Introduction
The equatorial ionization anomaly (EIA) is an important characteristic of the
low-latitude and equatorial ionosphere. The EIA is produced by the equatorial
plasma fountain effect that elevates the equatorial and low-latitude
ionospheric plasma to higher altitudes in the F region. The fountain effect
is a daytime phenomenon characterized by the upward plasma drift that results
from interactions between electric and magnetic fields in the E region. In
the F region, the transport processes are important because of the
small recombination rates. Due to the action of gravity and the pressure gradient,
this plasma diffuses down along the magnetic field lines to latitudes away
from the geomagnetic equator (15–20∘), creating two crests with
larger ionospheric plasma density in both hemispheres north and south of the
magnetic equator (Namba and Maeda, 1939; Appleton, 1946; Rishbeth, 2000;
Abdu, 2005).
Global coverage measurements made by satellites have provided observational
studies which have shown the presence of four peaks in global longitudinal
structures from global local time observations of EIA. The wavenumber-4
(wave-4) longitudinal structure can be observed in several ionospheric parameters, for
instance electron density (Ne), total electron content (TEC) and
ionospheric F2-layer peak height (hmF2) (e.g., Lin et al., 2007a, b,
c; Liu and Watanabe, 2008; Lin et al., 2009; Liu et al., 2010). The wave-4
structures seen in the ionosphere are related to the DE3 wave (the eastward non-migrating diurnal tide with wavenumber 3, “E”
for eastward) that comes from latent heating in the troposphere. The wave-4 structure is a result of the
predominant wave-4 topography, which is reflected in the diurnal and
semidiurnal components of the latent heating rates due to deep tropical
convection (Forbes, 2007).
The four peaks in the longitudinal structure in the equatorial anomaly were
first observed by Sagawa et al. (2005) using atomic oxygen OI 135.6 nm nightglow emission images
obtained by far ultraviolet (UV) emission from IMAGE (Imager for
Magnetopause-to-Aurora Global Exploration) satellite observations from March
to June 2002. The authors suggested that the possible cause of these
structures could be attributed to non-migrating tides propagating from below
because factors like those related to differences between the geomagnetic and
the geographic equator or the declination angle of the geomagnetic lines
could not explain the observed four-peaked structure.
The Sagawa et al. (2005) supposition was confirmed by Immel et al. (2006) who
indicated the existence of good correlation between the Sagawa et al. (2005)
ionospheric imaging results and the upper atmosphere tidal parameters from
the GSWM (Global Scale Wave Model). According to the authors, the tidal
amplitudes decay in the E-region altitudes, making it impossible to affect the F region
directly. In this way, a possible scenario would be one in
which the tidal modulations could reach higher ionospheric altitudes by
the E-layer dynamo mechanism.
Hagan et al. (2007) reported a series of simulations with the TIME-GCM
(Thermosphere Ionosphere Mesosphere Electrodynamics general circulation
model) which reproduced the IMAGE observational results and confirmed
that tides coming from the lower atmosphere could affect the EIA and
consequently impact the ionosphere. The results identified that the
non-migrating tidal component responsible for the EIA four-peak structure is
the DE3.
Non-migrating tides are global-scale waves with periods that have harmonics
of a solar day and their origin is associated with interactions between
tides and gravity waves, gravitational pull of the Sun, large-scale latent
heat release, absorption of solar radiation, nonlinear interactions between
particular sets of global-scale waves and other interactions (Hagan and
Forbes, 2003). The migrating tides move with the apparent motion of the Sun
when they are observed from the ground, and their origin is associated with
the absorption of infrared light in the troposphere and the UV light by
ozone in the stratosphere.
The wave-4 structures can be noticed in periods of low and high solar
activity. According to Scherliess et al. (2008), this is indicative of the fact that
the coupling between the lower atmosphere and the ionosphere occurs independently of
solar flux conditions. The authors used more than 5 million low-latitude TEC
observations from the TOPEX (Ocean Topography Experiment) satellite, from
August 1992 until October 2005. They found that the longitudinal TEC structure
was not changed by the solar cycle conditions.
Oberheide et al. (2011a), using TIMED (Thermosphere Ionosphere Mesosphere
Energetics and Dynamics) temperature and wind observations, from SABER
(Sounding of the Atmosphere using Broadband Emission Radiometry) and TIDI
(TIMED Doppler Interferometer) instruments, respectively, studied the
contributions to the four-peak structure in the ionosphere of the
following non-migrating tides: DW5, DE3, SW6, SE2, TW7 and TE1 (D, S and T
for diurnal, semidiurnal and terdiurnal tides, respectively. W and E are for
westward and eastward, respectively; the numbers are related to the tidal
wavenumbers). All these waves can be observed as having a four-peak structure
by the quasi-Sun-synchronous satellite using satellite sampling. Besides the
tidal waves, the study also considered the stationary planetary wave with wavenumber
4 (SPW4). They demonstrated that the wave-4 structure in the E region
was caused not only by the DE3 but also by a number of other waves, mainly
the SE2 tidal component and SPW4. They also concluded that the modes TE1,
DW5, SW6 and TW7 did not add substantially to the wave-4 amplitude.
Using the NCAR (National Center for Atmospheric Research) TIME-GCM model,
Pedatella et al. (2012) performed numerical simulations using DE3, SE2 and
SPW4 waves, for solar minimum conditions during the September equinox, in order
to study the formation of the four-peak structure in the low-latitude
ionosphere. Their study presented results indicating that SE2 is not a
contributor to the wave-4 longitude variation. The authors indicated that the
SPW4 could be a result of a nonlinear interaction between DE3 and DW1
(the migrating diurnal tide, “W” for westward). It was also shown that the four-peak structure noticed in
the September equinox was carried mainly by a combination of DE3 and SPW4
waves.
Magnetic latitude interval of observations. Illustration of the
ionospheric data used in the present study, which includes observations
in the region ±20∘ around the magnetic equator. The example above is
the NmF2 data taken during a time interval between 12:00 and
14:00 LT.
Chang et al. (2013), using 2007–2011 COSMIC (Constellation Observing System
for Meteorology, Ionosphere, and Climate) observations, noticed that the wave-3 and wave-4 longitudinal structures seen in the TEC data are due to a
combination of DE2 and SPW3, and DE3 and SPW4, respectively. The authors did
not observe significant contributions from SE1 and SE2 modes. It was found that
DE3 and SPW4 relative amplitudes have their maximum and minimum in 2008 and
2010, respectively, differing from the absolute amplitudes, which were
correlated with the solar flux observations, possibly caused, according to the
authors, by interannual variability in the DE3 component in the mesosphere–lower thermosphere
(MLT) region due to lower and middle atmospheric sources such as ENSO (El
Niño–Southern Oscillation) and QBO (quasi-biennial oscillation). The relative
amplitudes were calculated by normalizing the tidal and planetary waves'
amplitudes by the maximum zonal mean TEC in the low-latitude region in order
to remove the solar cycle influence. DE3 showed major amplitudes in the Southern
Hemisphere in comparison to the Northern Hemisphere. Waves' amplitudes considered
in their study also showed differences for both hemispheres.
The main subject of the present study is to observe the wave-4 structures in
ionospheric parameters during different solar activity periods around the September equinox and verify which waves will result using bi-spectral
Fourier analysis from NmF2 (the density of the F layer peak),
hmF2 and Ne observations. It is know that large ionospheric
variabilities in these parameters come from variation in the electric fields
and winds, mainly due to ion–neutral coupling and electrodynamic
perturbations (Abdu, 2016). The hmF2 is related to effects of
ionospheric dynamo, which consequently is responsible for plasma uplift from
low to high altitudes in the ionosphere. The fountain effect elevates
ionospheric plasma in the regions near the magnetic equator to high
altitudes, where it diffuses down through the geomagnetic field lines to latitudes
located around 15–20∘ from the magnetic equator. In this way it affects
the electron density distribution. It will be possible to see which waves are
more related to the wave-4 pattern, mainly when it appears more prominently.
Studies in the literature have discussed that waves of tropospheric origin
could modify the ionosphere through the dynamo mechanism or through direct
upward propagation (Forbes, 1996). Some ionospheric perturbations can also be
generated in situ.
Observations and results
The Formosa Satellite 3, also known as Constellation Observing System for
Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC or F3/C), is a set of
six microsatellites that monitor atmosphere and space weather with
instruments of radio occultation observations at altitudes ranging from the
troposphere to the ionosphere (Lin et al., 2007c). The satellites have their
final orbit at an altitude of 800 km and a GPS receiver is used to obtain the
atmospheric and ionospheric measurements through phase and Doppler shifts of
radio signals. From the Ne profiles, the NmF2 and hmF2 can
be calculated. The COSMIC satellites give around 24 h of LT coverage
globally and ∼ 2200 electron density vertical profiles per day. The
COSMIC observations provide global coverage of electron density profiles with
uniform distribution and good resolution. More details can be found in
Pancheva and Mukhtarov (2010, 2012) and Ely et al. (2012). The COSMIC data
are available for download from the website:
http://cdaac-www.cosmic.ucar.edu/cdaac/products.html.
NmF2 LT contour plots. (a) LT × longitude
NmF2 contour plots from 2007 to 2015 around September equinoxes.
(b) Amplitudes obtained for non-migrating tides indicating their
frequency (1/day) and respective zonal wavenumber. The corresponding year for
each plot is indicated in the graphs.
The ionospheric parameters hmF2 and NmF2 observed around
September equinoxes from 2007 to 2015 were used to study the wave-4
structures in the ionosphere during distinct solar activity periods. These
two parameters were used because the low-latitude ionospheric morphology is
greatly affected by the electric fields and neutral winds which define its
dependence on latitude, longitude and local time (Pancheva and Mukhtarov,
2012). During the equinox it is known that DE3 and SPW4 reach their maxima
amplitudes (Pedatella et al., 2012). During this season the effects of the
interhemispheric wind are low and the vertical E×B drift is the main factor
related to the longitudinal density structure formation (Oh et al.,
2008).
Using an empirical model from Alken and Maus (2007), Lühr et al. (2008)
studied the non-migrating tides' influence on EEJ (equatorial electrojet)
intensity, and they noticed that the DE3 wave was the main contributor to EEJ
longitudinal variation during equinoxes. In the present study, about 90
days (45 days before and after the September equinox) of data
collected in the region ±20∘ around the geomagnetic equator were
selected for each year. Figure 1 illustrates the distribution of individual
NmF2 data taken for 2008 for a fixed local time (LT) interval
between 12:00 and 14:00. Using a 2 h interval, it was possible to obtain a large amount of data to set up the
temporal and longitudinal averages for each parameter, as we can see in
Fig. 1. After this process, the data were interpolated in order to obtain a
32 × 32 matrix. So, for each year, NmF2 and hmF2
intensity matrices from 0 to 24 h in time, and from -180 to 180∘
in longitude, were obtained. Applying the two-dimensional fast Fourier
transform (fft2) analysis over these matrices, phases and amplitudes of the waves
present in these distributions were obtained. NmF2 and hmF2
variations were plotted in Figs. 2 and 4, respectively, with their
corresponding amplitudes calculated from spectral analysis results.
NmF2 relative amplitudes. NmF2 relative amplitudes
calculated for periods around September equinoxes from 2007 to 2015 for some
waves' signatures observed in Fig. 2, mainly those related to the wave-4
structure, such as SE2, DE3 and SPW4, and those related to the offset between
the geographic and geomagnetic coordinates, such as SPW1 and DW2. The F10.7 solar
flux is plotted in the top graph, together with the DW1 relative amplitude.
Figure 2 shows the LT versus longitude maps averaged for NmF2
variations (left) and the frequencies (1/day) and zonal wavenumbers of the
main non-migrating tides' amplitudes obtained by two-dimensional Fourier
spectral analysis, or fft2 analysis (right). The variations mean that zonal
mean NmF2 was subtracted from observations in order to obtain
variations around zero. The fft2 is a MatLab routine and it computes Fourier
coefficients in one dimension, in time for example, and then using the
resulting coefficients, a new set of coefficients is computed in the other
dimension, in this example, in space. In the continuous Fourier transform if
a quantity with a unit A is computed in the frequency space, then the unit
is A Hz-1. Since the fft2
routine used in this study is a discrete Fourier transform, the units are the
same as the original data. These results encompass all the years used in this
study. In the graphs on the left the maximum intensities of NmF2 are
usually observed during the daytime when the photoionization is higher. For
higher solar periods, NmF2 increases due to variations in the
radiation intensity that cause changes in the electron–ion pairs' production
in the ionosphere. Visually the wave-4 patterns can be observed in 2008, 2009
and 2010. These four peaks are usually located in the Central Pacific
(∼ -180 to -120∘ longitude), South America (∼ -120
to -60∘ longitude), Africa (∼ -20 to 20∘ longitude)
and Southeast Asia (∼ 80 to 120∘ longitude) (Lin et al.,
2007c). From 2011 onward these structures are not well defined in comparison
with those observed during the low solar activity years.
The panels on the right indicate the main waves detected by the fft2. The unit is
the same as the data. We submitted the matrices to the fft2 MatLab routine. From
the results obtained we identify to which wavenumber and frequency the
spectral intensity was related. In the plots what we can see is the wave
amplitude calculated from fft2 coefficients. The following nomenclature
will be used: D, S and T for diurnal, semidiurnal and terdiurnal tides,
respectively; and W and E for westward and eastward propagation, respectively.
SPW refers to stationary planetary waves. The numbers after these capital
letters will be related to the zonal wavenumbers. Positive wavenumbers refer
to westward propagation, while negative wavenumbers refer to eastward
propagation. Some of these wave signatures possibly are observed in the
ionospheric parameters due to modulations in the ionospheric dynamo or
direct changes in the density composition. DW1 in thermospheric altitudes is
caused by EUV (extreme ultraviolet radiation) forcing.
Observing the panels on the right in Fig. 2, it can be noticed that the DE3 wave is
more pronounced during 2007, 2008 and 2015. In 2009 and 2010, the DE3 amplitude
does not show a significant presence. In 2011 and 2012, DE3 appears but with
small amplitude in comparison with other waves. In 2013 and 2014, DE3 seems
to be associated with other tidal modes, like SE1. SW1 is seen in all the
years and is more pronounced during 2009, 2010 and 2012. This wave results
from the coupling between SPW1 and SW2 (Jones et al., 2013). DW2 can be noticed
in 2007, 2008, 2013, 2014 and 2015. Other waves like SPW1 and SPW4 can also be
observed in some of these figures.
The wave-4 structures are usually related to DE3, SPW4 and SE2 non-migrating
waves, which are of tropospheric origin. The fft2 analysis plots in the
Fig. 2 do not indicate SE2 features. According to Jones et al. (2013), DW2 and
D0 are generated in situ in the upper thermosphere due to the non-dipole nature
of the geomagnetic field. These two non-migrating tides come from the
interaction between SPW1 (related to the offset between the geomagnetic and
geographic poles) and DW1 (migrating diurnal tide). The offset between the
magnetic and geographic equator and differences in magnetic declination are
the cause for the longitude variation of the neutral winds in the equatorial
region (Lei et al., 2007).
Figure 3 exhibits DE3, SPW4, SE2, SPW1, DW1 and DW2 relative amplitudes
obtained from the results presented in Fig. 2. The relative amplitudes were
calculated using the ratio between the amplitude of the selected wave and the
maximum zonal mean because the absolute amplitudes reflect the semiannual and
solar cycle variations of the Ne zonal mean. The upper plot shows the diurnal
migrating tide (DW1) relative amplitude and the F10.7 solar flux. We can see
that the solar flux increased from 2007 to 2015, whereas the DW1 relative
amplitude showed an opposite behavior. In the bottom plot of Fig. 3, we can see
some non-migrating and stationary waves' relative amplitudes. DE3 and SPW4
show decreasing values as the solar flux increases. SE2 is a wave of
tropospheric origin, also related to wave-4 structure, but in this example it
does not show substantial variation. DW2, which appeared in several spectral
plots in Fig. 2, also does not show great variation through the years, and the SPW1
wave has its maximum in 2011, when the high solar activity period starts.
(a) LT × longitude
hmF2 contour plots from 2007 to 2015 around September equinoxes.
(b) Amplitudes obtained for non-migrating tides indicating their
frequency (1/day) and respective zonal wavenumber. The corresponding year for
each plot is indicated in the graphs.
Figure 4 shows longitudinal local-time-averaged distributions for
hmF2 (left) with their respective non-migrating (right) tidal
amplitudes. In the panels on the left it is possible to see the wave-4
structures in practically all years, except 2010 and 2011. The F-layer
elevation after 18:00 LT (high hmF2 values) for high solar activity
periods is a signature of the fountain effect intensification after the
sunset because of the pre-reversal peak in the vertical drift (see, for
example, Batista and Abdu, 2004). The right plots of Fig. 4 show that the DE3
wave is present practically in all the years, being more pronounced during
2007, 2008, 2009, 2013 and 2014. In other years its amplitude is smaller or
not evident in the graphs. DW2 mode has significant amplitudes in all the
years. The SPW4 wave is clearly visible from 2007 to 2010 and in 2015. The SW1
mode is present in the years 2007, 2011, 2014 and 2015. SW1 and SW3 are supposed
to be produced by the SPW1 and SW2 (semidiurnal migrating tide) coupling.
SPW4 can also be noticed in the right figures.
hmF2 relative amplitudes
for periods around September equinoxes from 2007 to 2015. The same waves
amplitudes plotted in the Fig. 3 were considered here for the same reasons
pointed earlier. F10.7 solar flux is plotted in the top graph with the DW1
relative amplitude. The results show different results showed in the Fig. 3.
Figure 5 shows the hmF2 relative amplitudes calculated as the ratio
between the selected mode amplitude and its zonal mean. In the top figure,
DW1 shows the same behavior as the F10.7 solar flux. In the bottom figure, DE3
has its maximum in 2013. SPW4 and DW2 have a similar variation, whereas SPW1
and SE2 show steady values over the years. These relative amplitudes do not
exhibit the same behavior observed by the NmF2 relative amplitudes.
This can be due to the fact that the F region height and density respond
differently and at different time constants to mechanical forcing, as
discussed in Batista et al. (2011).
Electron densities (Ne) for 2 years with different levels of solar
activity, 2008 and 2013, around the September equinox, were selected in order to
see which wave signatures will appear in these ionospheric parameters at
different altitude intervals. Ne average matrices taken at each 50 km
interval, from 100 to 800 km, were created. Each Ne matrix was
transformed in an LT × longitudinal contour plot and submitted to a
Fourier two-dimensional spectral analysis. The results are presented in the
forthcoming Figs. 6 and 7.
2008 September equinox period Ne LT contour plots.
(a) LT × longitude maps for electron densities (Ne) for
some altitudes of the 50 km interval, during the 2008 September equinox period.
(b) Amplitudes obtained for their respective non-migrating tides
indicating their frequency (1/day) and respective zonal wavenumber. The
corresponding altitude intervals used are indicated in the plots. The color
scale, in el cm-3, is
shown at the right side of each graph.
Figure 6 shows some of the local time versus longitude electron density
contour plots observed during the 2008 September equinox (left) and their
respective amplitudes for the main non-migrating tides (right). The Ne data
were averaged considering a 50 km altitude interval. The wave-4 pattern is observed in all the altitude
intervals for the 2008 September equinox. The right plots show that the DE3
wave is present from the 200–250 km to the 700–750 km interval. From
300 to 350 km, DW2 starts to have significant amplitudes which can be noticed
up to 750 km. SPW4 is visible in almost all the altitude intervals, except
in the 200–250 km interval. Clearly we see that DE3, SPW4 and DW2 waves are
present from altitudes of ∼ 300 km upwards. It is noticed that in this
case, geomagnetic effects cannot be ignored since DW2 results from the interaction
between SPW1 and DW1. The fft2 analysis for the 100–150 km altitude interval
indicates a large number of waves compared to other altitude intervals. This
shows that many waves reach the E-region altitudes, but only some of them are
able to disturb the ionosphere at higher altitudes.
2013 September equinox Ne LT contour plots.
(a) LT × longitude contour plots for electron density (Ne)
for some altitudes of the 50 km interval, during the 2013 September
equinox period. (b) Amplitudes obtained for non-migrating tides
indicating their frequency (1/day) and respective zonal wavenumber. The
corresponding altitude intervals used are indicated in the plots. The color
scale, in el cm-3, is shown at the right side of each frame.
Figure 7 shows the electron density contour plots in different altitude
intervals observed around the 2013 September equinox (left) and their
amplitudes. Each graph in Fig. 7 has its own color scale. The DE3 component
is present in all the altitude intervals. The SPW4 appears between 100 and
200 km and from 400 to 800 km. Above 500 km only DE3 and SPW4 stand out
among the other non-migrating waves. In some altitude intervals it is
possible to see DW2 (∼ 600–750 km) and SW1 (∼ 150–600 km)
waves. For 2013, DW2 was not observed to be as prominent as in 2008, mainly at
altitudes higher than 500 km. The wave-4 structures are clearly present from
∼ 400 km and above. In the results of the fft2 spectral analysis of
amplitudes, the same behavior observed for the 100–150 km altitude interval during 2008
was also observed for 2013, indicating that many non-migrating tides'
signatures present in the E-region altitudes could not reach the upper
ionosphere.
Figure 8 shows the vertical profiles of the absolute amplitudes of the
diurnal migrating tide and the main non-migrating tidal waves observed around
the 2008 and 2013 September equinoxes. Figure 8 is a compilation of some of the results
presented in Figs. 6 and 7. During 2008, the DE3, SPW4 and DW2 wave modes
have similar profiles and their maximum occurs at 300 km. During 2013, DW1,
DE3 and SPW4 show higher amplitudes in comparison to 2008. The exception is
the DW2 wave. The peak altitude also changes for DW1, DE3 and SPW4. DE3, SPW4
and DW2 do not have the same behavior as seen in 2008 profiles. The peak
altitude of DE3 and SPW4 is 450 km. DE3 has a “secondary peak”
at 250 km, while SPW4 suffers an accentuated decrease. They start to have
similar variation above 400 km. It is important to mention that the DW2
shows more prominence during 2008 than 2013. SPW1 has an amplitude peak
∼ 300 km, and this wave seems to influence the DE3 and SE2 wave. DW2
and SPW4 both decrease around this altitude. Amplitude profile plots showed
that in 2008, DW2 had amplitudes close to DE3 and SPW4, and in 2013, the same
did not happen. In 2008, SPW1 presented smaller values around the F region
peak than in 2013, when its amplitude was largest around 300 km. It can be
noticed that the SPW1 signature is more prominent in 2013, while DW2 is more
prominent in 2008.
Amplitudes' vertical profiles of some waves observed in Figs. 6 and
7. Panel (a) is related to amplitudes observed in the 2008 September
equinox period, while (b) is related to those observed in the 2013 equinox period. The same waves' signatures used in Figs. 3 and 5 were also
considered here. DE3, SE2 and SPW4 are supposed to be related to the wave-4
structures in the ionosphere. SPW1 and DW2 are supposed to be related to the
offset between the geographic and geomagnetic coordinates. The black
horizontal bars on the DE3 plot represent the standard deviation calculated for
some altitude intervals.
Observed and reconstructed NmF2 for 2008 and 2013 around
September equinoxes. Panels (a, c) are related to 2008, while
(b, d) are related to 2013. Panels (a, b) are the
observed NmF2 variation contour plots, also present in Fig. 2.
Panels (c, d) were reconstructed using SPW4, DE3 and migrating tide
waves' (DW1, SW2 and TW3) signatures observed in the spectral analysis (not
shown) at ionospheric altitudes. The reconstruction done for 2013 clearly
shows a wave-4 structure, while the same was not observed for 2008.
It is important to say that in the graphs of Figs. 6 and 7, the four-peak structures become more evident from ∼ 250 km, and at higher altitudes,
the DE3 and SPW4 waves are of more significance in comparison with other
non-migrating waves. Jones et al. (2013) demonstrated that the non-migrating
tidal components could be generated in situ in the upper thermosphere due to
ion–neutral coupling. According to Oberheide et al. (2011b), DW2 and D0 arise
from hydromagnetic coupling between SPW1 and DW1. According to Jones et
al. (2013), the hydromagnetic coupling processes affect tidal winds as a
result of the Lorentz force in the horizontal momentum equation. For a more
complete explanation about this subject, see studies of Richmond (1971) and
Forbes and Garrett (1979). Hagan et al. (2009) showed that the nonlinear
interaction between DE3 and DW1 produces SE2 and SPW4.
Figures 6, 7, and 8 showed that both DE3 and SPW4 were the most prominent
waves in altitudes above the F region peak altitudes. In order to see the
importance of DE3 and SPW4 waves in the wave-4 structure formation, we
decided to use these waves combined with the diurnal, semidiurnal and
terdiurnal migrating tidal modes to build local time versus longitude maps
for NmF2, hmF2 and Ne for 2008 and 2013. Ne was plotted for
three altitude intervals: 100–150, 200–250 and 400–450 km. These results
can be found in Figs. 9, 10 and 11, respectively.
Observed and reconstructed hmF2 for 2008 and 2013 around
September equinoxes. Panels (a, c) are related to 2008, while
(b, d) are related to 2013. Panels (a, b) are the
observed hmF2 variation contour plots, also present in Fig. 4.
Panels (c, d) were reconstructed using SPW4, DE3 and migrating tide
waves' (DW1, SW2 and TW3) signatures observed in the spectral analysis (not
shown) at ionospheric altitudes. Both reconstructions in bottom figures
clearly show a wave-4 structure.
Figure 9 shows the LT × longitude plots obtained for NmF2
during 2008 (left) and 2013 (right) September equinoxes. The upper plots show
the results presented previously in Fig. 2 for the same years. The bottom
plots in the figure show the respective 2008 and 2013 LT × longitude maps
reconstructed using the DE3 + SPW4 + migrating tides. The diurnal
(DW1), semidiurnal (SW2) and terdiurnal (TW3) migrating tides were used in
the reconstructed plots. Using this type of reconstruction, we intend to see
whether it is possible to explain the wave-4 pattern with only these waves. For
2008 the wave-4 pattern is not as pronounced as for 2013. We can conclude that
the sum of DE3 + SPW4 + migrating tides is not enough to explain the
wave-4 pattern observed in the NmF2 data in 2008. Other waves were
also included in other tests not presented here (DW2, SE2, SPW1), but the
plots did not show better results. The plots related to 2013 in Fig. 9
indicated better results than those found for 2008. In the right plots we can
easily observe the four peaks in the reconstructed graph. A possibility taken
into account is that during high solar periods, the DE3, DW1 and SPW4
absolute amplitudes for NmF2 are higher than those observed during
low solar periods. During 2013 it is clear that a combination of DE3, SPW4
and the diurnal, semidiurnal and terdiurnal migrating tides can
satisfactorily reproduce the wave-4 structures observed in the data.
Observed and reconstructed Ne for 2008 and 2013 around September
equinoxes. In this figure plots related to the 2008
September equinox period are in (a) panels, while those related to
2013 are in (b) panels. Three intervals were selected: 100–150,
200–250 and 450–500 km. For each one of these altitude intervals, we show
the observed and reconstructed variations. The reconstructions were done using
SPW4, DE3 and migrating tide waves' (DW1, SW2 and TW3) signatures observed in
the spectral analysis (not shown) at ionospheric altitudes. The altitude
intervals related to each plot in the figure are identified in the plots. The
best comparisons were found in the 450–500 km altitude interval for both
years 2008 and 2013.
Figure 10 shows the hmF2 LT × longitude maps for 2008
(left) and 2013 (right) September equinoxes. Similar to Fig. 9, the upper plots
in the Fig. 10 show hmF2 observed previously in Fig. 4, and the
bottom plots show their respective contour graphs reconstructed using only
the sum of DE3 + SPW4 + migrating tides (diurnal, semidiurnal and terdiurnal). For hmF2, we can observe the wave-4 pattern in reconstructed
data for both years 2008 and 2013. This means that the
DE3 + SPW4 + migrating tides are enough to explain the wave-4 pattern
observed in the hmF2 data. The differences between the bottom and
upper plots are due to the other wave modes. In general, the best comparison
between the observed and reconstructed parameters was found when the DE3 and
SPW4 wave modes had significant amplitudes in the
frequency × wavenumber distributions. In this work we concentrated
on the qualitative aspect of the comparison. The tilt to the east observed in
the reconstructed plot for 2013 could be due to the DE3 wave.
Figure 11 shows the electron density (Ne) LT × longitude contour
plots obtained for around 2008 (left) and 2013 (right) September equinoxes.
The upper plots relate to the 100–150 km interval, the middle plots
relate to the 200–250 km interval, and the bottom plots relate to the 450–500 km
interval. For each altitude interval, we can see the observed data and the
reconstructed contour plots using the sum of DE3 + SPW4 + migrating tides.
For the 450–500 km interval, both years show good agreement between the
observed and reconstructed graphs. This shows that the DE3 and SPW4 modes have
great importance at this altitude and that they are the main waves responsible for
the wave-4 pattern observed. Ne profiles plotted in Fig. 8 showed that this
interval was situated above the altitude of the peaks of the main observed
migrating tides. In the interval of 200–250 km, we can see some similarities
between the upper and bottom plots. The agreement is better for 2008 than
2013. In the 100–150 km interval it can be observed for 2008 that the sum
of DE3 + SPW4 + migrating tides did not result in the wave-4 pattern,
meaning that other mechanisms are necessary to explain the four-peak structure observed. Considering the year 2013, the sum resulted in a wave-4
pattern, but did not show a significant similarity with the observed data.
For altitudes below the E region, it can be seen in the study of Yue et al. (2010)
that a pseudo and reversed-phase wave-4 pattern was evident in the altitudes around E region layers due
to the effect of the Abel inversion. Above 800 km the effects due to the plasma
drift are weakened.
The results found in this study showed that the wave-4 structures in
the ionosphere can be observed in both years of low and high solar activity
around September equinoxes. The Ne contour plot showed that the wave-4 structure is
not well defined below the ionospheric F-region peak situated around
∼ 300 km. Above this altitude the wave-4 pattern is more clearly
visible. Spectral analysis indicated that the main non-migrating waves in the
higher altitudes of the ionosphere were the DE3 and SPW4 waves for both years 2008
and 2013. We conclude that these are the main waves responsible for the
wave-4 structures seen in the ionosphere.
Our analysis for different altitude intervals showed that the wave-4 structures
are associated mainly with DE3 and SPW4. Longitudinal variations in the low-latitude ionosphere can also be related to the in situ generation of waves by
ion–neutral coupling (Jones et al., 2013). Pedatella et al. (2012) observed
that DE3 and other waves could propagate directly to the thermosphere,
causing a wave-4 perturbation driven by in situ changes in neutral
composition or in the meridional neutral winds; they concluded that the
wave-4 structure results from a combination of E×B drifts, neutral winds
and neutral composition.
Discussion and conclusions
This study has shown that the DE3 wave is the main non-migrating tide responsible
for the wave-4 longitude variations observed in satellite data. We also
indicated that the wave-4 structure becomes more prominent at altitudes above
250 km, where the DE3 and SPW4 amplitudes were larger than other
non-migrating modes. The focus of this work is not related to details about
how the wave-4 structures are created in the ionosphere or the mechanisms
associated with that, but it is related to investigating which waves are present in these structures and
which are the main ones responsible for the four peaks in the longitudinal
structure.
We demonstrated with some very good results that the wave-4 structures
observed at ionospheric altitudes result from the interaction between
migrating tides (which have the largest amplitudes) and non-migrating tides,
mainly the DE3 wave. Since these structures were more prominent at altitudes
from ∼ 250 up to ∼ 800 km, it is very clear that the wave-4
structures were caused by these waves via E-region dynamo modulation,
consequently affecting the plasma distribution. It is important to mention
here that the waves were identified by fft2 analysis, which
identified all the waves present in 00:00–24:00 LT and -180 to 180∘
distribution.
The diurnal, semidiurnal and sometimes the terdiurnal migrating tides
usually had the largest amplitudes. The main non-migrating tides identified
by the fft2 analysis were plotted with their corresponding parameters in this
study. Some of these waves were not related to a wave-4 structure, but they
have a significant presence, like DW2. This means that more studies are
necessary to understand the detailed mechanisms associated with wave-4
structures in the ionosphere.
In this study we have shown that the wave-4 structures in the equatorial and
low-latitude ionospheric parameters can be observed during both low and high
solar activity around September equinoxes. These four peaks appeared more
prominently in Ne data above F-region peak altitudes. By doing
two-dimensional fft2 analysis and selecting some wave signatures to build
LT × longitude contour plots, it was verified that the DE3 and SPW4
waves were the main wave modes responsible for this structure in most of the
cases.
In the amplitudes calculated from fft2 spectral analysis contour plots, we
noticed the presence of DW2, SPW1 and other waves' signatures not necessarily
related to waves of tropospheric origin. SPW1 is related to the offset
between the geomagnetic and geographic coordinate, and DW2 is a combination
between SPW1 and DW1. DW2 could be observed during all the periods selected
in hmF2 amplitudes. NmF2 amplitudes exhibited this wave
signature only in periods of high solar activity. DW2 signatures also
appeared in Ne fft2 contour plots for both 2008 and 2013 September equinox
periods. Vertical amplitude Ne profiles showed that in 2008, DW2 amplitudes
were more prominent than in 2013, mainly at altitudes above 250 km. In 2013,
SPW1 had larger amplitudes around 300 km.
It is well known that atmospheric waves have significant effects on the
ionosphere. In this work we did not use observational data from the MLT
(mesosphere–lower thermosphere) region, and we perform averages considering
a wide range (∼ 20∘) of magnetic latitudes around the equator from
both the Northern and Southern Hemisphere. The wave parameters such as the vertical
and horizontal wavelengths were not calculated because the focus of this work
is on the zonal wavenumbers that produce the wave-4 structures, though
it was possible to show that effects from below could reach high altitudes.
It is important to keep in mind that the wave-4 structures observed are mainly strongly related to DE3 and SPW4 waves. The detailed way as to how
this happens remains unknown. It is possible that the DE3, propagated from lower altitudes by tide
modulation to the E region, creating electric fields that combined with
thermospheric winds and geomagnetic field lines, gives rise to ionospheric
dynamo. The effects caused by this, such as vertical plasma drifts, were possibly
responsible for releasing tropospheric, thermospheric and geomagnetic
influences into higher altitudes of the ionosphere. For the first time, the wave-4 structure
over different altitude intervals and two different periods of solar cycle activity was
investigated. Spectral analysis done for NmF2,
hmF2 and Ne showed that these structures are mainly related to DE3
and SPW4 waves.