern crest

35

40

45

50

55

60

65

Cre

st m

ag

nitu

de

(T

EC

u)

Southern crest

10 15 20 25 30 35 40 45 504

5

6

7

8

9

Delta H (nT)

Cre

st p

ositio

n

(0 N

)

10 15 20 25 30 35 40 45 508

9

10

11

Delta H (nT)

Cre

st p

ositio

n

(0 S

)

0.67

(a)

(b)

0.88

(a)

0.68

(b)

0.81

Page 23 of 24



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24

509

Fig. 12. Variations of interplanetary and magnetic parameters during from 29 to 31 October 510

2013. 511

512

Fig. 13. Response of the EIA to the Halloween storm of 30 October 2013. 513

-15

-10-505

10

IMF

Bz

(n

T)

-5

--2.5

0

2.5

IEF

y

(m

v/m

)

-90

-60

-30

0

Dst

(nT

)

0 6 12 18 24 6 12 18 24 6 12 18 24-80

-40

0

40

80

120

UT = LT -3 (Hour)

De

lta

H (

nT

)

29 / 10 / 2013 31 / 10 / 2013 30 / 10 / 2013

0

20

40

60

8029 October 2013

0

20

40

60

8030 October 2013

0

20

40

60

8031 October 2013

0

20

40

60

0

20

40

60

0

20

40

60

0

20

40

60

VT

EC

(T

EC

u)

0

20

40

60

0

20

40

60

-20 -15 -10 -5 0 5 10 150

20

40

60

Dip latitude (0)

-20 -15 -10 -5 0 5 10 150

20

40

60

Dip latitude (0)

-20 -15 -10 -5 0 5 10 150

20

40

60

Dip latitude (0)

18 UT 18 UT

19 UT

20 UT

19 UT

17 UT

20 UT

19 UT

18 UT

17 UT 17 UT

20 UT

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for review only...for review only 1 1 variability of the african equatorial ionization anomaly (eia)...

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