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Journal of Atmospheric and Solar-Terrestrial Physics 173 (2018) 14–22
Contents lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics
journal homepage: www.elsevier.com/locate/jastp
Asymmetric DE3 causes WN3 in the ionosphere
Jinzhe Jiang a, b, c, d, Weixing Wan a, b, c, d, *, Zhipeng Ren a, b, c, d, Xinan Yue a, b, c, d
a
Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China
Institutions of Earth Science, Chinese Academy of Sciences, Beijing, China
c
Beijing National Observatory of Space Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China
d
College of Earth Sciences, University of Chinese Academy of Sciences, Beijing, 100049, China
b
A B S T R A C T
This study investigates a mechanism to generate the wavenumber-3 longitude variation in the ionosphere, using the simulations with the Global Coupled Ionosphere
Thermosphere Electrodynamics Model, developed by the Institute of Geology and Geophysics, Chinese Academy of Sciences (GCITEM-IGGCAS). Due to the asymmetry
of geomagnetic field, the asymmetric Hough mode of diurnal eastward wavenumber-3 (DE3) also produces the WN3 structure in the ionosphere by coupling with the
magnetic line. The densities of the neutral mass and the plasmas in the ionosphere are studied in detail. The results show a clear WN3 pattern driven by tide's electrodynamical coupling. We then conclude that the asymmetric component of the DE3 can also cause the WN3 structure in the ionosphere, which confirms the assumption
that more than one source could generate WN3 structure in previous studies.
1. Introduction
Ionosphere-thermosphere coupling is a complicate and important
process in the upper atmosphere. Some activities in the mesosphere and
low thermosphere (MLT) region can affect the variations in low latitude
ionosphere by ionospheric dynamics. The researches about it have shown
lots of interesting results such as the longitudinal wavenumber-3/
4(WN3/4) structure found in the ionosphere and thermosphere. WN4
structure was reported by Sagawa et al. (2005), which found four longitudinal hotspots in the equatorial ionospheric anomaly (EIA) in the
nighttime airglow intensity data of IMAGE satellite. Using the observational data of radio occultation from satellites, the researches of electron
density showed the wavenumber 4 patterns more obviously (Lin et al.,
2007; Liu et al., 2009b; Pancheva and Mukhtarov, 2010). The plasma
density by other means data displayed the same phenomenon, like satellite in-situ observation from ROCSAT-1, CHAMP and DMSP (Lühr et al.,
2007; Kil et al., 2007, 2008; 2009; Liu and Watanabe, 2008; Pedatella
et al., 2008; Ren et al., 2008, 2009a; 2009b; Liu et al., 2009a; Bankov
et al., 2009; Huang et al., 2010). More following works about it indicated
that many relevant parameters in ionosphere and thermosphere also had
the characteristic of WN4 structure, for instance, the total electron content (TEC) (Scherliess et al., 2008; Wan et al., 2008, 2010), zonal wind at
400 km (Lühr et al., 2007), thermospheric neutral temperature (Forbes
et al., 2009) and thermospheric mass density (Liu et al., 2009a; Kwak
et al., 2012).
Immel et al. (2006) suggested that ionospheric WN4 structures may
be caused by the non-migrating tidal mode diurnal eastward
wavenumber-3 (DE3), which is one of the largest diurnal tides in MLT
region. DE3 tide is considered to be excited by latent heat release and
cumulus convection heating in the troposphere, then the tide can propagate upward into ionosphere region to affect the variations there. By
analyzing the WN4 structure's features and comparing with that of DE3
tide, many studies proved the connection between them (Wan et al.,
2008, 2010; England et al., 2009; Immel et al., 2009; Wu et al., 2009).
More detailed researches about the source of WN4 separated DE3 tide
into symmetric and antisymmetric components to study the cause of its
formation. Ren et al. (2010) found that DE3 tide's symmetric wind
component played a more important role than antisymmetric wind in
controlling WN4 structure in the simulations. The same conclusion of two
components' effect on WN4 structure was shown in (Wan et al., 2010),
which studied the relationship between WN4 and DE3.
Compared to the WN4 structure, less attention was paid to the WN3
variation in the ionosphere. However, there did exist some discovery
about it using the similar methods as used in WN4 investigations. Pedatella et al. (2008) found that the longitudinal structure of the EIA plasma
density obtained from the CHAMP in-situ data became WN3 longitude
structure during January and December, and the main reason was
thought to be the considerable DE2 tide's modulation on the dynamo
electric fields in northern winter. England et al. (2009) observed an
obvious WN3 structure during July in the airglow data and it had the
same maximum with DE2 tide in northern summer. With the researches
of the coupling between thermosphere and ionosphere going on, more
* Corresponding author. Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China.
E-mail address: wanw@mail.iggcas.ac.cn (W. Wan).
https://doi.org/10.1016/j.jastp.2018.04.006
Received 9 January 2018; Received in revised form 10 April 2018; Accepted 10 April 2018
Available online 11 April 2018
1364-6826/© 2018 Published by Elsevier Ltd.
J. Jiang et al.
Journal of Atmospheric and Solar-Terrestrial Physics 173 (2018) 14–22
Fig. 1. Simulations at 406 km altitude and 15:00 local time under low solar activity conditions. The contours in the left column indicate the normalized density
averaged in month, and the data in Fourier transform is showed in the right column. The top row represents the symmetric Hough mode's results and the bottom row is
for the antisymmetric one.
thermospheric WN4 waves at low solar activity. In this simulation, the
effects influences on the thermosphere and ionosphere from below with
above mentioned two mechanisms were separated by controlling the
different combinations of lower boundary conditions and the electric
fields. This method shows the upper response in the MLT and ionosphere
to the below clearly and makes great contributions to the research on the
coupling between ionosphere and thermosphere.
Previous work about the WN4's origin used the view of symmetric
properties of DE3 and assumed two different mechanisms affecting the
upper ionosphere separately. Many valuable results of the WN4's characteristic have been obtained. But the DE3's antisymmetric part was
ignored by researchers as the response to its symmetric part is much
stronger and fits WN4's structure well. In order to know how the lower
tides affect the ionosphere, each component and mechanism needs to be
considered. Hence, in this study we will use the similar way as Wan et al.
(2010) with Hough Mode as the lower boundary in the simulations,
combining two components of DE3 tide and two effect mechanisms to
investigate the DE3 tide's influence on ionosphere in detail.
other tide modes were involved in this area. When analyzing the annual
variation of the WN4 structure, H€ausler and Lühr (2009) proposed that
WN3 structure could be caused by DW4, DE2, SW5, SE1 and SPW3 tide
modes. Kil et al. (2010, 2013) studied the WN3 structure's characteristics
in the plasma density, vertical E B drift and O/N2 ratio to analyze the
connection of the ionosphere longitudinal structure with thermosphere
tide, and suggested that DE2 is probably the main source of the ionospheric WN3 structure and other process may also contribute. It is proved
by (Pancheva and Mukhtarov, 2011; Pancheva et al., 2012), which
concluded that DE2 was the main factor but SPW3, DW4 and SE1 waves
had considerable effects causing the WN3 structure using COSMIC and
TIMED data. However, other tides, which may also affect the ionospheric
WN3 structures through the nonlinear mechanisms, was not considered.
The response in the ionosphere and thermosphere due to the nonmigrating atmospheric tide in the MLT could be produced by two
mechanisms: the upward tidal wave propagation and the electrodynamical coupling. Oberheide and Forbes (2008) suggested that the
DE3 tide could propagate into upper ionosphere straight forward and
affect ionospheric variations as a local source (Oberheide and Forbes,
2008; Oberheide et al., 2009). The electro-dynamical mechanism means
that the non-migrating tides influence the longitudinal structures in the
F-region by modulating the electric fields in E-region, which was
demonstrated in some model simulations (England et al., 2008, 2010; Jin
et al., 2008; Oh et al., 2008; Fang et al., 2009; Ren et al., 2010). Moreover, traveling planetary waves like the quasi-2 day wave and the 6.5 day
wave impact the ionosphere status by modulating the dynamo, changing
thermospheric O/N2 ratio and interacting with tides (Gan et al., 2015;
Yue et al., 2016).
Wan et al. (2012) run the GCITEM-IGGCAS model to simulate the
DE3-WN4 coupling process. They concluded that the electro-dynamical
coupling controls all of the ionospheric WN4 and most thermospheric
WN4, and the upward tidal wave propagation mechanism affects the
2. Result
In this work, we use GCITEM-IGGCAS model to investigate ionospheric response to non-migrating tides below through simulations. This
3-D model is a self-consistent one that can calculate the main parameters
in the ionosphere and thermosphere, including the number densities and
temperature of neutral components, ions and electron, as well as neutral
wind and electric field vectors. The parameters distribute at a spherical
geographical coordinate frame with the resolution of 5 in latitude vs.
7.5 in longitude, at the height dimension from 90 km to 600 km in
altitude. Furthermore, as a self-consistent part of GCITEM-IGGCAS, the
electric fields model is an independent one in the model and can be drove
by other input. It makes great convenience to design our simulations for
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J. Jiang et al.
Journal of Atmospheric and Solar-Terrestrial Physics 173 (2018) 14–22
(F107 ¼ 210), to consider the solar radiation effect on the ionosphere.
The neutral and plasma density ratio of ionosphere response to the
“quiet” condition as the normalized values are chosen to show the influence of DE3 tide's Hough mode on ionospheric variations. The wave
number distribution is calculated by Fourier transform.
the purpose of studying the effects induced by two mechanisms
mentioned before separately. The readers are referred to Ren et al.
(2009b) for the details of the model.
The lower boundary tide adopts the DE3's Hough mode at 90 km.
According to the assumption of the classic tidal theory, we can deduce
the Laplace's tidal equation, and its Eigen solutions are called Hough
modes, which represent the latitudinal structures of tides. Each tide has
different orders of Hough modes with various shapes and scales in latitude, and is separated into symmetric and antisymmetric structures with
respect to the equator. Because we focus on the ionospheric responding
of tides below at low latitude, so the first orders of DE3 tide's symmetric
and antisymmetric mode are selected to drive the simulations. They
represent the most normal variation form in the actual case and their
amplitudes peak in the target area exactly. Then each mode is used in the
simulation independently.
Our simulation is designed in the similar way as Wan et al. (2012) to
separate two coupling mechanisms' effects. Using single tide at the
boundary in one simulation is the main method of our study, though the
superposition of multiple waves could cause some other effects, such as
the horizontal gradients and wavenumber features (Ward et al., 2010), it
is reasonable to do this approach because we focus on the ionospheric
response to the specific tide. And without other tides' disturbance, single
tide's effect on ionosphere could be considered as linear, then the linear
transform on the input tide would not change the results' features. First,
we define a reference “quiet”, during which the model runs without tides
at the lower boundary and the electric fields calculated self-consistently
in the simulation. This “quiet” data provides a background weighing the
coupling effects' magnitude and the input “quiet” electric fields used in
other simulations. Then we put different kinds of tide modes into the
simulation as a “disturbance” situation. The difference between “disturbance” and “quiet” is considered as the influence of each tide mode. The
simulations run in two solar activity levels, low (F107 ¼ 70) and high
2.1. Different DE3 components’ effect
We know non-migrating tides' amplitudes are weaker than those of
migrating tides at the bottom boundary, so in order to show different
modes' influence obviously, we enlarge the amplitude of input Hough
mode equal to the migrating tide's scale and keep it constant in the
simulation during whole year.
First we use DE3's symmetric and antisymmetric Hough mode as the
boundary below to “disturb” the ionosphere. Fig. 1 shows the “disturbed”
wave patterns of thermosphere neutral mass density as mentioned
before, WN4 structure is mainly caused by DE3's symmetric component
as shown in contours (a) in Fig. 1, and the wave pattern distributes
around equator from 30 to 30 symmetrically. Contours (b) in Fig. 1
show the wave number distribution calculated by Fourier transform. As
indicated, WN4 is the dominated pattern. The antisymmetric component's result is a bit complicated. The density pattern in contour (c) does
not show a regular pattern and the amplitude is lower than that in (a). But
in contour (d) the amplitude of WN3 is visible and even comparable with
WN4. Different from (b), WN4 occurs only within 30 latitude with a
smaller amplitude symmetrically. It is easy to understand WN4's pattern,
because the anti-symmetric mode in this area modulates the ionosphere
in each hemisphere. But the WN3 part can't be explained easily with the
existed theories regarding WN3. So we will study the anti-symmetric
mode's influence in two mechanisms separately: the upward tidal wave
propagation and the electro-dynamical coupling.
The “tide disturbance” case is devised as following: the anti-
Fig. 2. Thermosphere neutral mass density response simulated with DE3 symmetric and antisymmetric Hough modes as the boundary for low solar activity in two
mechanisms. Contour (a) (b) represent the electro-dynamics coupling and tide upward propagation's effects with symmetric mode, (c) (d) represent the results with
antisymmetric modes.
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Journal of Atmospheric and Solar-Terrestrial Physics 173 (2018) 14–22
Fig. 3. Thermosphere neutral mass density response and wave number distribution simulated with DE3 antisymmetric Hough mode for two solar activities. (a) (b)
represent the results for low solar activities for F107 ¼ 70, (c) (d) represent those for high condition as F107 ¼ 210.
symmetric Hough mode is input as the boundary and the electric fields
mode uses the “quiet” electric fields mentioned before. Obviously this
one offers the data driven by tides below only that presents the upward
tidal wave propagation. The case “total disturbance” means that the
model runs with the tide's Hough mode and the self-consistent electric
fields, with two mechanisms considered together this time. According to
the definitions above, we can take out the diverse response of each
process by combining any two runs: the difference between “tide
disturbance” and “quiet” is the influence of direct upward propagation of
tides; the difference between “total disturbance” and “tide disturbance”
is the effect of electro-dynamical coupling mechanism. Fig. 2 shows the
wave patterns of thermosphere neutral mass density simulated at 406 km
altitude and 15:00 local time under low solar activity by these two
mechanisms. The four contours in the picture all indicate the normalized
density with yearly average versus geographic longitude and latitude.
The top and bottom rows represent the ionospheric responses to DE3's
symmetric and anti-symmetric Hough modes, respectively. Two columns
represent electro-dynamical coupling mechanism and tidal upward
propagation effect, respectively. Contours (a) and (b) in Fig. 2 show the
marked WN4 pattern concentrating in the region from latitude 30 to
30 . The amplitude of contour (a) is smaller than that of (b), which
means that the tidal propagation is a more important role in generating
WN4 structure than electro-dynamical coupling at low solar activity. This
conclusion is consistent with Wan et al. (2010)'s simulation results, in
which he found that WN4 pattern in thermosphere could be caused by
both mechanisms of DE3 tide below, and was dominated by tidal upward
propagation during low solar activity, while with the increasing solar
activity, propagation component became weaker than the other one,
therefore the electro-dynamic coupling controlled the wave pattern
during high solar activity. The wave pattern's characteristics also depend
on solar activity in our result, and some will be shown later.
The amplitude in subplots (c) and (d) are comparable, but they are all
much smaller than those of (a) and (b). Furthermore, the patterns in (c)
and (d) appear a significant distinction from (a) and (b), not showing a
traditional WN4 pattern. The different influence on ionosphere between
DE3's symmetric and anti-symmetric component have been discussed by
Wan et al. (2010) and Ren et al. (2010) in their simulations. They suggested that the symmetrical zonal wind of DE3 was the main driver of the
WN4 structure. DE3's anti-symmetric mode does not contribute to the
WN4 structure but shows some interesting characteristics in our simulation. The electro-dynamic coupling effect of DE3's anti-symmetric
Hough mode displaying in contour (c) appears a symmetric structure
about equator, and seems like a WN3 pattern concentrating in the region
from latitude 60 to 60 . The study about the influence of DE3's
anti-symmetric component before only pointed out the missing of WN4
but there was no more specific information of it such as the shape or the
intensity. Moreover, WN3 structure was thought to be generated by DE2
tide below mainly, and other tides like SE1, DW4 and SPW3 also provided some contributions according to the works on the coupling of
thermosphere and ionosphere (H€ausler and Lühr, 2009; Pancheva and
Mukhtarov, 2011, 2012). So, the “strange” structure driven by DE3's
anti-symmetric Hough mode seems like a new source of WN3 in the
ionosphere. The tidal upward propagation effect of DE3's anti-symmetric
Hough mode are shown in contour (d). Different from the other three
contours, this pattern is anti-symmetric about equator and the visible
wave covers all the latitude range. Each hemisphere exists a broken WN4
pattern with eastward shift from equator to pole. The anti-symmetry of
the data is consistent with the input Hough mode's property, indicates
that the tidal upward propagation may be a local mechanism——the tide
only affects the area over. But it is difficult to explain the shift of wave
pattern. We then attribute the phenomenon to the tides' interaction in
equator that effects the position of the wave structure. Four combinations
between two mechanisms and two Hough modes enable separating the
DE3's effects on ionosphere in brief. Then we investigate the characteristic of WN3 pattern driven by DE3's anti-symmetric Hough mode in
detail.
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Journal of Atmospheric and Solar-Terrestrial Physics 173 (2018) 14–22
Fig. 4. Wave number distributions of netural density simulated by DE3's symmetric (a), antisymmetric component (b) and them together (c) for low solar activities.
The bottom plot shows WN3 amplitude averaged in latitude.
Fig. 5. Same as Fig. 4 but for a high solar activity F107 ¼ 210.
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J. Jiang et al.
Journal of Atmospheric and Solar-Terrestrial Physics 173 (2018) 14–22
by both. The WN3 amplitude averaged in latitude is plotted on the bottom row with respect to months.
The top row in Fig. 4 shows that the wave number 4 is the main
response to the symmetric Hough mode of DE3 and it occurs from latitude 30 to 30 symmetric about equator. The peak value appears at
August and September, while minimum occurs in December and January.
In previous works about WN4's features, its amplitude regularity shows
an annual variation with a peak around June to October and a minimum
near winter solstice. Our results of symmetric component's simulation
accord with the annual variation perfectly, which proves the importance
of symmetric component to the WN4 structure in the ionosphere. The
second row is the results simulated with HM2 only. The same as shown in
section 2.1, WN3 dominates the response. The amplitude is smaller than
that driven by HM1, and the annual variation is much different from the
WN4 structure. There are two peaks occurring in February and October,
and two minimum in May and January. But the WN3 pattern in the
ionosphere shows a semi-annual variation with two peaks in May and
December in previous studies (Pedatella et al., 2008; England et al.,
2009). The deviation between our simulation and previous conclusion
needs further study. The simulations with HM1 and HM2 together are
displayed on the third row and the contours have the similar configuration to the top row. It means that the symmetric component controls
the response as we mentioned before.
Fig. 5 illustrates the wave number distributions simulated with the
same way of Fig. 4 but for the high solar activity. The entire amplitude of
Fig. 5 is weaker than that of Fig. 4, consisting with the solar activity
restrain effect and has the same annual variation with Fig. 4. The pattern's latitude range on first row becomes larger than the condition for
low solar activity, from latitude 60 to 60 . This result suggests a new
characteristic of WN4 with solar activity: the WN4 pattern expands in
latitude with the increasing solar activity.
The wave number distributions of plasma density obtained with the
2.2. WN3 structure caused by DE3
The studies of DE3's influences on ionosphere focused on the relationship with WN4. We mainly discuss the WN3 structure caused by
DE3's antisymmetric component here. Fig. 3 shows the WN3 patterns
simulated with DE3's anti-symmetric Hough mode for two solar activities. The contours in the left column indicate the normalized density
averaged during the whole year, and the data after Fourier transform is
showed in the right column. The top row represents a low solar activity
that F107 is 70 and the bottom row is for a high solar activity that
F107 ¼ 210. The same as the case for low solar activity, contour (c)
representing result for the high level shows a WN3 pattern concentrating
from latitude 60 to 60 , symmetric about equator. The right two plots
show this phenomenon more obviously: being the most prominent
component, and the WN3 dominates the distributions. We know that the
increasing solar activity could restrain the WN4 structure intensity in
ionosphere. In Fig. 3 the WN3 structure appears the same dependency on
solar activity, and its amplitude in low F107 condition is larger than that
in high condition.
The WN3 structure could be driven by tide DE3's asymmetric Hough
mode in ideal conditions as shown in Fig. 3, then we wonder what
happens in actual case when the boundary is specified by the empirical
DE3 tide. This DE3 tide are fitted from the data of TIMED/SABER in a 60
day window centered at the September equinox, and then averaged from
2002 to 2008. Using the least square method, we calculate the empirical
DE3 tide's Hough mode components, its first two solutions are the symmetric Hough mode HM1 and anti-symmetric Hough mode HM2. Besides
HM1 and HM2 being the input tide separately, the sum of HM1 and HM2
is also used in the simulation as the lower boundary in contrast. Fig. 4
illustrates the wave numbers distribution of the neutral mass for
12 months at low solar activity. The top two rows display the results
simulated by HM1, HM2 separately, and the third row is that simulated
Fig. 6. Wave number distributions of plasma density simulated by DE3's symmetric (a), antisymmetric component (b) and them together (c) for low solar activities.
The bottom plot shows WN3 amplitude averaged in latitude.
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Journal of Atmospheric and Solar-Terrestrial Physics 173 (2018) 14–22
Fig. 7. Same as Fig. 6 but for a high solar activity F107 ¼ 210.
Wan et al. (2010) showed the different process of symmetric and
anti-symmetric component effecting on the upper layer well. The symmetric wave mode produces the same electric field distribution in the
northern and southern hemispheres, and it can be added to the F layer
and maintains the original trend. In contrast, a different direction or
phase of the electric field will be generated by the wave mode in the
same simulations are displayed in Fig. 6 and Fig. 7, for two solar activities, respectively. The plasma density's patterns on the first row appear a
symmetric structure with the equator occurring at about 15 –30 latitude, following the normal WN4 annual variation of one peak in autumn.
The anti-symmetric component's results shown on second row are more
complex than first row. Different from that of the neutral density, the
WN3/4 driven by anti-symmetric component have the comparable
amplitude. The HM2's WN3 amplitude have three peaks in February, July
and October at low solar activity, and two peaks in February and October
at high solar activity. The third row concludes that symmetric component
controls the responding in the plasma density too. The plasma's diversity
of wave number pattern in latitude proves that solar activity could
expand the distribution in the meridional direction.
3. Discussion
This work mainly studies the influence of the bottom tides on the
upper layer by the electro-dynamics coupling mechanism. In the simulations DE3's antisymmetric wave mode can product WN3 structure. But
its character is not same as the empirical WN3. The annual variation of its
amplitude peaks in February and October, which is much different from
previous conclusion. While the HM2 calculated in our simulations has the
similar variation and the symmetric mode's result is consistent with the
HM1. So we believe that tide's intensity controls the WN3 structure's
annual variation generated by the electro-dynamics coupling mechanism. Another issue is the tiny amplitude of this WN3 during whole year.
But it is easy to understand by the relevant researches, because we know
that the main effect of the DE3 tides on the upper layer is the symmetrical
wave mode and the anti-symmetrical mode provides little contribution. It
seems that the WN3 structures produced by antisymmetric mode of DE3
are significantly smaller and cannot be regarded as a significant contribution to the WN3 structure of the ionosphere. But the mechanism
causing this structure is an interesting and useful theory.
Fig. 8. DE3 antisymmetric mode's wave number amplitude averaged in latitude
of netural density of for different magnitude conditions. The blue line represents
the results in Earth magnetic field, the green line represents the dipole field with
magnetic dip angle 0 and the red line represents the dipole field with magnetic
dip angle 11 . (For interpretation of the references to colour in this figure
legend, the reader is referred to the Web version of this article.)
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Journal of Atmospheric and Solar-Terrestrial Physics 173 (2018) 14–22
phenomenon provides new possibilities. Tides' antisymmetric modes can
create other waveforms through the coupling of the magnetic lines.
4. Conclusion
We have simulated the ionospheric responses to different DE3's
Hough mode as the boundary below. The analysis of ionospheric parameters' variation obtained in the simulations are consistent with the
conclusions that the symmetrical wave mode of DE3 tides is the main
driver on the upper layer, and the symmetric mode is the main contributor to WN4 structure. We also find that the asymmetric component is a
contributor of WN3 structure. By coupling with the Earth magnetic fields
the anti-symmetric mode of DE3 generates a WN3 pattern and the intensity is the main factor affecting the structure's annual variations.
Acknowledgments
This work was supported by National Science Foundation of China
(41474133,41674158, 41621063, 41427901, 41504119), Youth Innovation Promotion Association CAS(2014057), Thousand Young Talents
Program of China and the Opening Funding of Chinese Academy of
Sciences dedicated for the Chinese Meridian Project.
Fig. 9. The amplitudes of WN3 structures generated from DE2, DE3, SE1, DW4
and SPW3 as boundary below in the simulations.
north and south hemispheres, and the electric fields cancel out by
coupling with magnetic line, then it could weaken the influence of the
anti-symmetric mode below. By analyzing the antisymmetric mode's
coupling way, Wan et al. (2010) think this mechanism in F layer can
make the antisymmetric mode's effect disappearance. However in the
simulation we find that the amplitude is weakened a lot truly, but not
disappeared simply. The results show three wave's structure instead of
four waves. As known, the Earth magnetic field is not a standard dipole
field, so we think the reason may be that the asymmetry of the Earth
magnetic field prevent the canceling out perfectly, then form a WN3
structure. To prove the guess, two groups of simulation are made in a
dipole field with magnetic dip angle 0 and 11 in contrast. Fig. 8 shows
the wave number amplitude of DE3's antisymmetric mode averaged in
latitude of three conditions, the blue line represents the results in Earth
magnetic field, the green line represents the dipole field with magnetic
dip angle 0 and the red line represents the dipole field with magnetic dip
angle 11 . The much smaller amplitudes of red and green lines support
the assumption above. The result of dipole field with 0 dip angle is easy
to understand: the field is symmetric with equator, so the effects from
two hemispheres are canceled out entirely, there is no WN3 structure
created. The case of dipole field with 11 has the similar result, the input
tide in the simulation spreads on middle and low latitude and affects the
same region, in this area the field shows the same form with 0 dip angle's, then there is no WN3 either. So the DE3's anti-symmetric mode can
produce WN3 structure by coupling with the Earth's asymmetry magnetic
fields.
This mechanism differs from the previous theory of three wave
structure's generation. The tide wave modes, such as DW4, SPW3, DE2
and SE1, are widely considered to be the sources of the WN3 structure. So
we make a comparison of WN3 structure generated from these tides
(DE2, SE1, etc.) using similar way to show the contribution of DE2's WN3
in Fig. 9. There are 5 tides used separately in the boundary to generate
WN3 structure in the simulations, the tides are from the same source of
empirical DE3. In the figure the blue line represents the WN3 from DE2,
which is largest of all, the peak is about 2.1% appearing in June and the
valley value is about 0.7% in March. The carmine line represents the
SPW3's effect, it is a bit weaker than DE2's, and the peak is in July of
about 1.6%. The black line represents SE1's effect, which has similar
magnitude with SPW3's. Its peak appears in May, the value is about 1.5%.
The green line represents WN3 from DW4, its value is much smaller with
a peak about 0.2% in July. The weakest one is red line which represents
DE3's WN3, its peak is about 0.1% in September. So we can know that
WN3 structure in ionosphere are mainly caused by DE2, SE1 and SPW3.
DE3 and DW4 provide little contribution to WN3. However, this
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