JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, A09309, doi:10.1029/2007JA012308, 2007
Simulation of equatorial electrojet magnetic effects with the
thermosphere-ionosphere-electrodynamics general circulation model
V. Doumbia,1,2 A. Maute,3 and A. D. Richmond3
Received 29 January 2007; revised 9 April 2007; accepted 21 May 2007; published 28 September 2007.
[1] In this work, the magnetic variations simulated by the NCAR thermosphere-
ionosphere-electrodynamics general circulation model (TIE-GCM) in the vicinity of the
magnetic equator are examined to evaluate the ability of this model to reproduce the
major features of the equatorial electrojet (EEJ) as observed on the ground as well as on
board low-altitude orbiting satellites. The TIE-GCM simulates electric currents of
various origins and reproduces their associated magnetic perturbations. We analyze the
diurnal and latitudinal variations of the EEJ magnetic effects calculated on the ground in
West Africa under approximately the same solar activity condition as in 1993 for the
March equinox and June and December solstices. The latitudinal and local time structures
of these simulated results correspond well to those that are observed. We also compare
longitudinal variations of the midday EEJ magnetic perturbations observed by the
CHAMP satellite with the model predictions. Although the simulations and observations
both show multiple maxima and minima in longitude, the locations of these extrema often
disagree. In the model most of the longitudinal variation of the magnetic variations
is associated with nondipolar structure of the geomagnetic field. We find that the modeled
contributions of the thermospheric migrating diurnal and semidiurnal tides to the
magnetic perturbations have large longitudinal variations, and we suggest that an increase
in the amplitude of these tides in the TIE-GCM may cause them to play a major role in
explaining the morphology of the EEJ longitudinal variation.
Citation: Doumbia, V., A. Maute, and A. D. Richmond (2007), Simulation of equatorial electrojet magnetic effects with the
thermosphere-ionosphere-electrodynamics general circulation model, J. Geophys. Res., 112, A09309, doi:10.1029/2007JA012308.
1. Introduction
[2] The equatorial electrojet (EEJ), a dayside current
sheet flowing eastward along the geomagnetic dip equator
at 105 km altitude, is associated with the regular dynamo
process in the low-latitude ionosphere. This current system
has been widely investigated through its magnetic signatures recorded along station chains across the dip equator
[Forbush and Casaverde, 1961; Fambitakoye and Mayaud,
1976; Hesse, 1982], and on board low-altitude orbiting
satellites [Cain and Sweeney, 1973; Onwumechili and
Agu, 1980; Agu and Onwumechili, 1981]. Thus most of
its spatial and temporal features were set forth on the basis
of ground-based and satellite magnetic measurements. During the International Equatorial Electrojet Year (IEEY), a
worldwide campaign of simultaneous measurements
[Amory-Mazaudier et al., 1993; Arora et al., 1993; Rigoti
et al., 1999] allowed to update those features [Doumouya et
1
Laboratoire de Physique de l’Atmosphère, Université de Cocody,
Abidjan, Ivory Coast.
2
Temporarily at High Altitude Observatory, National Center for
Atmospheric Research, Boulder, Colorado, USA.
3
High Altitude Observatory, National Center for Atmospheric Research,
Boulder, Colorado, USA.
Copyright 2007 by the American Geophysical Union.
0148-0227/07/2007JA012308
al., 1998], and to undertake a global study of the EEJ using
ground-based magnetic data [Doumouya et al., 2003]. The
advent of Oersted, CHAMP and SAC-C satellites makes it
possible to examine the longitudinal variation and other
global characteristics of the EEJ through the magnetic
records at various longitudes and local times [Jadhav et
al., 2002; Lühr et al., 2003; Doumouya and Cohen, 2004].
Although those studies have yielded increased progress in
understanding the EEJ phenomenon, most of its characteristics are still poorly interpreted. In effect, the day to day,
seasonal, and solar cycle variability, the reversal known as
counter-electrojet, and the unexpected behaviors in its
longitudinal variation in certain longitude sectors, need
more objective explanations. Furthermore, the question of
how the circuit of the EEJ closes, or how it is coupled with
large-scale current systems like the planetary Sq system or
the meridional current evidenced by Maeda et al. [1982] in
Magsat dusk data, are not yet resolved.
[3] The main purpose of this work is to research the
causes of some of the above EEJ features with the help of
the National Center for Atmospheric Research (NCAR)
thermosphere-ionosphere-electrodynamics general circulation model (TIE-GCM) [Richmond et al., 1992]. The TIEGCM is designed to calculate the coupled dynamics,
chemistry, energetic, and electrodynamics of the global
thermosphere-ionosphere system between about 97 km
and 500 km altitude. Its inputs include solar ultraviolet
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radiation flux intensity, parameterized by the F10.7 index,
and upward propagating atmospheric tides at the lower
boundary. In particular, the TIE-GCM calculates ionospheric electric fields and currents and their associated
magnetic perturbations at the ground and at low-Earth-orbit
altitudes, using a realistic geomagnetic main field, under
various seasonal and solar activity conditions. Because of
longitudinal variations of the main field, the local-time
patterns of TIE-GCM ionospheric currents and geomagnetic
perturbations show longitudinal variations, even when the
upward propagating atmospheric tides input at the lower
boundary have a fixed local time pattern, the so-called
‘‘migrating’’ tides. Longitudinal variations can also be
produced by nonmigrating tides [e.g., Immel et al., 2006].
In this study we will focus on the migrating tides as first
step.
[4] The present work evaluates how well the TIE-GCM
accounts for the EEJ observed features, using standard
model inputs that include migrating tides. For that purpose,
the diurnal, latitudinal and longitudinal patterns of the
simulated EEJ magnetic signature are analyzed and compared with ground-based and satellite measurements of the
EEJ magnetic effects. We examine the influences of the
migrating tides and of the geomagnetic main field structure
on the EEJ in order to determine their roles in the EEJ
longitudinal variation and its seasonal variability. We do not
consider here the influences of ‘‘non-migrating’’ tidal components, for which the daily cycle varies with longitude.
Even though observations indicate that non-migrating tides
may have substantial amplitudes in the lower thermosphere
[e.g., Talaat and Lieberman, 1999; Oberheide and Gusev,
2002; Manson et al., 2004; Huang and Reber, 2004], these
tidal components have not yet been carefully evaluated in
the TIE-GCM.
2. TIE-GCM Ionospheric Current
Configurations and Electrodynamic Parameters in
Equatorial Latitudes
[5] For a description of the different physical processes
included in the TIE-GCM we refer to earlier publications
and the references therein: Dickinson et al. [1981, 1984],
Roble et al. [1982, 1988], and Richmond et al. [1992].
Although the model features are described in these
references, the most relevant ones for this study are
mentioned in the following. Especially the low-latitude
electrodynamics and current configuration are described
for a better understanding of the subsequent magnetic
perturbations calculation.
2.1. Basic Assumptions
[6] The TIE-GCM uses a realistic geomagnetic field
model (International Geomagnetic Reference Field 2005),
with magnetic apex coordinates according to Richmond
[1995]. In the low-latitude region the geomagnetic field
lines are assumed to be equipotential, which results in a
symmetric electric potential about the geomagnetic equator.
Within the polar cap (above/below ±75° latitude), the
electric potential distribution is prescribed by the Heelis et
al. [1982] model. At the lower boundary, approximately 97
km, tidal perturbations can be included. At the upper
boundary, approximately 500 – 600 km, the vertical O+ flux
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is specified, which approximates the plasma exchange
between the ionosphere and the plasmasphere.
[7] In this paper, we examine the current and magnetic
perturbations only at low latitudes. We focus on simulations
for the March equinox and the December and June solstices,
for moderate solar activity conditions (F10.7 = 160 sfu,
where 1 sfu = 1022 Wm2 Hz1), and for geomagnetic
quiet time conditions. The integrated hemispheric power of
auroral electrons is set to 15 GW and the cross polar cap
potential is kept constant at 45 kV. For most of the model
runs, we specify migrating diurnal and semidiurnal upward
propagating tidal perturbations at the lower boundary as
calculated by the Global Scale Wave Model (GSWM) from
Hagan and Forbes [2002, 2003], but do not include the
nonmigrating components. The TIE-GCM is run for several
days to reach a diurnally reproducible state.
2.2. Current Systems in the Low-Latitude Regions
[8] To determine the magnetic perturbation at the ground
and above the ionosphere we need to know the current
system. The TIE-GCM solves for the electric potential
assuming equipotential geomagnetic field lines. Knowing
the electric potential, we can calculate the electric current
in the ionosphere and the geomagnetic field-aligned current
at the top of the conducting ionosphere. The horizontal
current density between 90 km and 600 km is integrated in
height and represented as a thin current sheet at 110 km.
Divergence of this horizontal current is assumed to connect
to purely field-aligned current above the current sheet.
[ 9 ] We treat the quasi-dipole current components
[Richmond, 1995] as if they were true current components
on a spherical Earth with a dipolar geomagnetic field, so
that we can use spherical harmonic analysis as described by
Richmond [1974] to calculate the equivalent current function and the associated magnetic perturbation at the ground.
The equivalent current is a fictitious divergence-free horizontal sheet current that produces the same magnetic
perturbation at the ground as the true three-dimensional
current system. To calculate the equivalent current function
we use spherical harmonics of degree and order up to M =
24 and N = 72, which ensures that the equatorial electrojet
can be resolved. The total sheet current can be expressed by
an equivalent current and a residual current. We define the
residual current as the difference between the sheet current
and the equivalent current. For the magnetic effects associated with currents induced within the Earth, we assume that
at 600 km depth there lies a perfectly conducting layer,
where the vertical component of the magnetic perturbation
field vanishes. For calculating the magnetic perturbation
above the ionospheric current sheet layer, we have to take
into account, in addition to the equivalent current, the
residual and the field-aligned current. The residual current
couples with the field-aligned current to form a divergencefree current system that produces no magnetic perturbation
on the ground. Shortly above the current sheet the magnetic
perturbation due to the residual current is DBres where
DBres = mo.bzxKres, with Kres the residual current, bz
the unit upward vector, and mo the permeability of free
space. At higher altitudes the meridional component of the
magnetic perturbation associated with the residual currents
is assumed, for simplicity, to be constant in height, while the
zonal component is adjusted in height to account for the
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3.1. Local Time and Latitudinal Variations
[11] Figures 1a, 1b and 1c show the daily variations of the
D, H and Z components simulated at the magnetic stations
of the West African meridian network [Doumouya et al.,
1998; Vassal et al., 1998] in March, June and December,
respectively, along with the corresponding seasonal averages of the observations during the IEEY in 1993 (see
Table 1 for the coordinates of the 10 stations along the West
African meridian chain across the magnetic dip equator).
Figures 1a – 1c show the consistency of the diurnal variations of TIE-GCM-simulated EEJ magnetic components
with the observations from one station to another. Indeed,
the diurnal variations of H and Z present the expected shape
from the Southern Hemisphere to the Northern Hemisphere,
including the amplitude variation with respect to the latitude
of the stations. Note that the average value of F10.7 for 1993
was about 110 sfu, smaller than that used in the simulations.
[12] The simulated D variation is weaker than the observed one in December; and the diurnal patterns are also
different. For March and June, the computed D amplitude is
stronger than in December, and better capture the local time
variation of the observations in the stations located to the
north than in those to the south of the magnetic equator. The
Figure 1a. Diurnal variation of the horizontal (magnetic
east D and north H) and vertical Z components during the
March equinox. The dotted red lines represent the seasonal
averages of the magnetic quiet time data recorded in West
Africa during the International Equatorial Electrojet Year
(IEEY) (1993), and the solid blue lines represent the TIEGCM simulation in approximately the same medium solar
activity and seasonal conditions as in 1993. Only six
stations of the network are represented.
horizontal component of the field-aligned current flowing
between the current sheet and the altitude in question. Once
the magnetic perturbations have been calculated in the
dipolar geometry, they are treated as though they are
quasi-dipole components of the magnetic perturbation field
in order to map them over the Earth.
3. Results
[10] In this section, the diurnal variations of the geographic and magnetic northward components (respectively
X and H), the eastward components (Y and D), and the
vertical downward component Z, are analyzed as functions
of magnetic local time and geographic latitude and longitude at different seasons.
Figure 1b. Same as Figure 1a but for the June solstice.
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Figure 1c. Same as Figure 1a but for the December solstice.
latitudinal variation of the computed D component seen in
Figure 2 is relatively small and is most of the time
comparable with the observations except for stations situated to the southern side of the magnetic equator where they
exhibit larger variations, especially near 8°N geographic
latitude. The contour maps (Figures 3a and 3c) show that
the simulated D component in the Northern Hemisphere is
eastward (positive) in the morning and westward (negative)
in the afternoon, while the opposite configuration is
observed to the south during March and December; in June
(Figure 3b) two different orientations are observed in the
morning (weakly eastward to the north and westward to the
south), while D is entirely westward in the evening.
Table 1. Coordinates of the 10 Stations Along the West African
Meridian Network Across the Magnetic Dip Equator During the
International Equatorial Electrojet Year
Geographic Location
Stations
Code
Latitude, °N
Longitude, °W
Dip Latitude, deg
Tombouctou
Mopti
San
Koutiala
Sikasso
Nielle
Korhogo
Katiola
Tiebissou
Lamto
TOM
MOP
SAN
KOU
SIK
NIE
KOR
KAT
TIE
LAM
16.733
14.508
13.237
12.356
11.344
10.203
9.336
8.183
7.218
6.233
3.000
4.087
4.879
5.448
5.706
5.636
5.427
5.044
5.241
5.017
6.90
4.03
2.50
1.41
0.12
1.20
2.30
3.82
4.99
6.27
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Figure 2. Latitudinal variation of the horizontal (magnetic
east D and north H) and vertical Z components during
equinox. The dotted red lines represent the seasonal
averages of the magnetic quiet time data recorded in West
Africa during the IEEY; the solid blue lines represent the
TIE-GCM simulation for approximately the same solar
activity and seasonal conditions. The small vertical lines
indicate the position of the magnetic equator.
[13] These structures of simulated D could be related to
the planetary Sq current system rather than to the EEJ.
Although these patterns are analogous to those observed in
West Africa during the International Equatorial Electrojet
Year (IEEY) by Doumouya et al. [1998], there are some
differences with the observations. For the observation data,
the structure of D seems to contain an additional component
that closes on the edges of the EEJ, as confirmed in Figure 4
representing the D component recorded in central Africa in
1969 [Fambitakoye and Mayaud, 1976]. The 1969 experiment in central Africa was made along a meridian chain of
9 magnetic stations that expanded on a large latitude profile
Figure 3. Contour map representation of the TIE-GCM
simulation of the D component, (a) for the equinox, (b) for
the June solstice, and (c) during the December solstice for
medium solar activity conditions as observed during the
IEEY.
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Figure 4. Examples of a typical pattern of the D component diurnal variation in the EEJ influence area,
observed in central Africa in 1969 on different days. The contour interval is 5 nT.
of about 3000 km, an advantage which allows a comprehensive representation of the structure of the D component
[Doumouya et al., 1998]. The contour map of January 31,
1969, exhibits four distinct peaks with opposite signs: a
positive peak (20 nT) at 8°N dip latitude and a negative
peak (±20 nT) at 6°S for the morning hours; a negative
peak (20 nT) at 8°N and a positive peak (25 nT) at 6°S
in the early afternoon. For the other contour maps similar
peaks are often, but not always present. Such a structure of
the D variation suggests there may sometimes exist a
current system that flows in a horizontal plane, with two
vortices flowing respectively clockwise to the south, and
anticlockwise to the north, focused at about 6°S and 8°N on
either side of the dip equator. Such a current system flowing
in the restricted equatorial area is not reproduced by our
TIE-GCM simulations, and needs to be investigated further.
[14] The TIE-GCM computed H and Z components
present more significant amplitudes during the day time
(Figure 1), arising from dawn to dusk, with the maximum
around noon. The amplitude of the computed H (Figure 2)
is stronger near the dip equator, while Z is maximum near
the southern edge and minimum near the northern edge of
the EEJ, around 3° from the magnetic equator. These shapes
of H and Z are typical magnetic signatures of an eastward
EEJ around noon. Nevertheless the model underestimates
the amplitude of the observations and there is a shift
between the times of the maxima. For the observations
the maximum occurs earlier.
[ 15 ] Beside the normal eastward EEJ effects, the
computed H and Z components are reversed around
0700 LT in the morning (Figures 1a, 1b, and 1c), and this
reversal is found at all seasons, with varying amplitude as
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Figure 5. Diurnal variation of the H component of the TIE-GCM– simulated EEJ magnetic effect in 30°
increments along the dip equator between 175° and 155° geographic longitude. The solid blue line
represents December solstice, the dashed green line represents June solstice, and the dotted red line
represents March equinox.
for the normal eastward EEJ, stronger in March (equinox)
and weaker in December and June (solstices). In contrast to
the model, the observations exhibit only weak reversals in
March, with amplitudes and durations smaller than those of
the model. The reversals of H and Z are interpreted, in terms
of equivalent current, as a current system flowing westward
along the dip equator. This equatorial westward current has
been observed at other longitudes, and is referred to as
counter-electrojet [Gouin and Mayaud, 1967].
3.2. Longitudinal Variation
3.2.1. Description of the Longitudinal Pattern of the
TIE-GCM – Simulated EEJ Magnetic Effect
[16] We have computed the diurnal variation of the
magnetic perturbations along the dip equator in different
seasons. Figure 5 represents the simulated diurnal variation
of the H component. The amplitude of the H component
depends on season, being highest in March. This plot shows
a net change in the amplitude and the shape from one 30°
longitude sector to another. A 0:30 to 1:30 hour shift of the
maximum is observed for June and December, depending
on the longitude sector. The counter-electrojet (CEJ) effects
occur in the morning, the amplitudes of which also change
with longitude. The largest amplitudes of the CEJ are
located between 90°W (90°E) and 65°E geographic longitude, covering South America, the Atlantic Ocean and
Africa, with the strongest amplitudes in South America. In
the afternoon there are no apparent reversals attesting the
occurrence of afternoon CEJ.
[17] The longitudinal profiles (Figure 6) of the H, D and
Z noon values at the dip equator show that all the components vary with longitude. The H component exhibits two
significant maxima respectively west of South America
(85° geographic longitude) and in the eastern Asia-Pacific
area (135°). There is a broad minimum between 20° and
60°, with a slight tertiary maximum in Africa (30°) during
March and June, and a secondary minimum in the midPacific (±180°). The D component shows a longitudinal
variation that exhibits a strong amplitude (50 nT) at the
dip equator in the Atlantic-South American longitude sectors, with a maximum around 80° and a minimum around
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Figure 6. Noontime longitudinal variation of the TIEGCM – simulated EEJ magnetic effects: (top) H component,
(center) D component, and (bottom) Z component, in
equinox (dotted red line), June solstice (solid green line),
and December solstice (dot-dash blue line).
30°. The D component oscillates around 0 nT elsewhere,
with weaker amplitude (less than 10 nT).
[18] The existence of a nonzero D component at the
magnetic equator has two causes. First, we find that the
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vortices of equivalent current in the Northern and Southern
hemispheres can extend across the magnetic equator.
Figure 7 shows the equivalent current function for two
universal times near the times when the noontime D
component in Figure 6 minimizes or maximizes: 1300 UT
and 1700 UT, when local noon is at 15° longitude or 75°
longitude, respectively. It is seen that around local noon the
equivalent current flows from the Southern to the Northern
Hemisphere at 1300 UT, producing a negative D component, while the noontime equivalent current flows from the
northern to the Southern Hemisphere at 1700 UT, producing
a positive D component.
[19] A second factor that affects the D component at the
magnetic equator is the nonperpendicularity of the magnetic
equator to the direction of the main geomagnetic field. Since
the EEJ current flows along the magnetic equator, the
strongest horizontal magnetic perturbations are perpendicular to the magnetic equator. The D component is measured
in the direction perpendicular to the main field, but this
direction is not necessarily perpendicular to the strong EEJ
magnetic perturbations, and so the D component senses a
part of the EEJ magnetic perturbation. Figure 8 shows
(dotted black line, labeled ‘‘angle’’) the angle y between
the direction in which D is measured and the magnetic
equator, where a positive angle means that the D direction is
to the north of the equatorial direction, such that an
eastward electrojet would contribute positively to D at the
ground. The solid blue line shows Dangle = Hsiny at local
noon, which is approximately the contribution to D from the
strong EEJ component of magnetic perturbation perpendicular to the magnetic equator. The total D component from
Figure 5 has been replotted in Figure 8 in dashed red. Dangle
comprises a minor, but nonnegligible, part of the total D
component. The dashed-dotted line in brown shows the
difference between the total D perturbation and Dangle, and
is due to the flow of equivalent current across the magnetic
equator, as discussed in the previous paragraph. It is
interesting to note that this latter (dashed-dotted line in
brown) component tends to oscillate with longitude in a
similar manner as the (solid blue line) Dangle component but
with the peaks and valleys shifted 10° – 30° in longitude.
[20] The Z component varies with very weak amplitude at
the dip equator (Figure 6), as expected. However, it is
amplified in the longitude sector of South America, where
a minimum of about 20 nT is noticed around 50°
longitude.
[21] In section 3.2.2, the TIE-GCM-simulated longitudinal
variation of the H component will be compared with the
CHAMP satellite borne EEJ magnetic signature.
3.2.2. Comparison of the TIE-GCM – Computed EEJ
Longitudinal Variation With CHAMP Satellite
Observations
[22] In Figure 9, the noontime magnitudes of the H
component at 430 km altitude above the magnetic equator
from the TIE-GCM are shown as a function of longitude
(blue dotted lines) during the March equinox and the June
and December solstices, along with the corresponding
CHAMP satellite observed mean longitudinal variations
(red solid lines) of the magnitude of the EEJ total magnetic
perturbation (jDFj). The scattered dots represent the single
values of jDFj at the dip equator for passes occurring
between 11 and 1300 LT. The EEJ magnetic perturbation
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Figure 7. Equivalent current function [kA] representing the Sq current system at (top) 1300 UT and
(bottom) 1700 UT over geographic longitude and latitude. Equivalent current flows counterclockwise
about the northern minimum and clockwise about the southern maximum. The geomagnetic equator is
indicated by the dashed line. The model result is for March equinox with F10.7 = 160 and migrating
diurnal and semidiurnal tides prescribed at the lower boundary.
DF has been isolated from the total satellite magnetic
records by first extracting the main geomagnetic field total
strength (F) using the IGRF 2000 model coefficients. The
resulting residuals for each pass over the dip equator were
fitted with a polynomial function, excluding the values
close to the dip equator, in order to identify and remove a
background signal that is superimposed on the EEJ contribution. This background signal is intended to include all the
magnetic perturbations due to non-EEJ sources (crustal
field, magnetospheric currents, Sq current, field aligned
current, etc.). The remaining signal attributed to the EEJ
depends on this processing technique. Doumouya and
Cohen [2004] have discussed the CHAMP satellite magnetic data processing in more detail. Since the TIE-GCM
values do not have a corresponding background value
removed, we cannot directly compare the magnitudes of
the simulations and observations, but will instead focus on
the respective longitudinal variations.
[23] In the equinox and the June solstice the satellite
observations in Figure 9 exhibit four maxima, at about
170°, 85°, 10° and 100°. In June the maximum around
10° is enhanced and shifted westward. Four minima at
about 130°, 45°, 40° and 150° are also observed.
Similar observations were made by Jadhav et al. [2002]
during the March equinox and June solstice. As described in
section 3.2.1, the TIE-GCM simulations show only two
main peaks (85° and 135°), a broad minimum (20° to
60°) with a tertiary maximum (30°), and a secondary
minimum (±180°). For the two results, only the maxima
around 85° coincide. A shift of about 40° between the
peaks in Africa and Asia is observed. In the December
solstice, the satellite observation exhibits three maxima
around 135°, 45° and 60°, two minima around 100°
and 20°, and an almost flat minimum between 100° and
140°, whereas the TIE-GCM gives two peaks at 90° and
140°, a minimum at ±180° and a flat minimum between
30° and 70°.
[24] The above results show poor agreement between the
TIE-GCM simulation and the observed longitudinal variation of the EEJ at satellite height. The TIE-GCM longitu-
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Figure 8. Contributions to the longitudinal variation of the D component at the magnetic equator. The
dashed red line represents the TIE-GCM total D component; the black dotted line represents the angle
between the direction of the D component and the magnetic equator; the solid blue line represents the
D variation due to projection onto the magnetic east direction of the EEJ magnetic component perpendicular
to the magnetic equator. The dashed-dotted brown line represents the effect of the equivalent current that
extends across the magnetic equator. The green line represents the magnetic equator.
dinal variation of the EEJ is dominated by the geomagnetic
main field structure. Note that the satellite observed average
longitudinal variation of the EEJ magnetic perturbation is
subject to various error sources, such as the processing
method and the day to day variability of the EEJ that is
shown by the scatter of the dots in Figure 9. The standard
deviations with respect to mean variations are respectively
6 nT for the equinox, 8 nT for the June solstice and 7 nT for
the December solstice.
3.3. Effects of the Migrating Thermospheric Tidal
Forcing on the EEJ Magnetic Field
[25] The migrating diurnal and semidiurnal tidal effects
on the EEJ are studied by running the TIE-GCM till we get
a diurnally reproducible result for a given season. To isolate
the tidal effect, we have computed the TIE-GCM EEJ
magnetic effects along the dip equator, with and without
the tidal forcing at the lower boundary of the model.
Differencing the two results allows us to isolate the tidal
forcing effect. Figure 10 shows the diurnal variation of the
H component in different longitude sectors at the ground
level for the March equinox. The dotted red lines represent
the total H component including the tidal effect, the dashed
black lines (H0) show the H component without tidal
forcing referred to as ‘‘tidal-free component’’; and the solid
blue lines show the difference Ht (Ht = H H0), representing
the tidal contribution. Note that the red dotted lines in
Figure 5 and Figure 10 are the same. It can be noticed that
the tidal contribution is an important component of the EEJ
magnetic signature. The amplitude of its contribution can
exceed 35 nT on the ground around noon. Furthermore,
according to its daily variation, the tidal contribution seems
to play also the most dominant role in the CEJ occurrences.
Indeed, the reversals are mainly carried by the tidal contribution (solid blue lines). Although the tidal contribution
exhibits reversals in the afternoon as well as in the morning,
the total magnetic perturbations do not show any apparent
afternoon CEJ effect. This is confirmed in Figure 5 for the
three seasons (March equinox, June and December Solstices)
and different longitude sectors. The lack of afternoon
reversals of the total magnetic perturbation is due to the
fact that the amplitude of the afternoon reversals in the tidal
component is much weaker than that in the morning, and is
not enough to counterbalance the tidal-free component, and
then to cause the reversal of the total perturbation.
[26] Figure 11 shows the longitudinal profiles of the H
component at local noon, in the three cases mentioned
above for March equinox. The dotted red line, with tidal
effect, has been described section 3.2.2; the dashed black
line, without tidal effect, exhibits two maxima, one in South
America and the other in eastern Asia, and a flat minimum
over Africa. The tertiary maximum around Africa seen in
the results that include tides is not observed when tides are
removed. This feature seems to be exclusively related to the
tidal effect (the solid blue line). Notice that the tidal effect
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mostly amplifies the different undulations of the EEJ
longitudinal variation.
[27] We compare in Figure 12 the longitudinal variation
of the noontime tidal effect on the H component with that of
the total EEJ magnetic perturbation observed by the
CHAMP satellite, for various seasons. Figure 12 shows
the magnitudes of these (negative) perturbations, with
different scales shown on the right and left. For the March
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equinox (top), the (positive) tidal effect on ground from the
model is also shown (dashed red curve). The CHAMP
observations (solid green line) show a similar number of
peaks in longitude as the simulation of the tidal effects, but
except for the maximum at 85° in all three curves, there
are significant shifts between the simulated and observed
minima and maxima, yielding sometimes to opposite
phases. At 430 km altitude, the longitudinal profile of the
tidal effects (dotted blue line) shows the same phases as on
ground, except for the minimum at 140° at ground that is
shifted westward to 175° at 430 km. For the June and
December solstices, there is less agreement between the
model and the satellite observations. In June the longitudinal changes of the tidal effect are very weak between 40°
and 100°, whereas the satellite observations exhibit very
strong variations, more nearly out of phase than in phase
with the model variations, especially between 0° and 180°.
In December the oppositions between the two curves are
mostly between 180° and 0°.
3.4. Effects of the Geomagnetic Main Field Structure in
the EEJ Longitudinal Variation
[28] The EEJ as an ionospheric current system is related
to the ionospheric regular dynamo process and in turn
depends on the local magnitude of the geomagnetic main
field through the ionospheric conductivity as given by
Ohm’s law in the low-latitude ionosphere [Davis et al.,
1967]. Both the Pedersen conductivity (s1) and Hall
conductivity (s2) depend on the intensity of the main field
(B) through the gyrofrequency [Baker and Martyn, 1953].
The longitudinal inequalities in the main field induce
longitudinal changes in the EEJ intensity. In addition,
longitudinal changes in the main field direction and
curvature affect the EEJ. In this section, we verify the
importance of the geomagnetic main field structure on the
EEJ (Figure 13) by comparing the ground-level TIE-GCM
result for March at noon at the dip equator (black dotted
lines) with that in which the IGRF geomagnetic field (B) is
replaced by a tilted dipole model field (red solid lines). The
H component of the simulated EEJ magnetic effect related
to the tilted dipole field is nearly constant with respect to
longitude. Only the D component is changing with longitude in a sinusoidal function. These results are very different
from those obtained with the IGRF model. For the dipole
field model, no significant effects of the tides are noticed in
the shape of the longitudinal variation.
4. Discussion
[29] According to the morphological analysis of the EEJ
magnetic signature simulated by the TIE-GCM, it can be
Figure 9. Comparison between the CHAMP satellite
observed longitudinal variation of the amplitude of the
(negative) EEJ magnetic effect (solid red line) and the TIEGCM simulations (dotted blue line) at the dip equator
during (top) March equinox, (center) June solstice, and
(bottom) December solstice. The red dots represent the
noontime amplitude of the EEJ magnetic signature at the dip
equator for each single satellite pass, and the solid red line
represents the average longitudinal variation.
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Figure 10. Contribution of the thermospheric migrating diurnal and semidiurnal tides to the EEJ
magnetic effects at the ground in different longitude sectors for the March equinox. The dotted red line
represents the total magnetic effect including the tidal effect, the dashed black line represents the
magnetic effect without tidal effect, and the difference (solid blue line) represents the exclusive tidal
effect.
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Figure 11. Longitudinal variation of TIE-GCM– simulated EEJ magnetic effects at the dip equator
during the March equinox at local noon on the ground, with tidal contribution (dotted red line), without
tidal contribution (dashed black line), and the difference representing the exclusive tidal effect (solid blue
line), scaled on the right side.
concluded that the TIE-GCM reproduces many of the main
features of the EEJ reasonably well, and also qualitatively
accounts for some of its variations such as its local time,
latitude, longitude, and seasonal dependence. We have
shown that the H and Z components of the EEJ magnetic
effect are approximately reproduced, as observed at the
ground and low-Earth-orbit altitudes. The TIE-GCM produces also a diurnal variation in the D component at the
magnetic equator, induced mainly by meridional current
flow across the dip equator. The success of the model in
reproducing observed EEJ features qualifies it for addressing several questions relating to the total configuration of
the EEJ, in particular how its circuit closes, how it connects
to the planetary current system, and its relation to thermospheric winds. For example, this work has shown that the
migrating diurnal and semidiurnal tides are the main cause
of the counterelectrojet occurrence and its longitudinal
dependence in the model. The fact that differences are
identified between some details of the TIE-GCM predictions and the observations indicates that the model and its
inputs can still be improved. For example, a different
amplitude, phase, or distribution of the tides input to the
TIE-GCM could affect the EEJ magnitude and time of
maximum, and could alter the amount of current crossing
the magnetic equator that affects the equatorial D variation.
To further investigate such effects goes beyond the scope of
this study.
[30] The TIE-GCM reproduces less well the longitudinal
variation of the EEJ as obtained through CHAMP satellite
observations. We have shown that the nondipolar structure
of the main field is the primary cause of the undulations in
the EEJ longitudinal variation in the TIE-GCM. In particular,
the migrating tides interact with this nondipolar structure in a
manner that produces a tidal contribution to the longitudinal
variations of the EEJ strength that shows multiple longitudinal maxima somewhat similar to those in the CHAMP
observations. However, the modeled strength of these
maxima is generally less than the observations indicate,
and the longitude of their positions is sometimes different.
This suggests the possibility that migrating thermospheric
tides may play a more important role in the shape of the
longitudinal profile of the EEJ strength than our TIE-GCM
results predict, if these tides are underestimated in the TIEGCM. Nonmigrating tides, though not included in our
simulations, may also play a significant role. Indeed, Immel
et al. [2006] have shown that nonmigrating tides may help
explain the presence of four longitudinal maxima in the
strength of the nighttime equatorial ionization anomaly
observed by Sagawa et al. [2005]. Whether this longitudinal
structure in the nighttime ionosphere is closely related to
the longitudinal structure in the daytime EEJ remains to
be determined. The disagreement between the TIE-GCM
simulation of the EEJ longitudinal variation and the satellite
observations may also be related in part to the way the EEJ
signature was extracted, as well as to the nature of these
data. Indeed the data used here come from CHAMP scalar
measurements. Furthermore, the method used to extract the
EEJ signature could have underestimated the EEJ contribu-
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tion and even missed it at certain places. For instance, we
have remarked that the longitudinal variations of the mean
satellite data are smaller than the modeled ones, in contrast to
ground-based measurements. Especially, this underestimation could happen in the areas where the dip equator is
strongly tilted, as in South America-Atlantic longitude sector.
Since the satellite orbit is polar, its passes in that region of
strong dip equator slope askew the EEJ profiles.
[31] On a quantitative basis, not only does the model
underestimate the amplitude of the EEJ ground-based magnetic effects, a local time shift of the extrema of the diurnal
variation of the H and Z components is observed. Most of
the time, the H maximum occurs earlier in the observations
than in the model results. Rastogi [2004] has also shown
that the maximum of the H component occurs about one
hour before noon. One possible cause of the disagreement
between the observations and the TIE-GCM results can be
the relative permanence and overestimation of the counterelectrojet effect by the model.
[32] The strong amplitude of the D component in the
longitude sector of South America is caused primarily by
the structure of equivalent Sq current. A hemispheric
invasion of the current vortices, characterized by a current
flow across the dip equator, is observed in this sector where
the geomagnetic field is weak and much distorted, with a
strong declination. The Z component is also influenced by
this effect. The hemispheric asymmetry of the equivalent
Sq current vortices was also modeled by Takeda [2002]
using ground-based magnetic observations.
[33] Contributions of the tides are also noticed on the D
and Z components at the dip equator, with strong amplitudes in the South American-Atlantic sector. For the D
component, the tidal effect at noon is opposite to the global
effect, westward to the west of the intersection of the
geographic and magnetic equators, and eastward to the east.
This is shown by an opposite meridional current flow with
respect to the equivalent Sq current mentioned above. The
tidal effect on the Z component keeps the same upward
orientation at noon as for the global equivalent current,
peaking around the intersection of the geographic and dip
equators.
[34] Notice that we did not compare the simulations of the
D and Z component longitudinal variations with observations to confirm such a behavior of the current systems.
Such a comparison requires vector measurements, and may
be possible by using the CHAMP satellite and ground-based
magnetic data. Our immediate future work consists in using
satellite vector magnetic data in order to pursue this study.
For that purpose, we will adopt a more objective approach
to analyze CHAMP satellite data. A better adapted internal
Figure 12. Comparison between the amplitude of the
(negative) H component of the TIE-GCM tidal effects and
the corresponding CHAMP satellite-borne longitudinal
profile of the EEJ scalar magnetic signature at local noon.
(top) Tidal effect represented on the ground (dashed red
line) and at 430 km (dotted blue line) with CHAMP
observation (solid green line) during March equinox. Tidal
effect represented at 430 km (dotted blue line) and CHAMP
observation (solid green line) at the (center) June and
(bottom) December solstices.
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field model will be used to isolate the ionospheric contributions from the satellite magnetic measurements. The
POMME model [Maus et al., 2005] could be used for this
purpose. The last version of this model includes many
interesting features, such as the attitude corrections, which
will bring more accuracy to CHAMP vector measurements.
[35] Acknowledgments. This study was partly supported by the
NASA Sun-Earth Connection Theory Program. Vafi Doumbia’s visit at
the National Center for Atmospheric Research (NCAR) was financed by a
Fulbright grant supervised by the Council for International Exchange of
Scholars (CIES). Note that V. Doumbia’s previous papers are authored as
V. Doumouya.
[36] Amitava Bhattacharjee thanks Subramanian Gurubaran and another
reviewer for their assistance in evaluating this paper.
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Figure 13. Effect of the main geomagnetic field structure
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local noon for March. The solid red line represents the TIEGCM simulation of the EEJ magnetic effects using a tilted
dipole main field; the dotted black line represents the TIEGCM simulation of the EEJ magnetic effects using the
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V. Doumbia, Laboratoire de Physique de l’Atmosphère, Université de
Cocody, 22 BP 582 Abidjan 22, Côte d’Ivoire. ([email protected])
A. Maute and A. D. Richmond, High Altitude Observatory, National
Center for Atmospheric Research, Boulder, CO 80307-3000, USA.
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