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Communications/Navigation Outage Forecasting System
observational support for the equatorial E × B drift velocities
associated with the four‐cell tidal structures
Eduardo A. Araujo‐Pradere,
1,2
David N. Anderson,
1,2
Mariangel Fedrizzi,
1,2
and Russell Stoneback
3
Received 16 November 2010; revised 31 March 2011; accepted 15 April 2011; published 13 July 2011.
[1]Previous studies have established the existence of a four‐cell longitude pattern in
equatorial Fregion ionospheric parameters such as total electron content and electron
densities and in daytime, equatorial E×Bdrift velocities. This paper, for the first time,
quantifies the longitude gradients in E×Bdrift associated with the four‐cell tidal
structures and confirms that these sharp gradients exist on a day‐to‐day basis. For this
purpose, we use the Ion Velocity Meter (IVM) sensor on the Communications/Navigation
Outage Forecasting System (C/NOFS) satellite to obtain the daytime, vertical E×B
drift velocities at the magnetic equator as a function of longitude, local time, and season.
The IVM sensor measures the E×Bdrift velocity in three dimensions; however, we
only use the E×Bdrift observations perpendicular to Bin the meridional plane. These
observations can be used to obtain the vertical E×Bdrifts at the magnetic equator by
mapping along the geomagnetic field line. The period initially selected for this work
covers several days in October, March, and December 2009. We find, on a day‐to‐day
basis, that (1) sharp E×Bdrift gradients of −1.3 m s
−1
deg
−1
exist in the western Pacific
sector during equinox, (2) sharp E×Bdrift gradients of +3 m s
−1
deg
−1
are observed in the
eastern Pacific sector during equinox, and (3) sharp E×Bdrift gradients of −1.7 m s
−1
deg
−1
exist in the eastern Pacific sector during December solstice.
Citation: Araujo‐Pradere, E. A., D. N. Anderson, M. Fedrizzi, and R. Stoneback (2011), Communications/Navigation Outage
Forecasting System observational support for the equatorial E×Bdrift velocities associated with the four‐cell tidal structures,
Radio Sci.,46, RS0D09, doi:10.1029/2010RS004557.
1. Introduction
[2] In the Earth’s ionospheric Fregion, between 200 and
800 km altitude, the daytime distribution of electrons
and ions as a function of altitude, latitude, longitude and
local time are determined by ionospheric production, loss
and transport mechanisms. Production is primarily through
photoionization of atomic oxygen by solar EUV (l<
91.1nm) radiation, and loss is through charge exchange of
O
+
ions with N
2
and O
2
, to give NO
+
and O
2
+
, followed by
recombination with electrons. Transport of ionization per-
pendicular to B is due to E×Bdrifts and transport parallel
to B is due to ambipolar diffusion and the component of the
neutral wind parallel to B. At low latitudes, the primary
transport mechanism is via E×Bdrifts in the vertical and
meridional plane. At the magnetic equator, these E×B
drifts are upward in the daytime and primarily downward at
night. The daytime upward drifts are responsible for pro-
ducing crests in the Fregion peak electron density, N
max
,
at +/−15° to 18° magnetic latitude, known as the equatorial
anomaly [Hanson and Moffett, 1966; Anderson, 1973].
[3] A recent technique has been developed to infer the
daytime, vertical E×Bdrift velocity from ground‐based
magnetometer observations [Anderson et al., 2002]. Uti-
lizing a magnetometer located on the magnetic equator
(Jicamarca, Peru) and one off the magnetic equator at 6°N
mag. lat. (Piura, Peru), Anderson et al. [2004] developed
various relationships between the observed difference in the
H component, DH(H
Jic
−H
Piura
), and the vertical E×Bdrift
velocity observed by the Jicamarca Unattended Long‐Term
Ionosphere Atmosphere (JULIA) coherent scatter radar
measuring the Doppler shift of 150 km echo returns. These
150 km E×Bdrifts have been shown to be essentially
equivalent to Fregion E×Bdrift velocities by comparing
them with the Jicamarca Incoherent Scatter Radar (ISR)
E×Bdrifts. Anderson et al. [2004] developed a neural
network technique that gave realistic, daytime E×Bdrift
velocities. The neural network was trained with over
450 quiet and disturbed days between 2001 and 2004, using
5 min observations of DH and JULIA E×Bdrift velocities
between 09:00 and 16:00 LT. Figure 1 compares the average,
1
CIRES, University of Colorado, Boulder, Colorado, USA.
2
NOAA Space Weather Prediction Center, Boulder, Colorado, USA.
3
Hanson Center for Space Sciences, University of Texas at Dallas,
Richardson, Texas, USA.
Copyright 2011 by the American Geophysical Union.
0048‐6604/11/2010RS004557
RADIO SCIENCE, VOL. 46, RS0D09, doi:10.1029/2010RS004557, 2011
RS0D09 1of7
DH‐inferred E×Bdrift velocity for equinoctial, quiet days
with the Scherliess and Fejer [1999] climatological E×B
drifts in the Peruvian, Philippine and Indian longitude sec-
tors. The excellent comparisons give us confidence that
realistic E×Bdrifts can be obtained from the DH technique.
A subsequent paper by Anderson et al. [2006], demonstrated
that realistic E×Bdrift velocities could be obtained with the
Peruvian sector‐trained neural network, when applied to
other longitude sectors where appropriately placed magnet-
ometers existed, such as in the Philippine, Indonesian and
Indian sectors.
[4] Recently, several observational studies have identified
the existence of a low‐latitude, four‐cell longitude pattern in
various ionospheric parameters. The first evidence emerged
from IMAGE satellite FUV (135.6 nm) radiance observa-
tions after sunset (20:00 LT) that clearly showed enhance-
ments in airglow‐inferred N
max
values at the crests of the
equatorial anomaly in four specific longitude zones during
March–April 2001 [Immel et al., 2006]. They attributed the
four‐cell pattern to the effects of a four‐cell pattern in
daytime, vertical E×Bdrift velocities associated with the
diurnal, eastward propagating, nonmigrating, wave number
3 (DE3) tidal mode [Hagan and Forbes, 2002]. Since the
IMAGE observations were at night, the authors could not
rule out a four‐cell pattern in the prereversal enhancement in
E×Bdrift that occurs after sunset. A subsequent paper by
England et al. [2006], however, established that the four‐
cell pattern was observed in CHAMP satellite in situ elec-
tron densities at 12:00 LT. Further, the four‐cell pattern has
also been observed in ROCSAT‐1, daytime electron den-
sities at 600 km [Kil et al., 2008] and in COSMIC occul-
tation observations [Lin et al., 2007; Liu et al., 2010] and
ground‐based, global ionospheric total electron content
(TEC) maps –GIMs [Wan et al., 2008]. In addition, four‐
cell patterns in daytime, vertical E×Bdrift velocities have
been reported from DMSP observations [Hartman and
Heelis, 2007; Kil et al., 2008] and ROCSAT‐1 observa-
tions [Kil et al., 2007, 2008].
[5] More recently, Scherliess et al. [2008] analyzed
TOPEX/TEC observations from 1992 to 2005 and binned the
data for quiet days into equinox, June solstice and December
solstice periods and by local time. The local time period
from 12:00 to 16:00 LT displayed the four‐cell pattern in the
same specific longitude sectors seen in the previous studies
during the equinoctial season (see Scherliess et al. [2008] for
details). The TOPEX/Poseidon satellite incorporates a dual‐
frequency altimeter operating at 13.6 GHz and 5.3 GHz to
observe ocean surface heights. The dual‐frequency allows
the total electron content (TEC) to be measured from the
satellite altitude of 1336 km to the ocean surface. The data set
of TEC observations studied by Scherliess et al. [2008]
covers the period from August 1992 until October 2005.
For the current study, a subset of their database has been
used covering the years 2001–2002. As discussed by
Scherliess et al. [2008], difficulties arise for a statistical
analysis of the TOPEX TEC values owing to the slow pre-
cession of the satellite orbit (2°/d). In their paper, Scherliess
et al. [2008] normalized the TEC data to a common baseline
Figure 1. Average DH‐inferred E×Bdrifts (red curve) compared with the Fejer‐Scherliess climatolog-
ical model (blue curve) in the (a) Peruvian longitude sector, (b) Philippine longitude sector, and (c) Indian
longitude sector.
ARAUJO‐PRADERE ET AL.: E×BDRIFTS ASSOCIATED WITH 4‐CELL PATTERN RS0D09RS0D09
2of7
in order to circumvent this problem and the same normali-
zation has been applied in the current paper. In a nutshell, the
normalization is accomplished by first finding the maximum
TEC value for each ascending and descending pass between
+/−30° geomagnetic latitude. Next, the peak values are
longitudinally averaged to give daily values, again for
ascending and descending passes, separately. These daily
values were used as normalization factors, where each 18 s
TEC data point was divided by its corresponding normali-
zation factor. More detail concerning this applied normali-
zation is given by Scherliess et al. [2008], whose figures
refer to “relative”TEC values that have been “normalized”
using these factors.
[6] In order to quantify the daytime, vertical E×Bdrift
velocities at different longitude sectors that might explain the
TOPEX/TEC observations, Anderson et al. [2009] analyzed
the magnetometer‐inferred, daytime E×Bdrift velocities in
the Peruvian, Philippine, Indonesian and Indian sectors for
both the years 2001 and 2002. They binned all of the quiet‐
day, E×Bdrifts into three seasons and found the average
E×Bdrift pattern for each of the three seasons and in each of
the four longitude sectors. Figure 2a presents the average,
daytime vertical E×Bdrift velocities as a function of local
time in four longitude sectors for equinox periods in 2001
and 2002. Note the large difference in maximum E×Bdrift
velocity between the Philippine sector and the Indonesian
sector, 22 m s
−1
versus 15 m s
−1
. These two locations are
only 15 degrees apart in longitude. In Figure 2b, the TOPEX/
TEC values are plotted for equinox, 2001 and 2002, between
12:00 and 16:00 LT. The bottom portion of Figure 2b is
simply the average of the Northern and Southern Hemisphere
TEC values plotted as a function of geographic longitude and
absolute value of the geomagnetic latitude, to emphasize the
location of the longitude gradients. The longitude locations
of the Peruvian (blue curve), Philippine (red curve), Indo-
nesian (purple curve), and Indian (green curve) sectors are
indicated in Figure 2a. Note the very sharp gradient in TEC
between the Philippine (∼125°E geographic longitude) and
the Indonesian (∼140° geographic longitude) sectors. This
sharp gradient at the edge of the cell is presumably caused by
the sharp gradient in the daytime E×Bdrift velocity
between these two sectors. From Figure 2b, there also appear
to be sharp longitude gradients at 220°E and 320°E, which
would imply sharp gradients in the daytime, vertical E×B
drift velocities at these longitudes.
[7] The results presented in Figure 2 are unique and the
comparisons between ground‐based, inferred E×Bdrift
velocities and the satellite TEC observations that relate to
the four‐cell pattern and its seasonal and longitudinal
dependence have not previously been compared. This paper
addresses two fundamentally important and specific scientific
questions: (1) How sharp are the longitude gradients in day-
time, vertical E×Bdrift velocities that define the boundaries
of each of the four cells? (2) Quantitatively, are these sharp
longitude gradients in E×Bdrift velocities observed on a
day‐to‐day basis? Previous studies have shown that the four‐
cell pattern exists day to day [Sagawa et al., 2005; Immel
et al., 2006; England et al., 2006; Wan et al., 2008; Immel
et al., 2009], but this is the first study to determine, quanti-
tatively, that the sharp gradients in E×Bdrift occur on
aday‐to‐day basis. Answering these questions from an
observational standpoint will set the “benchmarks”that are
needed by the theoretical modelers to understand the physical
mechanisms and to compare model results with observations.
2. Approach
[8] In order to answer the two scientific questions listed
above, we have incorporated E×Bdrift observations from
the Ion Velocity Meter (IVM), which is one of the Coupled
Ion‐Neutral Dynamics Investigation (CINDI) sensors on
board the Communications/Navigation Outage Forecasting
System (C/NOFS) satellite [de la Beaujardiere and C/NOFS
Science Definition Team, 2004]. The Coupled Ion‐Neutral
Dynamics Investigation (CINDI) on board the C/NOFS
satellite is composed of two sensors, the Ion Velocity Meter
Figure 2. (a) Average E×Bdrifts for equinox 2001–2002 in the Peruvian, Philippine, Indonesian, and
Indian sectors (see text for details). (b) TOPEX relative TEC as a function of geographic longitude and
magnetic latitude for equinox 2001–2002 and 12:00–16:00 LT (see text for details).
ARAUJO‐PRADERE ET AL.: E×BDRIFTS ASSOCIATED WITH 4‐CELL PATTERN RS0D09RS0D09
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(IVM) and the Neutral Wind Meter (NWM). The IVM
sensor measures (1) cross‐track E×Bdrift velocities
with an accuracy of 2 m s
−1
and a sensitivity of 1 m s
−1
and
(2) along‐track, with respective E×Bdrift velocities of
10 m s
−1
and 5 m s
−1
. The NWM sensor measures neutral
wind velocities (1) cross‐track, with an accuracy of 5 m s
−1
and sensitivity of 2 m s
−1
, and (2) along‐track, with
respective velocities of 10 m s
−1
and5ms
−1
.TheC/NOFS
IVM sensor has been used to obtain the daytime, vertical E×
Bdrift velocities at the magnetic equator as a function of
longitude, local time and season. The IVM sensor measures
the E×Bdrift velocity perpendicular to Band can be used to
obtain the vertical E×Bdrifts at the magnetic equator by
mapping along the geomagnetic field line. This is possible
because the magnetic field lines are equipotentials and the
daytime E×Bdrifts at the magnetic equator vary linearly with
altitude between 200 and 800 km [Pingree and Fejer, 1987].
[9] There are a number of constraints we have adopted in
organizing the IVM E×Bdrift observations: (1) We
incorporate IVM observations only between 10:00 and
12:00 LT because this is the approximate LT window for the
maximum E×Bdrift velocities at all longitudes. (2) We
only utilize IVM observations below 500 km, which is low
enough to ensure that O+ is the major ion. (3) The IVM
observations are averaged over each degree of longitude.
(4) For this study, the periods for IVM observations are
primarily in October, March, and December 2009.
3. Results
[10] In Figures 3a, 4a, 5a, and 6a, we display the IVM
E×Bdrift velocities as a function of geographic longitude
for a number of consecutive days in October, March, and
December 2009. In Figures 3b, 4b, 5b, and 6b, the longitude
coverage is overlaid on the TOPEX contours of relative
TEC values. Bear in mind that the beginning of each curve
on the left corresponds to 10:00 LT, while the end of each
curve corresponds to 12:00 LT. Figure 3a displays two
curves for 6 and 7 October, respectively. The geographic
coverage for the 6 October, 10:00 to 12:00 LT coverage is
from 65° to 90°E, while the 7 October coverage is from 85°
to 115°E. The altitude of the C/NOFS satellite varies from
414 km at 10:00 LT to 405 km at 12:00 LT for both days
while the respective geomagnetic latitudes vary from 1° to
−5°. The IVM observed E×Bdrift velocities are roughly 35
to 40 m s
−1
. Between 65° and 115°E longitude, there are no
sharp longitude gradients in E×Bdrift implying that this
longitude window is in the middle of one of the four‐cell
patterns (Indian sector). This is confirmed by Figure 3b,
which overlays the longitude window on the TOPEX/TEC
contour plot.
[11] In Figure 4, three consecutive days in October (11,
12, and 13 October) present IVM E×Bdrift velocities
in the western Pacific sector between 112° and 147°E
geographic longitude. For each of the curves, the sharp
longitude gradient in E×Bdrift occurs between ∼133° and
145°E longitude. Note that the slope in E×Bdrift velocity
versus geographic longitude is roughly equivalent for all
3 days and is about −1.3 m s
−1
deg
−1
. The altitude of the
C/NOFS satellite between 10:00 LT and 12:00 LT decreases
from 450 km to 410 km while the geomagnetic latitude
increases from −9° to −15°. Pingree and Fejer [1987] have
shown that over this height range, the vertical E×Bdrift
velocity is essentially constant. The change in geomagnetic
latitude when E×Bdrift sharply decreases between 135 and
145°E longitude is only 2° from −10° to −12° geomagnetic
latitude meaning that the sharp change in E×Bdrift with
longitude is not due to the change in geomagnetic latitude.
The boundary of this western Pacific sector cell is located
roughly at the longitudes discussed in section 1, where the
ground‐based magnetometer‐inferred E×Bdrift velocity
gradients were observed. Again, we have overlaid the lon-
gitude window with the TOPEX/TEC contours in Figure 4b.
[12] In contrast, Figure 5 depicts the IVM‐observed E×B
drift velocity gradient in the eastern Pacific sector between
Figure 3. (a) IVM E×Bdrifts versus geographic longitude for 6–7 October 2009 (see text for details).
(b) TOPEX relative TEC as a function of geographic longitude and magnetic latitude for equinox 2001–
2002 and 12:00–16:00 LT (see text for details).
ARAUJO‐PRADERE ET AL.: E×BDRIFTS ASSOCIATED WITH 4‐CELL PATTERN RS0D09RS0D09
4of7
∼240° and 265°E longitude for three consecutive days on
19, 20, and 21 March 2009. For these 3 days the slopes in
the gradients are roughly + 3 m s
−1
deg
−1
. The altitude and
magnetic latitude changes are from 405 km to 450 km and
from −1° to 11°, respectively. Figure 5b clearly demon-
strates that the boundary of the eastern Pacific sector cell is
being observed in going from ∼240° to 250°E longitude.
Figures 3–5 display the day‐to‐day consistency in the IVM
observed E×Bdrift velocity (1) within one of the four‐cell
structures and (2) at the boundaries of two of the four‐cell
structures for equinox conditions.
[13] Figure 6 displays boundary conditions for the eastern
Pacific sector for 2 days during the December solstice
period, 8 and 9 December 2009. For this case the longitude
gradient in E×Bdrift is exactly opposite to the gradient
pictured in Figure 5 and is approximately −1.7 m s
−1
deg
−1
.
Scherliess et al. [2008] observed that during the December
solstice period, there were apparently only three cells
observed rather than the four‐cell patterns during equinox
and June solstice periods. This DE2, nonmigrating pattern
has also been reported by Forbes et al. [2008]. The altitude
and magnetic latitude variations are from 406 km to 415 km
and from −7° to 0.5°, respectively.
4. Summary and Future Work
[14] This paper has addressed the two important, scientific
questions posed in the Introduction. Using the C/NOFS/IVM
Figure 5. (a) Same as Figure 3a but for 19–21 March (see text for details). (b) Same as Figure 3b (see
text for details).
Figure 4. (a) Same as Figure 3a but for 11–13 October 2009 (see text for details). (b) Same as Figure 3b
(see text for details).
ARAUJO‐PRADERE ET AL.: E×BDRIFTS ASSOCIATED WITH 4‐CELL PATTERN RS0D09RS0D09
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observations of the meridional E×Bdrift velocities for
specific days in October, March, and December 2009, we
have demonstrated that extremely sharp longitude gradients
in E×Bdrift velocity exist in the eastern and western Pacific
longitude sectors and that these longitude gradients appear
on a day‐to‐day basis in each of these sectors. Table 1 lists
the longitude gradients in E×Bdrift velocities for the days
in October, March, and December 2009 and their respective
longitudes. It is also shown that these sharp longitude gra-
dients exist on a day‐to‐day basis as depicted in Figures 4–6.
The constraints listed in section 2 limited the number of
useful days for C/NOFS/IVM observations.
[15] We have tentatively established that the longitude
gradients in E×Bdrift velocities at the boundaries of the
four‐cell structures are very sharp and that they occur on a
day‐to‐day basis. The next logical step is to determine
whether or not existing theoretical, self‐consistent, atmo-
spheric‐ionospheric models can account for these sharp
gradients in observed E×Bdrift velocities at the boundaries
of the four‐cell structures. If they can, then the model results
can be analyzed to determine the causes of the sharp
boundaries and their longitude dependence.
[16] We intend to theoretically investigate these sharp
gradients by incorporating the IDEA model. This theoretical
model has been described in a paper by Fuller‐Rowell et al.
[2008] to demonstrate the impact of terrestrial weather on
the upper atmosphere. The Integrated Dynamics through
Earth’s Atmosphere (IDEA) model consists of the Whole
Atmosphere Model (WAM) –a general circulation model
(GCM) –built on the operational U.S. National Weather
Service (NWS) Global Forecast System (GFS) model and
extends to an altitude of 600 km by increasing to 150 layers
and taking into account the different physical processes that
occur in the upper atmosphere (see Fuller‐Rowell et al.
[2008] for details). The resolution of the model is 1.8° ×
1.8° in latitude and longitude. The WAM model is coupled
to the Global Ionosphere Plasmasphere (GIP) model through
a self‐consistent, global electrodynamics solver that pro-
vides GIP with global electric fields as an input. Successful
comparisons between observed sharp E×Bdrift gradients
and the theoretically calculated E×Bdrift gradients would
represent a “landmark”capability in explaining the inter-
actions between tropospheric and ionospheric mechanisms.
[17]Acknowledgments. The C/NOFS mission is supported by the
Air Force Research Laboratory, the Department of Defense Space Test Pro-
gram, the National Aeronautics and Space Administration (NASA), the
Naval Research Laboratory, and the Aerospace Corporation. At the Univer-
sity of Colorado, this work is supported by AFOSR grant FA9550‐09‐0408.
This work is supported at the University of Texas at Dallas by NASA grant
NAS5–01068.
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