RESPONSE OF GPS-TEC IN THE AFRICAN EQUATORIAL REGION … · Department of Physics, Kebbi State...
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http://www.iaeme.com/IJCIET/index.asp 1773 [email protected]
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
http://www.iaeme.com/IJCIET/index.asp 1774 [email protected]
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
Ikubanni S.O, Adebiyi S.J, Adebesin B.O, Dopamu K.O, Joshua B.W, Bolaji O.S and Adekoya
B.J
http://www.iaeme.com/IJCIET/index.asp 1775 [email protected]
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.
Response of GPS-Tec in the African Equatorial Region to the Two Recent St. Patrick‟s Day
Storms
http://www.iaeme.com/IJCIET/index.asp 1776 [email protected]
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)]
Ikubanni S.O, Adebiyi S.J, Adebesin B.O, Dopamu K.O, Joshua B.W, Bolaji O.S and Adekoya
B.J
http://www.iaeme.com/IJCIET/index.asp 1777 [email protected]
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
Response of GPS-Tec in the African Equatorial Region to the Two Recent St. Patrick‟s Day
Storms
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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.
Ikubanni S.O, Adebiyi S.J, Adebesin B.O, Dopamu K.O, Joshua B.W, Bolaji O.S and Adekoya
B.J
http://www.iaeme.com/IJCIET/index.asp 1779 [email protected]
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.
Response of GPS-Tec in the African Equatorial Region to the Two Recent St. Patrick‟s Day
Storms
http://www.iaeme.com/IJCIET/index.asp 1780 [email protected]
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.
Ikubanni S.O, Adebiyi S.J, Adebesin B.O, Dopamu K.O, Joshua B.W, Bolaji O.S and Adekoya
B.J
http://www.iaeme.com/IJCIET/index.asp 1781 [email protected]
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.
Response of GPS-Tec in the African Equatorial Region to the Two Recent St. Patrick‟s Day
Storms
http://www.iaeme.com/IJCIET/index.asp 1782 [email protected]
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.
Ikubanni S.O, Adebiyi S.J, Adebesin B.O, Dopamu K.O, Joshua B.W, Bolaji O.S and Adekoya
B.J
http://www.iaeme.com/IJCIET/index.asp 1783 [email protected]
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.
Response of GPS-Tec in the African Equatorial Region to the Two Recent St. Patrick‟s Day
Storms
http://www.iaeme.com/IJCIET/index.asp 1784 [email protected]
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
Ikubanni S.O, Adebiyi S.J, Adebesin B.O, Dopamu K.O, Joshua B.W, Bolaji O.S and Adekoya
B.J
http://www.iaeme.com/IJCIET/index.asp 1785 [email protected]
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.
Response of GPS-Tec in the African Equatorial Region to the Two Recent St. Patrick‟s Day
Storms
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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
Ikubanni S.O, Adebiyi S.J, Adebesin B.O, Dopamu K.O, Joshua B.W, Bolaji O.S and Adekoya
B.J
http://www.iaeme.com/IJCIET/index.asp 1787 [email protected]
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
Response of GPS-Tec in the African Equatorial Region to the Two Recent St. Patrick‟s Day
Storms
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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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