ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-35-39-2017Induction effects of geomagnetic disturbances in the geo-electric field
variations at low latitudesDoumbiaVafivafid@yahoo.frBokaKouadioKouassiNguessanGrodjiOswald Didier FranckAmory-MazaudierChristinehttps://orcid.org/0000-0002-5961-6331MenvielleMichelLaboratoire de Physique de l'Atmosphère, Université Felix
Houphouet Boigny, Abidjan, Côte d'IvoireLPP, CNRS, Ecole polytechnique, UPMC Univ Paris 06, Univ.
Paris-Sud, Observatoire de Paris, Université Paris-Saclay, Sorbonne
Universités, PSL Research University, 4 place Jussieu, 75252 Paris,
FranceInternational Center for Theoretical Physics, Trieste, ItalyUniversité Versailles St-Quentin; CNRS/INSU, LATMOS-IPSL, 4
Avenue de Neptune, 94107 Saint-Maur-des-Fossés, FranceVafi Doumbia (vafid@yahoo.fr)4January201735139511August201623October201622November2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://angeo.copernicus.org/articles/35/39/2017/angeo-35-39-2017.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/35/39/2017/angeo-35-39-2017.pdf
In this study we examined the influences of geomagnetic activity on the Earth
surface electric field variations at low latitudes. During the International
Equatorial Electrojet Year (IEEY) various experiments were performed along 5∘ W in West Africa from 1992 to 1995. Among other
instruments, 10 stations equipped with magnetometers and telluric electric
field lines operated along a meridian chain across the geomagnetic dip
equator from November 1992 to December 1994. In the present work, the induced
effects of space-weather-related geomagnetic disturbances in the equatorial
electrojet (EEJ) influence area in West Africa were examined. For that purpose, variations in
the north–south (Ex) and east–west
(Ey) components of telluric electric field were
analyzed, along with that of the three components (H,D and Z) of the geomagnetic field during the geomagnetic storm of 17 February 1993 and the
solar flare observed on 4 April 1993. The most important induction effects
during these events are associated with brisk impulses like storm sudden
commencement (ssc) and solar flare effect (sfe) in the geomagnetic field
variations. For the moderate geomagnetic storm that occurred on 17 February
1993, with a minimum Dst index of -110 nT, the geo-electric field
responses to the impulse around 11:00 LT at LAM are
Ex= 520 mV km-1 and
Ey= 400 mV km-1. The geo-electric field responses to the sfe that
occurred around 14:30 LT on 4 April 1993 are clearly observed at different
stations as well. At LAM the crest-to-crest amplitude of the geo-electric
field components associated with the sfe are Ex= 550 mV km-1 and
Ey= 340 mV km-1. Note that the sfe impact on the geo-electric field
variations decreases with the increasing distance of the stations from the
subsolar point, which is located at about 5.13∘ N on 4 April. This
trend does not reflect the sfe increasing amplitude near the dip equator due
the high Cowling conductivity in the EEJ belt.
Geomagnetism and paleomagnetism (geomagnetic induction)Introduction
Intense space weather events like geomagnetic storms and substorms are
potential sources of electric induction within the earth. These events cause
intense geomagnetic field variations which are expected to induce electric
fields and currents in the conducting layers of the Earth lithosphere.
Disruptions of technological systems dues to the “geomagnetically induced
currents (GICs)” have been known in the Scandinavian countries since the
mid-19th century (Pulkkinen, 2003). Due to these negative impacts on
technological devices, the GICs have been mostly investigated at high
latitudes (Pulkkinen et al., 2001, 2003a, b, 2007; Pirjola, 2000, 2002, 2005; Wik et al., 2008, 2009). Magnetosphere–ionosphere coupling through
geomagnetic field lines generates intense currents such as auroral
electrojets in the high-latitude ionosphere. These currents are extremely
enhanced during geomagnetic storms and substorms and cause very intense
geomagnetic field variations. As consequence, intense GICs cause
disturbances in technological devices like telecommunication and pipe lines,
power grids, and transformers.
Kappenman (2003) demonstrated the risk of large GIC occurrences associated
with large geomagnetic impulses like storm sudden commencement (ssc) at low-
and midlatitudes. There are reports of GIC causing perturbation in
technological structures in the mid- and low latitudes (Ngwira et al., 2008;
Torta et al., 2012; Trivedi et al., 2007). In addition to the global-scale
geomagnetic disturbances due to currents in the magnetosphere, it is now well
established that disturbances that originate from the high-latitude
ionospheric currents extend toward mid- and low latitudes during geomagnetic
storms. The effects of electric field prompt penetrations and the disturbance
dynamo (Blanc and Richmond, 1980; Fambitakoye et al., 1990; Zaka et al.,
2010) as well as the mechanisms of different processes have been thoroughly
investigated. However, only little research work has been devoted to the
induction effects of space-weather-related geomagnetic field disturbances at
low latitudes. Most studies on the issue have mainly focused on magnetically
quiet-time induction effects of the equatorial electrojet (Fambitakoye,
1973; Ducruix et al., 1977). Nevertheless, to a large extent, most of those
studies have concluded that there is a very weak induction effect of the equatorial electrojet (EEJ). However,
measurements of GICs in electric power transformers at low latitudes in
Brazil were analyzed by Trivedi et al. (2007). For the geomagnetic storm
period of 7 to 10 November 2004, they reported values of the GIC that
attain 15A. In a recent study, Ngwira et al. (2013) investigated the
global behavior of the horizontal geomagnetic field and the induced
geo-electric field fluctuations during extreme geomagnetic events. Among
other things, the latitude threshold boundary was examined. From this
study, Ngwira et al. (2013) found the largest perturbations in the geomagnetic
and geo-electric fields at high latitudes and an important enhancement in the
EEJ influence area. Carter et al. (2015) also analyzed potential induction
effects of the so-called interplanetary shock events (ssc and solar flare effect, sfe) on
the basis of the time derivatives of geomagnetic field variations. They
emphasized potential threats of important GICs during these shock events near
the geomagnetic equator due to enhancements of geomagnetic disturbances
caused by the EEJ in this area.
The present work aims at examining the induced effects of space-weather-related geomagnetic disturbances in the EEJ influence area in West Africa.
For that purpose, variations in the north–south (Ex) and east–west (Ey)
components of telluric (geo-electric) field are analyzed, along with that of
the three components (H,D and Z) of the geomagnetic field during the
geomagnetic storm of 17 February 1993 and the solar flare on 4 April 1993.
The West African network of 10 stations for the geomagnetic and
telluric field measurements during the International Equatorial Electrojet
Year (IEEY).
Data and analysis
In the framework of the International Equatorial Electrojet
Year (IEEY; 1992–1995), many different instruments were
deployed in different longitude sectors (Arora et al., 1993; Amory-Mazaudier et
al., 1993). On that occasion, variations in the geomagnetic field and
various parameters of the low-latitude ionosphere were monitored with a view
to deepening our knowledge on equatorial and low-latitude electromagnetic
phenomena and underlying physical processes. Specifically, in West Africa, a
network of three ionosondes, a Fabry–Pérot interferometer (Vila et al.,
1998) and an HF radar (Blanc and Houngninou, 1998) were set up. In addition to
those instruments, a meridian chain of 10 magnetic and telluric stations
was deployed across the geomagnetic dip equator, from Lamto (Côte d'Ivoire,
-6.30∘ dip latitudes) to Tombouctou (Mali, +6.76∘
dip latitudes), along the 5∘ W meridian. Figure 1 shows the IEEY
network of magnetic and telluric stations in West African (Amory-Mazaudier
et al., 1993; Doumouya et al., 1998; and Vassal et al., 1998). The
coordinates of the stations are given in Table 1. These stations collected
variations in the horizontal northward (H), eastward (D) and vertical
(Z) components of the geomagnetic field, as well as the north–south (Ex)
and east–west (Ey) components of the geo-electric field variations from
November 1992 to December 1994 (Doumouya et al., 1998 and Vassal et al.,
1998). The H and D components were measured with suspended magnet
variometers, and the Z component was recorded with a fluxgate
magnetometer. The Ex and Ey components of the geo-electric field were
measured using electric lines of 200 m. The measurements of all
components were performed at a sampling rate of 1 min.
Geographic coordinates of the magnetic stations installed
along the meridian 5∘ W in West Africa during the International
Equatorial Electrojet Year.
The Dst index from 16 to 18 February 1993. An ssc at 03:00 LT
starts the process of a moderate geomagnetic storm on 17 February 1993 with a daily Am = 64 nT.
At 11:00 LT another brisk increase started. During the main phase
of the storm, the Dst minimum value is -110 nT around
16:00 LT. The Dst data were copied from the website http://wdc.kugi.kyoto-u.ac.jp/dst_final/index.html.
As quiet-time geomagnetic field variations have been used to study the EEJ
(Doumouya et al., 1998), geomagnetically quiet-time variations in the
geo-electric field were dedicated to investigating induction effects of the
EEJ within the Earth. The first results based on the analyses of geomagnetic
field and geo-electric field observations have been published (Vassal et
al., 1998). The purpose of the present study is to analyze the effects of
geomagnetic disturbances in the geo-electric field variations at low
latitudes. Specifically, we examine the effects of the phases of a geomagnetic
storm and that of an sfe in the geo-electric field
variations in West Africa. To that end, data recorded on 17 February and on
4 April 1993 are considered. On 17 February 1993 a moderate geomagnetic
storm occurred with a daily mean value of the Am index of 64 nT. In addition
the effects of the sfe that occurred around 14:30 LT on 4 April 1993 are examined.
Dst index (a), variations in H(b), D(c) and Z(d)
components, and the time derivatives dH/dt(e), dD/dt(f) and dZ/dt(g) on 16–18 February 1993 at SIK (0.12∘ dip lat).
Geomagnetic field variations during the 17 February 1993 magnetic
storm
Figure 2 shows the Dst index from 16 to 18 February 1993. On 17 February,
a sharp increase in Dst index was observed at 03:00 LT, indicating the ssc, which suggests that the geomagnetic
storm process has started. Then an impulsive increase in Dst followed at 11:00 LT.
During the main phase of the storm the minimum value of the Dst is about
-110 nT around 16:00 LT. In the storm period, data were available at LAM, NIE,
SIK, KOU, SAN, MOP and TOM, but for this study we analyze data from LAM
(south), NIE and SIK (near the dip equator), and SAN and TOM
(north).
Geomagnetic field variations during the 17 February 1993 storm.
Variations in the H (panel a), D (panel b) and Z (panel c) component and its time derivative dZ/dt at LAM, NIE,
SIK, SAN and TOM.
The geomagnetic disturbance effects observed on 16–18 February 1993 at
SIK (0.12∘ dip lat). Dst index (a), variations in
H(b), the time derivatives dH/dt(c), dD/dt(d)
and dZ/dt(e), total variations in the Ex(f) and Ey(g) component of the geo-electric
field, and the geomagnetic disturbance effects Exd(h) and Eyd(i) in the
geo-electric field.
Geo-electric field variations during the 17 February 1993 storm.
The top panels show the time derivatives of H (left) and D (right) and
the other panels show variations in the Ex (left column) and Ey (right
column) at LAM, NIE, SIK, SAN and TOM.
As the time derivative dB/dt of the magnetic field is a potential
indicator of induction current occurrence, we first analyze the time
derivatives dH/dt, dD/dt and dZ/dt. These time derivatives are
calculated from 1 min data of the H, D and Z components. Figure 3
shows the Dst index and variations in H, D and Z components of the
geomagnetic field and their time derivatives dH/dt, dD/dt and dZ/dt at
SIK from 16 to 18 February 1993. On 17 February, disturbances associated
with the geomagnetic storm are observed from 03:00 LT to about 18:00 LT. At
03:00 LT, the ssc manifests itself in the sharp increase in H. The amplitude of
this ssc is about 35 nT in the H component, with a time derivative
dH/dt= 10 nT min-1. A secondary impulse observed at 11:00 LT causes an
increase of about 120 nT in the H component with a crest-to-crest
amplitude of dH/dt= 40 nT min-1. The signatures of the ssc and that
the daytime impulse are observed in D, Z, dD/dt and dZ/dt as well. Due
to the location of SIK near the dip equator, variations in Z and dZ/dt
are weak. Further fluctuations are observed during all the disturbance
periods. However the amplitudes of these fluctuations, including the main
phase of the storm, are weaker than the effects of the brisk impulse at
11:00 LT.
(a) Geo-electric field variations on 17 February 1993. The top
panels show the time derivatives of H (left) and D (right) and the other
panels show variations in the Ex (left column) and Ey (right column) at
LAM, NIE, SIK, SAN and TOM. The geomagnetic field disturbance effects are superimposed on the daily regular variation in Ex and Ey during the
geomagnetic storm. (b) Same as Fig. 7a, with the daily variations removed from Ex and
Ey.
Figure 4a, b and c present, respectively, variations in H, D and Z
components and their time derivatives dH/dt, dD/dt and dZ/dt at LAM,
NIE, SIK, SAN and TOM. The amplitude of ssc effects during the night does
not change much from one station to another. But the daytime impulse reflects
the influence of the equatorial electrojet in the H and Z components. In
the H component, the daytime impulse effects amplify as we get close to
the dip equator. In the Z component the impulse effects decrease as we get
close to the dip equator and reverse from Southern Hemisphere (positive) to
the Northern Hemisphere (negative). Its largest amplitude is observed at SAN
near the northern edge of the EEJ current sheet. For the D component,
negative impulses are observed and the amplitudes of these disturbances
increase more and more as we progress from south (LAM) to north (TOM).
Geo-electric field variations in response to the 17 February 1993
geomagnetic storm
Figure 5 shows the Dst index and variations in H, time derivatives
dH/dt, dD/dt, and dZ/dt and Ex and Ey components of the
geo-electric field at SIK from 16 to 18 February 1993. Ex (Fig. 5f) and Ey
(Fig. 5g) show daily variations (quasi 24 h period regular background signals
of opposite phases). While Ex decreases toward a minimum around noon, Ey
increases toward a maximum. Note that these daily variations are not
analyzed in the present study. The quiet period daily variations in Ex and
Ey are analyzed in another paper that is in preparation for a close future
submission. It appears that Ex and Ey are affected by the rapid
variations observed in geomagnetic field components.
The daily variations in Ex and Ey have been removed in order to better
highlight the geo-electric field responses Exd (Fig. 5h) and Eyd (Fig. 5i) to the
geomagnetic storm. Exd and Eyd show similar periodic variations to the
geomagnetic field time derivatives. In particular during the geomagnetic
storm on 17 February 1993, Exd and Eyd exhibit sharp impulses as
observed in the geomagnetic field time derivatives during the rapid phase of
geomagnetic field variation. The signatures of the ssc around 03:00 LT and
the brisk impulse at 11:00 LT in the geo-electric field components reflect
that of the geomagnetic field time derivatives, in such a way that Exd and
dD/dt exhibit a similar pattern and sign; their effect in Eyd and dH/dt
is also similar but with opposite signs. The brisk variations at 11:00
cause variations in the geo-electric field with crest-to-crest amplitudes of
about 20 mV km-1 in Exd and 120 mV km-1 in Eyd. Note that the effects of the
geomagnetic field brisk variations are observed in all the stations of the
network. Their latitudinal dependence will be examined in Sect. 3.
Geo-electric field variations in responses to the 4 April 1993 solar
flare effect
Figure 6 shows the variations in H and D, the time derivatives dH/dt
and dD/dt at KOU and the Ex and Ey components of the geo-electric
field at LAM, KOR, MOP and TOM on 4 April 1993. Around 14:30 LT, an sfe is
observed in the H component with a sharp impulse. The induction effects of
the brisk variation due to this sfe on the time derivatives
dH/dt and dD/dt of the magnetic field components and the Ex and
Ey components of the geo-electric field are observed at different stations. The
geo-electric field variations are particularly amplified at LAM, where the
crest-to-crest amplitude of Ex and Ey is, respectively, about 550 and 340 mV km-1. The sfe effects in Ey decrease from LAM (south) to TOM
(north); for Ex the sfe effects decrease from LAM to MOP and increase
at TOM. This latitudinal trend of the sfe signature in the geo-electric
field variations likely reflects the influence of the subsolar point
location at 5.13∘ N, which is about 1.10∘ south of LAM
(6.23∘ N). However, it is to be noted that the lateral
resistivity may cause non-negligible changes from one place to another.
After the sfe, fluctuations of the H and D components are observed during
the evening. These fluctuations also produce geo-electric field variations
that are weaker than the sfe effects.
On the dependence of the geo-electric field intensity on the dip latitude
The time derivatives dH/dt and dD/dt at SIK and the components Ex and
Ey of the geo-electric field observed on 17 February 1993 at LAM, NIE,
SIK, SAN and TOM are shown in Fig. 7a and b. In Fig. 7a, the
geo-electric field components Ex and Ey exhibit clear diurnal background
variations. The shapes and amplitudes of the Ex and Ey diurnal
variations change from one station to another. Their amplitudes are clearly
pronounced near the geomagnetic dip equator (NIE, SIK and SAN). As mentioned
in Sect. 2.2, the diurnal variations in Ex and Ey will be deeply
analyzed in an upcoming paper. After removing the daily background
variations, the fluctuations of Ex and Ey due to the geomagnetic field
brisk disturbances are shown in Fig. 7b. The signatures of the geomagnetic
field disturbances are observed in the two components of the geo-electric
field at the selected stations with a similar periodic pattern for respective
components. However the amplitudes of the storm effects in Ex and Ey change
drastically from one station to another. The responses of
geo-electric field to the ssc of 03:00 LT at LAM are Ex=-270 mV km-1 and
Ey=-150 mV km-1. For the impulse at 11:00 LT, the geo-electric field responses
attain crest-to-crest amplitudes of Ex=520 mV km-1 and Ey= 400 mV km-1 at
LAM. At TOM, the ssc effects at 03:00 LT are Ex=30 mV km-1 and
Ey=10 mV km-1; at 11:00 LT, Ex=95 mV km-1 and Ey=8 mV km-1. At NIE, the ssc
effects at 03:00 LT are Ex=30 mV km-1 and Ey=8 mV km-1; at 11:00 LT, Ex=70 mV km-1
and Ey=19 mV km-1. At SAN, the ssc effects at 03:00 LT are that Ex and
Ey are very weak; at 11:00 LT, Ex=10 mV km-1 and Ey=14 mV km-1. Finally at
SIK, close the EEJ center, the ssc effects at 03:00 LT are Ex=8 mV km-1 and
Ey=30 mV km-1; at 11:00 LT, Ex=20 mV km-1 and Ey=120 mV km-1.
It is to be noted that the most important impulses in the geo-electric
field variations are associated with brisk variations in the geomagnetic
field. Indeed, in addition to the ssc effects, the impulses between about
03:00 to 14:00 LT correspond to the brisk variations in the time
derivatives dD/dt and dH/dt in the same time interval. During the main
phase of the storm, additional fluctuations of weaker intensity are also
observed in the time interval from 15:00 to 16:00 LT.
Discussion
The present study analyzes geomagnetic field and geo-electric field
variations observed in West Africa during the IEEY campaign. The
observations demonstrate that intense space weather events are potential
sources of electric inductions within the Earth at low latitudes. The ssc
and sfe are intense and rapid geomagnetic field variations in time. The
effectiveness of the induction effects of these geomagnetic field variations
at low latitudes is clearly shown through the geo-electric field
observations. Furthermore the most intense induction effects are likely
associated with these brisk impulses in the geomagnetic field variations as
shown through our observations. Indeed, the moderate geomagnetic storm that
occurred on 17 February 1993, with the minimum Dst index of -110 nT,
produced non-negligible geo-electric field variations during the ssc
phase. At LAM, the geo-electric field responses to the daytime impulse
components are Ex= 520 mV km-1 and Ey= 400 mV km-1. On the other hand, the
geo-electric field variations associated with the sfe of 4 April 1993
also show special amplifications at LAM, where the crest-to-crest amplitude
of Ex and Ey is, respectively, about 550 and 340 mV km-1. These
observations clearly confirm the possibility of non-negligible GIC at low
latitudes, in case of severe space weather events, as demonstrated by
Kappenman (2003), Ngwira et al. (2013) and Carter et al. (2015).
Given the latitudinal dependence of the time derivatives of the geomagnetic
field, the daytime eastward induction effects should be expected to increase
as we get close to the geomagnetic dip equator. The observations show that
the magnitudes of the geo-electric field response to the geomagnetic
disturbances depend on the observational locations. However this dependence
does not show any special latitudinal trend for the 17 February geomagnetic
storm. The amplitudes of the geo-electric field variations at LAM and TOM
are totally different, the amplitudes at LAM are significantly more elevated
than that at TOM. The drastic change in the storm time geo-electric field
amplitude from one site to another may be related to the lateral variations in the Earth resistivity (Vassal et al., 1998). For the sfe effects, the
amplitudes tend to decrease from south to north for the two components with
the strongest amplitude at LAM and weakest amplitudes of Ey at TOM and
Ex at MOP. Note that the amplitude of Ex is higher at TOM than at MOP.
The latitudinal trend of the sfe signature in the geo-electric field
variations may be related to the location of the subsolar point, which is
located on the southern side of the chain of stations (at about
5.13∘ N on 4 April). This trend does not show any special
increase in the geo-electric field amplitude near the dip equator. A priori,
this observation contrasts with the latitudinal behavior of the sfe, which
increases when we get close to the dip equator (Rastogi et al., 1999), as
for most of the geomagnetic field disturbances. This increase is due to the
high electrical conductivity (Cowling conductivity) in the EEJ belt.
Concerning the latitudinal trend of the sfe signature in the geo-electric
field variations, the difference in the lateral resistivity from one
location to another may also contribute. Different lateral resistivity
possibly underlies the fact that Ex has a higher amplitude at TOM than at
MOP and also that the amplitudes of Ex and Ey are considerably more
elevated at LAM than elsewhere for the sfe signature as well as for
that of the ssc. Vassal et al. (1998) analyzed the lateral variations in
the Earth resistivity from LAM to TOM. They considered two models of
stratified Earth corresponding to the average resistive structure of the two
tectonic provinces across the area of concern: a sedimentary basin in the
north and a cratonic shield in the south. The apparent resistivity computed
according to those models was found to be stronger in the cratonic shield than
in the sedimentary basin. However the real structure of the Earth resistivity in
the area may be different from this simple scenario.
It is evident that the level of geomagnetically induced effects at low
latitudes, in terms of amplitude, cannot be compared with that at high
latitudes (Ngwira et al., 2013). However, the present study, based on a
moderate geomagnetic storm and solar flare, confirms that there exists a
risk of non-negligible GICs in conductive media at low latitudes during the
brisk phases of space weather events like sscs and sfes, as noticed by
Kappenman (2003), Ngwira et al. (2013) and Carter et al. (2015). Indeed, in
the light of the high amplitudes of the geo-electric field variations at LAM, the threat of intense geomagnetically induced current can be important for
highly conductive systems that are located in the vicinity of the EEJ. In
addition, the latitudinal variations in the geomagnetic field time
derivatives confirm the possibility of a potentially strong induction effect
due the EEJ near the geomagnetic dip equator.
Although the analyses of the diurnal variation in the geo-electric field
were not the purpose of this work, the observations clearly exhibit daily
variations in Ex and Ey, which can have different behavior from one
station to another. The amplitudes of the daily variations in Ex and
Ey are particularly enhanced at SIK (0.12∘ N dip lat), the
closest station of the network to the geomagnetic dip equator in 1993.
Special attention is given to this aspect in an upcoming work that will be
focused on large geomagnetic quiet-time data.
Data availability
The Dst index data were downloaded from the website of the WDC for Geomagnetism (2017),
Kyoto Dst index service:
http://wdc.kugi.kyoto-u.ac.jp/dst_final/index.html.
The geomagnetic and geo-electric field data belong to the IRD (Institut de
Recherche pour le Developpement) and to the IPGP (France) and Universite de
Cocody (Cote d'Ivoire). These research institutions collaborated in the IEEY
program in West Africa. The data are public. A CD copy of data can be obtained
if required by contacting the corresponding author (vafid@yahoo.fr).
Acknowledgements
The records of geomagnetic field and the geo-electric field variations were
created by the French research institutions IRD (Institut de Recherche pour
le Developpement) and IPGP (Institut de Physique du Globe de Paris), in
collaboration with Université de Cocody (Côte d'Ivoire) during the
International Equatorial Electrojet Year (IEEY).
The topical editor, E. Yizengaw, thanks two anonymous referees for help in evaluating this paper.
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