for review only...for review only 1 1 variability of the african equatorial ionization anomaly (eia)...
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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
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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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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
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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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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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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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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
Dip latitude (degree)
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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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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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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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
Dip latitude (degree)
-20 -15 -10 -5 0 5 10 15 2020304050
Dip latitude (degree)
-20 -15 -10 -5 0 5 10 15 2020304050
Dip latitude (degree)
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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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
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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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