RESPONSE OF GPS-TEC IN THE AFRICAN EQUATORIAL REGION … · Department of Physics, Kebbi State University of Science and Technology, Aliero, Kebbi State, Nigeria ... there is significant

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International Journal of Civil Engineering and Technology (IJCIET)

Volume 9, Issue 10, October 2018, pp. 1773–1790, Article ID: IJCIET_09_10_177

Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=9&IType=10

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication Scopus Indexed

RESPONSE OF GPS-TEC IN THE AFRICAN

EQUATORIAL REGION TO THE TWO RECENT

ST. PATRICK’S DAY STORMS

Ikubanni S.O.,Adebiyi S.J.,Adebesin B.O and Dopamu K.O

Department of Physical Sciences, Space Weather Group, Landmark University,

P.M.B. 1001, Omu-Aran, Kwara State, Nigeria

Joshua B.W

Department of Physics, Kebbi State University of Science and Technology,

Aliero, Kebbi State, Nigeria

Bolaji O.S

Department of Physics, University of Lagos, Nigeria

Department of Physics, University of Tasmania, Hobart, Australia

Adekoya B.J

Department of Physics, Olabisi Onabanjo University, Ago-Iwoye, Ogun State, Nigeria

ABSTRACT

The 2015 St. Patrick’s Day storm is one of the most intense geomagnetic storm in

this present solar cycle (SYM-H = -213nT). In this paper, we investigate the response

of the African low latitude ionosphere to this storm event using the Total Electron

Content (TEC) derived from four Global Positioning System (GPS) measurements in

the region. The responses were also compared with that of 2013 (SYM-H = -132nT).

The results obtained show that the deviation of TEC magnitude from quiet-time

average behavior is generally is larger in 2015 than 2013. The effect is much larger

during the recovery than the main phase for both storm events. In 2013, considerable

TEC enhancement ( for at least 3 consecutive hours) marks the

minimum depression of SYM-H at Libreville and the prenoon periods of the first day

of the recovery phase at MAL2 and ZAMB. Also, positive phases dominates at the

equatorial stations while negative phases dominates at the low-latitude station during

the 2013 storm recovery phase, suggesting the suppression of Equatorial Ionization

Anomaly (EIA) by inhibiting prereversal enhancement vertical ion drift. In 2015,

depletion marks the minimum downward excursion period while enhancement marks

the prenoon. Further depletion marks the postnoon and postsunset periods of the first

recovery day at the equatorial stations in 2015.For other recovery days, negative

Response of GPS-Tec in the African Equatorial Region to the Two Recent St. Patrick‟s Day

Storms


storm phases dominates low latitude (MBAR and MAL2), extending for about 36

hours, particularly around the midday, post-sunset and midnight. Comparison with

other works shows distinct responses at different sectors.

Keyword: Low-latitude ionosphere, Ionospheric storms, GPS- TEC, African sector

Cite this Article: Ikubanni S.O, Adebiyi S.J, Adebesin B.O, Dopamu K.O, Joshua

B.W, Bolaji O.S and Adekoya B.J, Response of Gps-Tec in the African Equatorial

Region to the Two Recent St. Patrick‟s Day Storms, International Journal of Civil

Engineering and Technology, 9(10), 2018, pp. 1773–1790.

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=9&IType=101.0

1. INTRODUCTION

Ionospheric storm, which is one of the consequences of space weather event, is a signature of

geomagnetic disturbance on ionospheric electron density. Geomagnetic disturbances result

from input of highly variable energy into the Earth‟s magnetosphere and they have adverse

effects on the operations of space-based technologies. One of the resulting geomagnetic

activities on the ionosphere is rapid variation in ionospheric electron content. In other words,

there is significant modification of ionospheric electron density away from the average quiet

behaviors. The variation, which can either be in form of enhancement or depletion of electron

density, is often referred to as positive and negative storm effects respectively.

The 17th March, 2015 geomagnetic storm that occurred on St. Patrick ‟s Day(thus

referred to as the St. Patrick‟s Day storm) reached the G4 level and was categorized as

“severe” storm (http://www.sswpc.noaa.gov/NOAAscales/). It is the most intense

geomagnetic storm event of this present solar cycle (i.e. solar cycle 24) (Astafyeva et al.,

2015), whose severity was underestimated by space weather forecasters around the world

(Jacobsen and Andalsvik, 2016). This is due to the relative weakness of the associated flares

and the perpendicular direction of the driving Corona Mass Ejections (CMEs) from the Sun

(Liu et al. 2015). Some of its attendance terrestrial effects are reported in several studies. For

example, Jacobsen and Andalsvik (2016) reported a severe scintillation in Global Navigation

Satellite System (GNSS) signals (measured as rate of Total Electron Content (TEC) index,

ROTI) in the middle latitude during the main and early part of the recovery phases. ROTI,

which is the measure of the time rate of differential phase of dual frequency Global

Positioning System (GPS) signals, is measured in 1 TEC unit per minute (TECU/min) (Pi et

al. 1997). Also, Astafyeva et al. (2015) conducted a multi-instrumental study of the global

ionospheric response to the 2015 St. Patrick‟s Day disturbance, the first of its kind for a

geomagnetic storm event of such magnitude in the 24th solar cycle. They reported that the

low latitude regions experienced the most dramatic positive effects in the morning and during

the post-sunset hours, particularly in the American and Eastern Pacific sectors. They also

reported hemispheric asymmetries at the mid-latitude for different sectors and recovery

phase-induced negative storm effect at high latitudes in all sectors. Sripathi et al. (2015)

documented observations over the Indian sector, which include positive ionospheric storm

phase during the main phase (MP), storm day suppression of anomaly crest, an intense

equatorial spread F irregularities, and substantial westward electric field orientation.

TEC enhancement or depletion during main phases of geomagnetic storm could be due to

dominating role of the Prompt Penetration Electric Fields (PPEF) or O/N2 increase and the

time of occurrence or location of the station while negative storm effects during recovery

phase could be due to poleward meridional wind disturbance, Disturbance Dynamo Electric

Fields (DDEF) and O/N2 decrease (Yue et al. 2016).The simulated PPEF and DDEF with

http://www.sswpc.noaa.gov/NOAAscales/

Ikubanni S.O, Adebiyi S.J, Adebesin B.O, Dopamu K.O, Joshua B.W, Bolaji O.S and Adekoya

B.J


other observational data (Nava et al. 2016) showed that magnitude of positive ionospheric

storm is a function of the local time of the station under observation with respect to the

commencement of the storm phase while the negative phase is influenced by decreasing O/N2

ratio and DDEF. Hairston et al. (2016)‟s analysis of data from the Coupled Ion Neutral

Dynamics Investigation (CINDI) onboard the Communications/Navigation Outage

Forecasting System (C/NOFS) spacecraft around the storm period revealed the absence of

prereversal enhancement (PRE) around the equator on the storm day, which was present

during each of the previous four days; suggesting the role of the DDEF . Since DDEF causes

a daytime westward enhancement and nighttime eastward enhancement in the zonal electric

field, the effect negates the quiet time ion drift pattern (Fuller-Rowell et al. 2008).Huang et

al. (2016) reported a 31-hour duration of strong DDEF starting from shortly after the MP till

about 20 hours into the recovery phase and coinciding with an approximately 31-hour long

large changes in the vertical ion drift. They also reported the variations of O+

density and

concentration as well as H+

density, where O+

concentration increased during storm-time by

over 30%. Storm-time O+

concentration increase was also observed to start around 4.7 hours

after the storm MP and lasted about 31 hours.

Kalita et al. (2016) investigated the effects of both storms under consideration in the

equatorial and low latitude for two different longitudinal sectors, about 13o apart. There was

inhibition of PRE leading to rapid TEC fluctuation in the 100oE sector low-latitude and a

saturated TEC at the EIA crest of the 77oE sector due to PRE effects. They also observe a

weakened EIA on 18 March 2013 and it absence on 18 March 2015 (marked by positive and

negative ionospheric storm effects at the equatorial and low latitude respectively) during the

onset of the recovery phase. Likewise, negative ionospheric storm observed in the low

latitude, which was stronger in 2015 than 2013 recover phase, was attributed to both

electrodynamics (in form of EIA inhibition) and thermospheric effects (in form of decreasing

O/N2 ratio). Kuai et al. (2016) identified the role of PPEF in the response of low latitude and

equatorial ionosphere of the American sector and Asia-Australia sector during the storm‟s

MP. The effect of the strong PPEF, leading to EIA intensification, is higher over the

American sector. The effects long lasting DDEF that dominates during the recovery phase, is

higher in the Asian-Australian sector. Among other observations, Ray et al. (2017)

documented TEC variations over India during the March 2015 storm. There was TEC

enhancements along the 77oE longitudinal sector around local morning time on 17 March

because of the prevailing action of PPEF, causing increase in electron density in the

equatorial/low latitude region. Zhong et al. (2016) reported a persistent TEC depletion

(lasting beyond 3 days) during the recovery phase at most sectors, with the exception being

the Pacific Ocean region. This depletion was observed to be greater around the local midnight

hours. TEC pattern during both main and recovery phases vary with altitude while the TEC

recovery rate has strong longitudinal dependence. Nava et al. (2016) reported TEC increase

during MP particularly at the EIA crests and TEC decrease that is stronger at middle and high

latitudes than equatorial latitudes in all sectors (American – 70oW, African – 10

oE and Asian

– 110oE) during the March 2015 storm. They however reported that there were substantial

differences in ionospheric response to the March 2015 storm at three different sectors. These

differences included very large TEC increase and a distinct EIA crest in the American sector;

very large TEC decrease in the Asian sector that begins towards the end of the MP and

extends from the high to low latitude, except the equatorial zone; a large depletion of the

American southern hemisphere during the recovery phase (18 – 20 March); well-defined EIA

crest in Asian and occasional formation of two crests in the African and American sectors;

and longer recovery phase at middle and high latitudes in Asian sector compared to other

sectors.


Storms


The 17th March 2013 St. Patrick storm has similar characteristics with that of the 2015.

For example, there were enhancements during the initial and main phases; but a long-lasting

(>17 hours) depletion in the Asian sector during the recovery phase as a result of pole-ward

meridional wind disturbances, disturbance dynamo electric field and decrease in O/N2 (Yue

et al. 2016). They both occurred on the same day of the month and almost the same time of

MP Onset (MPO). Although, they both have different degree of severity and also occurred at

different year, however the average annual radio solar flux of 10.7cm (F10.7) do not vary

much significantly (123 sfu in 2013 and 118 sfu in 2015).Rodriguez-Bouza et al. (2016)

reported similar ionospheric characteristics during the two events; a positive ionospheric

storm in the MP and a negative ionospheric storm in the recovery phase, with higher

magnitude and duration in 2015 than 2013 during the negative episodes, but higher

magnitude only in 2015 during the positive episodes. The negative ionospheric storm effects

over the Asian sector during the recovery phase of the March 2013 geomagnetic disturbance,

lasting over 17 hours, was reported to be stronger in the southern hemisphere (Yue et al.

2016). This hemispheric asymmetry coincides with hemispheric asymmetry in O/N2

depletion caused by the displacement between the geomagnetic and geographical poles.

Storms of similar characteristics give a good opportunity to critically examine the changes in

geospace and may provide information that are still relevant to better understand the storm

time driving mechanisms particularly in areas where ionospheric dynamics are most

prominent. Thus, in this investigation, we compared the ionospheric responses at stations

located in the African low latitude region to these storm events. Although several storm time

investigations have been conducted in the region, however, there are very limited works from

the African sector that investigate these two storms and their geospace effects. This is the

main intent for this work. To achieve this, section 2 presents the data and method of analysis,

the results were presented in section3. Chapter 4 discusses the results. The summary and

conclusion of the findings were highlighted in section 5.

2. DATA AND METHODOLOGY

In this paper, we have employed the Total Electron Content (TEC) data derived from GPS

measurements in our investigation. The GPS measurements are from four International GNSS

Service (IGS) receivers located in the south of the magnetic equator in the African sector.

The raw GPS measurements, stored in Receiver Independent Exchange (RINEX) files

format, were obtained from the archive of the Scripps Orbit and Permanent Array Center

(SOPAC) (ftp://garner.uscd.edu). Fig. 1 shows the locations of these receivers and the detail

geophysical information of these stations are also shown in Table 1.

Figure 1 Locations of the station used [Magnetic equator (middle black line), Magnetic EIA crest (red

lines)]

ftp://garner.uscd.edu/


B.J


Table 1 List of Stations used for this work

Location Station

ID Country

Geographic Geomagnetic LT

Latitude Longitude Latitude Longitude

Libreville NKLG Gabon 0.35o N 9.67

o E 8.05

o S 81.05

o E UT+1hr

Mbarara MBAR Uganda 0.60o S 30.74

o E 10.25

o S 102.36

o E UT+2hr

Malindi MAL2 Kenya 2.70o S 40.19

o E 12.10

o S 111.87

o E UT+3hr

Lusaka ZAMB Zambia 15.43o S 28.31

o E 26.28

o S 98.40

o E UT+2hr

The geomagnetic coordinates were obtained using the online calculator of the UK Solar

System Data Centre (UKSSDC) (http://www.ukssdc.ac.uk/cgi-bin/wdcc1/coordcnv.pl,

October 9, 2016). The Vertical TEC (VTEC) was obtained from the raw GPS data using the

GPS-TEC retrieval analysis software (version 2.9.3). The software was developed by Gopi

Krishna Seemala and the description is documented in GPS-TEC analysis applications user‟s

manual, 2009, Institute for Scientific Research, Boston College, Chestnut Hill,

Massachusetts. The software and other necessary information are available from

http://seemala.blogspot.com/.Other works to have employed this software include We have

included the satellite and receiver bias files obtained from Center for Orbit Determination in

Europe (CODE) (ftp://ftp.unibe.ch/aiub/CODE/) as additional files input into the software to

remove the hardware biases. To eliminate the error due to multipath, only satellite

observations whose elevation angle is greater than 30o were used for the investigation. In this

article, we henceforth refer to the processed VTEC simply as TEC. For quantitative

estimation of the storm effect, the deviation in the TEC value due to storm effect expressed in

percentage (% ) is obtained using the expression in equation 1:

(

)

where is the storm time TEC values while is the quiet time

average TEC values.

The quiet time average TEC values enable us to remove the quiet-time background

variation and it values represent the average of five most quiet days of the month of interest.

The quiet days were selected based on the information from the German Research Centre for

Geosciences database (http://www.gfz-potsdam.de/en/section/earths-magnetic-field/data-

products-services/kp-index/qd-days/qd-days-since-2010/). Several works had also adopted

the average of the five or ten quietest days of the month (Bolaji et al. 2013, Adebiyi et al.

2014 and Kuai et al. 2016). While the changes in the yearly average of F10.7 is a signature of

changing solar activity, its effect on ionospheric response to geomagnetic storm is yet to be

established. As recently suggested, neither F10.7 nor its derivatives track additional

radiations due to geomagnetic disturbance (Ikubanni and Adeniyi, 2017).This is validated by

the work of Nava et al. (2016), where F10.7 was observed to be constant despite the large

increases in Global Electron Content (GEC) between 14 and 21 March 2015. From equation

(1), positive value of implies enhancement of TEC while negative values indicates

depletion. In this study, the effect of the storm activity on the ionosphere is considered to be

strong if the value of exceeds ±25% and the disturbance last for a minimum period

of 3 hours, if otherwise, it is considered. Cander (2016) also employed ±25% as the reference

value for strong storm effect, since quiet time ionosphere has also shown day-to-day variation

from monthly average.

For quantitative description of these storm events, we have employed various

interplanetary and geomagnetic parameters. The interplanetary parameters include the

http://seemala.blogspot.com/

ftp://ftp.unibe.ch/aiub/CODE/

http://www.gfz-potsdam.de/en/section/earths-magnetic-field/data-products-services/kp-index/qd-days/qd-days-since-2010/

http://www.gfz-potsdam.de/en/section/earths-magnetic-field/data-products-services/kp-index/qd-days/qd-days-since-2010/


Storms


Interplanetary Magnetic Field, IMF-Bz (measured in nanotesla, nT) henceforth referred to as

Bz, the solar wind speed, v (measured in kilometers per second, km/s), proton density

(measured in Newton per cubed centimeter, N/cm3), and plasma temperature (measured in

Kelvin, K). The geomagnetic parameters are the Auroral Electrojet, AE (measured in

nanotesla, nT), and the symmetric disturbance index in the horizontal direction of the

magnetic field, SYM-H (measured in nanotesla, nT). SYM-H, a high resolution (1-minute)

widely used index for monitoring geomagnetic disturbances in the low and equatorial

latitudes, represents the magnetic field perturbations that originates from magnetospheric

currents, magnetopause, tail and ring currents (Wanliss and Showalter, 2006). These were

retrieved from the National Space Science Data Centre website

(http://nssdcgsfc.nana.gov/omniweb).

3. RESULTS

3.1. The geomagnetic disturbance of March 17, 2013

Figure 2 presents the solar and interplanetary parameters for the March 2013 event. The IMF-

Bz exhibits no significant northward orientation prior to the Storm Sudden Commencement

(SSC) (marked by the red dashed line) on 17 March around 0600 UT, where SYM-H index

reached a magnitude of 33nT. However, the IMF-Bz turned southward immediately after

the SSC and reached a peak magnitude of -14nT around 0900 UT and a corresponding

injection of ring current into the Earth‟s magnetosphere, causing a depression in SYM-H to

-104nT. Although, the Bz magnitude decreases, it remained southward and attained a new

peak ( -12nT) around 1800 UT while the SYM-H plot showed a sustained injection of ring

current until a minimum value ( -132nT) is reached around 2028 UT on 17 March, marking

the MP of the storm. The recovery phase began immediately and was complete by 0950 UT

on 18 March. As widely reported, this geomagnetic storm falls in the “intense” category “-

250nT minimum Dst -100nT” according to the classifications by Adebesin (2008).

The solar wind speed (V) and proton density (p+

density) revealed a sharp rise in

magnitude which coincided with the SSC, but no prior northward turning of IMF-Bz before

the rapid increase southward IMF-Bz magnitude. This suggests that the SSC is not driven by

interplanetary shock, rather by interaction of relatively slow solar wind and high speed

streams (Park et al. 2015).

A speed of 400 km/s and above is known to be sufficient to trigger shock on the Earth‟s

magnetosphere, but not the only condition. The speed, V, and p+

density increase from (414

km/s and 3.8 N/cm3) around 0500 UT on 17 March to (721 km/s around 1000 UT and 14.2

N/cm3 around 0800 UT) respectively on same day at SSC, while there is an abrupt rise in

temperature from 62,000 K to 530,000 K that coincides with the beginning of the SYM-H

depression. The plasma beta and plasma temperature plots show low values, implying that the

storm is driven by Interplanetary Coronal Mass Ejection (ICME) of magnetic cloud origin.

http://nssdcgsfc.nana.gov/omniweb


B.J


Figure 2 Solar, interplanetary and geomagnetic observations for March 16 – 20, 2013 storm (Day 075

– 079, 2013)

3.2. Ionospheric response to the March 17, 2013 storm

The response of the ionosphere to the storm is presented as a superposed plot of the quiet

time and storm time TEC morphology for the four stations (Figure 3). TEC deviation from

the quiet-time values is noticeable during the entire period of study; that is from the pre-storm

day (March 16, 2013) to the recovery phase. A consistent observation is that TEC magnitude

reached a maximum around the universal post-noon period for each day. However, the TEC

storm time variation is consistently more observable around and after the universal noontime,

which coincides with period of high AE and extended SYM-H depression in the MP as well

as afternoon/PRE period on other days. The magnitude of both quiet and storm time TEC

appears comparable at NKLG, MBAR, and MAL2 while there is about 28% reduction in the

peak magnitude at ZAMB. This distinct difference in the magnitude between observations at

NKLG/MBAR/MAL2 and ZAMB is not unexpected. While NKLG, MBAR and MAL2 are

stations in the equatorial ionization anomaly (EIA) region, ZAMB falls outside in the low

mid-latitude region. The higher ionization processes and density in the EIA region may have

been responsible for the larger TEC magnitude observed during both quiet and disturbed

periods.


Storms


Figure 3 Morphology plot of observed GPS-TEC quiet values (red dashed line) and disturbed values

(blue solid line) at three low latitude stations: Libreville (NKLG), Mbarara (MBAR), Malindi

(MAL2); and one mid-latitude station – Lusaka (ZAMB).

Figure 4 shows the quantitative deviation of storm-time from the quiet-time TEC

( ) observations due to the magnetic disturbance of March 2013. A 24-hour pre-storm

observation (i.e. 16 March) did not reflect any substantial storm effect. The deviations from

the average seen could be attributed to day-to-day deviations from quiet time average caused

by other background dynamical processes. The only pre-storm exception is at NKLG, where

substantial post-midnight TEC enhancement was observed. It should be noted that the pre-

storm event is produced under relatively calm ring current and Aurora Electrojet activity

marked by the near zero SYM-H and very low AE-index values. On 17 March, the SSC and

MPO did not lead to substantial storm effects on TEC at all four locations. Another TEC

enhancement, which coincides with the minimum SYM-H depression, was also observed at

NKLG. During the rapid stage of the recovery phase (between the minimum SYM-H

depression on 17th and 0800 UT on the 18th), there was earlier (0100 – 0300 UT) TEC

depletion (-38 to -61%) at NKLG and later (0400 – 0700 UT) enhancement (~ 33 to 40%) at

MAL2. Outside the EIA region (at ZAMB), the enhancement during this time is considered

not substantial. As the recovery phase continued on the 18th, data gaps at NKLG will not

allow for comprehensive observations, however, there was considerable but very weak

enhancements at MBAR between 2300 UT on 18th to 0300 UT on 19th. Likewise,

enhancement (48 to 78%) was observed at NKLG between 0100 and 0400 UT on 19th.

Generally during the recovery phase (19 – 20 March), enhancements mark the post-midnight

observations, which is greatest at NKLG (with an average of 70%), than at MBAR and

MAL2 (with averages 31 and 29% respectively) on 19th. Post sunset observations is marked

by negative phases, which is observable only at ZAMB between -33 and -46% (with an

average of ~ -39%) and between -29 and -54 (with an average of ~ -40%) for five hours

(1900 – 2300 UT) on 19th and 20th respectively.


B.J


Figure 4 Percentage deviation in TEC at NKLG, MBAR, MAL2, and ZAMB during the March 2013

geomagnetic storm. The dashed and dotted lines mark the +20% enhancement and -20% depletion

threshold values respectively.

3.3. The geomagnetic disturbance of March 17, 2015

Figure 5 shows the solar and interplanetary responses for the March 2015 event. The IMF-Bz

exhibits northward orientation to 20nT around SSC period (0447 UT) on 17 March (marked

by the red dashed line), where SYM-H index reached a magnitude of 60nT. However, the

IMF-Bz turned southward immediately after the SSC and reached a peak magnitude of -16nT

around 0800 UT on 17 March and a corresponding injection of ring current into the Earth‟s

magnetosphere, causing a downward depression in SYM-H to -101nT around 0937 UT on 17

March. The IMF-Bz turned northward for about 2 hours and reached a magnitude of 9nT. It

then overturned in the southward direction until it reached a new magnitude (-11nT) around

1300 UT and stabilizes at this magnitude till about 2300 UT, reaching a peak (-18nT) around

1400 UT(the duration of the southward IMF-Bz>-10nT is about 11 hours). The SYM-H plot

showed a sustained intensification of ring current until a minimum SYM-H value (-230nT) is

reached around 2300 UT on 17 March, marking the MP of the storm. The extended recovery

phase began immediately and was complete around 1500 UT on 19 March. As widely

reported, this geomagnetic storm falls in the “intense” category “-250nT minimum SYM-

H -100nT” according to classifications by Adebesin (2008). Generally, intense storms with

minimum SYM-H < -100nT is expected to have threshold values of southward IMF-

Bz>10nT with at least 3 hours duration (Gonzalez and Tsurutani, 1987; Adebesin, 2008). The

two SYM-H minimum peaks observed coincided with periods of significant southward Bz

turning.


Storms


Figure 5 olar, interplanetary and geomagnetic observations for March 16 – 20, 2015 storm (Day 075

– 079, 2015)

The solar wind speed (V) and proton density (p+

density) show sharp rise in values which

coincided with the SSC and the sharp northward Bz turning, indicative of the arrival of shock

in the interplanetary medium. A speed of 400 km/s and above can trigger shock on the

Earth‟s magnetosphere; although, there is gradual increase in plasma speed, V, prior to the

storm day. At SSC, the speed, V, and p+

density increase from (410 km/s and 15.9 N/cm3)

around 0400 UT on 17 March to (609 km/s around 1100 UT and 38.5 N/cm3around 0500 UT)

respectively on same day, while there is an abrupt rise in temperature from 38,000K

to111,000K around 0400 UT that coincided with the beginning of the SYM-H depression and

further rise to around 912,000K around 1100 UT on 17 March. This was followed by a rapid

decrease in temperature to around 53,000K three hours later and the temperature remained

low till the period of SYM-H depression. The plasma beta plot shows low values for the

entire duration of the storm MP. A coincidental high plasma temperature and low plasma beta

values indicate the presence of sheath in driving the storm, while a depressed plasma

temperature when the IMF is still southward indicates a geo-effective ICME. Therefore, the

storm can be classified as a sheat-ejecta driven type.

3.4. Ionospheric response to the March 17, 2015 storm

The response of the ionosphere to the storm is presented as a superposed plot of the quiet-

and storm-time TEC morphology for three EIA region stations (Figure 6); there was no data

at ZAMB during the periods of interest. TEC deviation from the quiet-time values is

noticeable during the entire period of study; from the pre-storm day (March 16, 2015) to the

recovery phase. A consistent observation is the attainment of maximum TEC magnitude

around the universal post-noon period for each day. However, the TEC storm time variation

is consistently more observable around and after the universal noontime. Further, the

magnitude of both quiet and storm time TEC is comparable around NKLG, MBAR, and

MAL2, particularly in the morning periods.


B.J


Figure 6 Morphology plot of observed GPS-TEC quiet values (red dashed line) and disturbed values

(blue solid line) at three low latitude stations (NKLG, MBAR, MAL2) [Data is not available at

ZAMB during this time].

Figure 7 Percentage deviation in TEC at NKLG, MBAR, and MAL2 during the March 2015

geomagnetic storm (There was no data at ZAMB). The dashed and dotted line mark the +25%

enhancement and -25% depletion threshold values respectively.


Storms


Figure 7 highlights the quantitative deviation of the storm-time from quiet-time TEC

( ) due to the magnetic disturbance of March 2015. A 24-hour pre-storm observation

(i.e. 16 March) reveals a negative phase at all stations between 0400 and 0900 UT, although

not substantial at NKLG. This pre-storm event shows small latitudinal variation. It should be

noted that the pre-storm event is produced under relatively calm ring current and Aurora

Electrojet activity marked by the near zero SYM-H and very low (< 300 nT) AE-index

values. On 17 March, there was no considerable effect of the storm during SSC and MPO.

Considerable depletion coincided with the minimum SYM-H, which coincided with post-

sunset/midnight hours. The depletion rangesfrom-43 to -67% between 2200 UT on 17th and

0100 UT on 18th at NKLG, from -33 to -57%and from -31 and -69% between 2100 UT on

17th and 0200 UT on 18th at MBAR and MAL2 respectively. The rapid stage of the recovery

phase of the storm on the 18th is marked by sharp change in TEC response, where rapid

enhancement (barely 2 hours after the depletion) reached peak values of 117% and 91% at

MBAR and MAL2 respectively by 0400 UT. A less pronounced enhancement, which peaked

(~34%) at 0800 UT (7 hours after depletion) was observed at NKLG. The magnitude of the

enhancement decreased gradually until depletion was again observed between 1300 and 1500

UT, and further depletion between 2000 UT/1900 UT (on 18th) and 0500 UT/0300 UT (on

19th) at MBAR and MAL2 respectively. Generally, the effect at NKLG is different from

those of the other two stations during the recovery phase on the 18th. The general trend at

MBAR and MAL2 is a pre-noon positive phase (average of 5 hours), a short post-noon

negative phase (average of 3 hours), and a longer negative phase (extending from 1800 UT

on 18th to several days after the storm). The negative phase is more substantial around post-

sunset periods for all days of the recovery phase. However, the largest depletion is observed

around the post-sunset/midnight of 18th March. Observations on 20 March at NKLG and for

the entire duration of the period of study at ZAMB were not presented due to lack of data at

these stations on these dates. Generally, the effect of the March 2015 storm on the low-

latitude ionosphere is more substantial during the recovery than the MP.

4. DISCUSSION

Our analysis attributed the source of the 2013 storm to ICME of magnetic cloud origin and

the 2015 storm to ICME driven by sheath. However, double-step geomagnetic storms, such

as the 2015 St. Patrick‟s Day storm, is suggested to be initiated by complex ejecta resulting

from interactions between two or more CMEs (Liu et al. 2014) and very geo-effective as a

result of their prolonged duration (Lugaz and Farrugia 2014; Mishra et al. 2015). Liu et al.

(2015) classified the drivers of 2015 storm under consideration in this paper as sheat-ejecta-

ejecta leading to a double-step geomagnetic storms resulting from interactions between two

CMEs; and a subsequent prolonged and larger effect as seen during the recovery phase.

The low-latitude/equatorial ionosphere variations during quiet time have been discussed

in relation to the electrodynamics that interplays in this region (e.g. Ikubanni et al. 2013,

2014). These include the electric fields due to the equatorial electrojet, neutral wind, and

thermospheric circulation. During the disturbance of the magnetosphere due to the impact of

solar events such as solar storm, the electrodynamics is modified causing the deviation of the

ionosphere from its quiet time average. These electrodynamics such as the electric fields may

be modified (enhanced or reversed) by either external electric fields of direct magnetospheric

origin – PPEF (Kelley et al., 1979; Fejer and Scherliess, 1997) or of indirect magnetospheric

sources – DDEF – which causes a reduction in eastward electric fields at equatorial latitudes

(Mendillo, 2006) as well as from neutral winds (Blanc and Richmond, 1980; Fejer and

Scherliess, 1995; Abdu, 1997).Wang et al. (2010) showed that enhanced daytime eastward

electric field in the initial phase due to PPEF can cause negative storm effects around the


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equator and a resulting positive storm effects at low and middle latitudes. Likewise, modified

thermospheric circulation leads to changes in thermospheric composition such as enhanced

O/N2, which may lead to prolonged positive phase, or reduced O/N2, which may drive

negative storm effects (Mendillo, 2006).

For the 2013 storm, the SSC and MPO had no considerable effect on TEC at all four

locations. Likewise, the minimum SYM-H did not leave a consistent signature on TEC at the

three equatorial stations. The different TEC patterns coincide with the time of minimum

SYM-H, which corresponds to the local post-sunset periods when PRE vertical drift modifies

the electron density profile. TEC behavior may be explained on the basis of the action of the

disturbed electric fields. During daytime, zonal electric field is eastward and due to the

release of additional energy, the electric field is enhanced during the storm MP. This should

cause the depletion around the equator and enhancement at low latitude. However, the

reverse was observed; rather than depletion at the equator, there was enhancement and there

was no considerable effect at the low and mid-latitude. This could be due to the inhibition of

the PRE as suggested by Kalita et al. (2016).This post-midnight depletion at NKLG followed

by some insubstantial enhancement during the early stage of the recovery phase in the

equatorial/low latitude can be attributed to DDEF, which suppresses or opposes the solar

quiet (Sq) electric field, thereby suppressing the EIA structure. This is similar to observations

in Asian sector, where weakened EIA was reported (Kalita et al. 2016). Also, midnight/post-

midnight enhancements after more than 24 hours into the recovery phase can be attributed to

transportation of neutral species from the high latitudes leading to atomic oxygen (O)

enrichment at low/equatorial latitudes (Mendillo, 2006). There were post-midnight

enhancements at NKLG, MBAR, and MAL2 while there was post-sunset depletion at

ZAMB. The post-sunset depletion/post-midnight enhancement during this period can be

attributed to the changes in thermospheric composition, presented as O enrichment at low and

equatorial latitudes and N2 enrichment at mid-latitude. ZAMB is closer to the lower boundary

of the mid-latitude (+30o

geomagnetic latitude).However, the thermospheric O/N2 map

(Figure 8) did not show any substantial difference within the African sector.Hence, as

suggested by Kalita et al. (2016), the negative phase is attributable to both the

electrodynamics and thermospheric effects.

Figure 8 GUVI images of the thermospheric O/N2 ratio map during 17 – 20 March 2013.


Storms


For the 2015 storm, there was no considerable response of the equatorial TEC to the

interplanetary shock between the time of SSC and the beginning of the minimum depression.

This could be attributed to the interaction between PPEF and DDEF. For example, PPEF is

expected to enhance PRE vertical drift on the storm day (17 March 2015), but that was not

observed, probably due to DDEF as suggested by Hairston et al. (2016). Huang et al. (2016)

reported evidence of increased upward vertical ion drift at 0245 LT on storm day (that is,

shortly before SSC); though the vertical ion drift still had upward orientation, the velocity

had decreased by 50ms-1

at 0430 LT (~ 1.75 hours later), but had turned downward with a

magnitude of ~80ms-1

between 0630 and 0730 LT. They explained that PPEF should have

caused significant downward vertical ion drift in the midnight-dawn sector during the first

and the second intensifications, but this did not occur due to the contributions of both PPEF

and DDEF to the vertical ion drift by the beginning of the storm and the dominance of DDEF

over PPEF several hours into the MP. This PPEF–DDEF interaction could be responsible for

the inconsiderable ionospheric storm effect between 0430 and 2000 UT before the minimum

depression and the considerable depletions during the minimum SYM-H depression around

midnight (Figure 7). The minimum SYM-H depression corresponds with the beginning of

considerable negative ionospheric storm, which lasted for about 5 hours. Likewise, DDEF

was suggested to influence the equatorial ionospheric response to the storm as early as the

end of the MP but the effect was masked by the substantial PPEF. The DDEF effect was not

visible until about 24 hours after the MPO (Tulasi Ram et al. 2016; Nayak et al. 2016).

Comparing our results with those of Kuai et al. (2016) reveals significant longitudinal

variation in low latitude ionospheric response. For example, Kuai et al. (2016) identified

stronger effect of PPEF over the American sector during MP and stronger effect of DDEF

over the Asian-Australian sector during recovery phase, while this work suggests that the

effect of PPEF was cancelled out by DDEF during MP over the African sector.

The onset of the recovery phase of the 2015 storm is marked by TEC enhancement, which

lasted for about 8 hours and whose magnitude increases away from the equator. The short-

lived TEC enhancement at this stage of the storm recovery may be interpreted as a function

of the effect of DDEF on the zonal electric field. During quiet time, post-midnight equatorial

is expected to experience downward ion drift leading to higher plasma density at the equator

than the low latitude. However, the observation suggests that DDEF reverses the downward

ion drift, leading to decrease around the equator and increase towards the low latitude. Huang

et al. (2016) had shown that the short-lived early morning enhancement occurred shortly after

DDEF caused enhanced upward drift of O+

to higher altitude. They identified that persistent

neutral disturbance winds generated during the storm MP, which continues to drive

significant ionospheric and magnetospheric field-aligned currents at high latitudes and

dynamo electric fields at middle and low latitudes, termed the “flywheel effect”, as the main

cause of long-lasting dynamo process during storm recovery phase. The “flywheel effect”,

which causes vertical ion drift and O+

concentration enhancement that lasted about 20 hours

after the storm MP (traversing the between 2300 UT on 17 March 2015 and 1900 UT on 18

March 2015) (Huang et al. 2016),The role of electrodynamics and thermospheric effects in

the ionospheric response during recovery phase were also identified over the Asian sector

(Kalita et al. 2016). A large decrease in O/N2 ratio extending to the low latitude was reported

for 18 March 2015 between the 100o – 130

oE, causing reduction in electron density

(Astafyeva et al. 2015). The GUVI image of the recovery phase (18 – 20 March 2015)

revealed thermospheric O/N2 variation due to the storm (Figure 9). Unlike the southern

hemisphere in the Asian sector where there is substantial decrease in O/N2 ratio over the

southern hemispheric low latitude on 18 March, there is very little change over southern

African low latitude region. While O/N2 ratio continues to increase substantially over the


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south Asian low latitude sector, the O/N2 differences between the three recovery days (18 –

20 March) over the African low latitude sector is small. Hence, the south Asian low latitude

ionosphere will be seriously affected by O/N2 depletion, compared to that of the African

sector.

Figure 9 GUVI images of the thermospheric O/N2 ratio map during 17 – 20 March 2015.

5. SUMMARY AND CONCLUSION

Two intense storms in the solar cycle 24 have been studied. These are the 2013 and 2015

storms which occurred on St. Patrick‟s Day. Of interest to this study is the identical nature of

the time of occurrence of the storm, and of different magnitudes and slightly different origins.

Our analysis of the solar and geomagnetic parameters showed that while the 2013 storm is as

a result of interaction of slow solar wind with high speed streams and followed by ICME of

magnetic cloud origin causing an intense storm of minimum SYM-H depression of -132nT,

the 2015 intense storm with a minimum SYM-H depression of -230nT is driven by sheath-

ejecta preceded by interplanetary shock. Our analysis showed smaller response in 2013 than

2015 both during the main and the recovery phases, particularly in the equatorial stations.

This may be due to the extent of transport of energy from the high latitude, which is expected

to be larger in 2015 than 2013 storm.

In 2013, there was no considerable effect during the MP, suggesting that the influence of

PPEF is masked. Oscillating positive/negative episodes highlight the earlier stage of recovery

phase while positive/negative effects at equatorial/low-latitude respectively highlight

observations more than 24 hours after the storm MP. In later periods of the recovery phase,

the only considerable enhancement in the EIA region was at NKLG where there were

considerable post-midnight and post-sunset enhancements and simultaneous considerable

post-sunset depletions at MAL2and ZAMB. This signifies PRE inhibition. Generally, a long-

lasting negative storm effects dominates outside the EIA. Within the EIA, there were only

small packets of significant storm effects; while there were considerable nighttime negative

storm effects in the region of transition from low to mid latitude. In 2015, there was no

considerable effect in the EIA region during the larger part of the storm MP except around

the minimum SYM-H where negative ionospheric phase corresponding to local midnight is

observed at all latitudes. The earlier stage of the recovery phase is marked by morning time

positive phase (lasting about 5 hours) and followed by a dominating negative phase between


Storms


1200 UT on 18th March and 2300 UT on 20th March (lasting about 36 hours) at the low

latitude stations of MBAR and MAL2 only. The negative phase is considerable around noon

and post-sunset/post-midnight.

Despite the similar day, time, and season of occurrence of the two storms, as well as the

days falling in the same solar flux level classification, there are distinct responses of the

equatorial ionosphere to the two storms, particularly during the recovery phase. This suggests

that storm origin and intensity do play important role in the modification of the

electrodynamics that affects the equatorial ionosphere during disturbed periods. The effect of

the substantial deviations in the ionosphere during storm time is becoming increasingly

important due to the increasing dependence on space-borne technologies. These deviations, if

not well understood and the effects mitigated, may have profound effects on applications

such as positioning, aviation, navigation, high frequency communication, and other

dependent services. Particularly, for most African countries, potential socio-economic loss

can be enormous during a certain extreme geomagnetic storm event. Recently, Geomagnetic

Induced Currents (GICs), which can cause massive damage to electricity and communication

infrastructure, was reported to be likely in the African low latitude during a severe storm

event (Adebesin et al. 2016).

ACKNOWLEDGEMENT

The authors sincerely appreciate the providers of the online solar and interplanetary data

(retrieved fromhttp://nssdcgsfc.nasa.gov/omniweb) as well as the International GNSS

Services (IGS) for the free availability of the data used for this work. The solar radio flux

(F10.7) data was retrieved from the archive of measurements of solar radio flux on Natural

Resource Canada website (26 March 2017), and can be found at

(ftp.geolab.nrcan.gc.ca/data/solar_flux/monthly_averages/solflux_monthly_average.txt).

The thermospheric O/N2 ratio presented are derived from GUVI instrument onboard the

National Space Administration (NASA)‟s Thermosphere Ionosphere Mesosphere Energy and

Dynamics (TIMED) satellite. The GUVI instrument is a product of The Aerospace

Corporation and The John Hopkins University.

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RESPONSE OF GPS-TEC IN THE AFRICAN EQUATORIAL REGION … · Department of Physics, Kebbi State University of Science and Technology, Aliero, Kebbi State, Nigeria ... there is significant