Introduction
The tendency of the nighttime thermospheric temperature is to decrease.
However, in the equatorial region around midnight the temperature increases
by 50–200 K (e.g., characteristics observed at Arequipa, Peru,
16.2∘ S; 71.5∘ W; ). This
phenomenon is called the midnight temperature maximum (MTM) and has been the
object of study since the 1970s through several instruments of observation,
which are detailed in the review by .
The MTM signature is described by in the F region.
Using an incoherent backscatter radar at Arecibo (18.47∘ N;
-66.72∘ W), they showed that the decrease in the height of the F
region was caused by the meridional wind. They also noted meridional wind
flowing equatorward before midnight. Then the meridional wind diminishes and
often reverses direction causing the downward motion of the F region
(referred to as midnight collapse). Studies made in the Indian sector by
and found the same
relationship among the MTM, thermospheric meridional wind, and F region
height motion near midnight as observed by at
Arecibo.
The airglow signature of the MTM is described by
and . They
observed that when the equatorward meridional wind reverses or there is an
abatement of the flow, the F region is drifted to lower heights, increasing
the dissociative recombination of OI6300 .
The current understanding of the formation of the MTM is due to the results
of the Whole Atmosphere Model (WAM) and the National Center for Atmospheric Research
Thermosphere Ionosphere Mesosphere Energetics Global Circulation Model
(TIMEGCM) that confirmed the theoretical explanation
of MTM presented by and .
The formation of the MTM starts from the day–night pressure gradient that
produces an eastward zonal wind toward the night terminator, combined with an
upward propagation tidal meridional wind and thermospheric tidal wind
produced in situ by EUV radiation at a subsolar point in the equatorial
region, the so-called pressure bulge, and supports the development of a
hydrostatic expansion. The hydrostatic expansion reverses the direction of
the meridional wind to poles. This reversion causes the midnight collapse of
the F region and the increase in the OI6300 relative intensity
.
made the first measurements of thermospheric
wind and temperature in Brazil from August to September 1982. Then,
and studied the
relation between the wind and the MTM at São José dos Campos
(23.2∘ S, 45∘ W) and Cachoeira Paulista (22.5∘ S,
45.0∘ W). Twenty years later, the project entitled RENOIR
installed two Fabry–Pérot interferometers, an
imaging system, and a GPS receiver at São João do Cariri
(7.4∘ S, 36.5∘ W) and Cajazeiras (6.9∘ S,
38.6∘ W) in order to study the equatorial thermosphere–ionosphere
coupling. Several papers have been published:
studied the relationship between Plasma bubbles and neutral wind;
and studied the
neutral wind associated with the solar activity;
compared the
neutral temperature and wind to the recent model predictions.
The nighttime climatology of the MTM in the equatorial region is reported in
the present work, based on Fabry–Pérot interferometer and Digisonde data from
February to December 2011. The main goal of this study is to analyze the
influences of the neutral winds along with the height and critical frequency
of the F region on the development and dynamics of the MTM.
Instrumentation and observations
In this work, two Fabry–Pérot interferometers (FPIs), located at
Cajazeiras and São João do Cariri, were used to measure thermospheric
winds, temperature, and relative intensity of the OI6300 emission at
∼ 250 km height with an accuracy of 5–10 m s-1 and 20 K
. Figure shows the location of
the observatories.
Each FPI consists of a 50 mm diameter interference filter with a 42 mm
diameter etalon having a fixed-gap spacing of 1.5 cm. The reflectivity of
the etalon coating was specified to be 77 % to enhance the transmission
of the OI6300 emission without much loss of spectral resolution. A 30 cm
focal length lens images 11.7 rings of the interference pattern onto an Andor
Technology DU-434 CCD camera using a 1024 × 1024 CCD chip, with each
square pixel having a dimension of
13 µm × 13 µm. The angular field of view for the
outermost ring of the observed ring pattern is approximately 1.8∘
.
In order to measure the airglow in a given region in the sky (zenith and
geographic north, south, east, and west) with an elevation angle of
45∘, a SkyScanner is placed above the FPI optics. The SkyScanner is a
dual-mirror system controlled by two Animatics
SmartMotors;
one mirror rotates to vary the elevation angle and the other to change the
azimuth angle. With this geometry of observation, the whole sky can be
covered.
The absolute coordinate calibration of each axis is determined by using
ephemeris of celestial objects (Sun, Moon, stars or planets) as the
reference. An absolute pointing accuracy of approximately 0.2∘ is
typically achieved. More details of the instruments and analysis procedure
used to estimate neutral winds can be found in
and .
The nighttime thermospheric winds are determined from the estimation of the
Doppler shifts in the observed OI6300 interference pattern image.
The OI6300 is emitted by the excited oxygen atom in state O (1D) that
decays to the ground state (3P), spontaneously releasing the energy
excess in an electromagnetic radiation form .
Data from a Digisonde portable sounder 4 (DPS-4) produced by the University
of Massachusetts Lowell's Center for Atmospheric Research (UMLCAR)
were used in the present work as well. The
ionosonde transmitter scans frequencies from 0.5 to 30 MHz with a peak power
of 500 W. This instrument uses a crossed delta transmitting antenna and four
crossed magnetic dipole receiving antennas . The DPS-4 basically operates vertically transmitting
electromagnetic signals and measures the time lag until the echo is received.
It generates a frequency versus height graph (ionograms), from which the
vertical electron density profile from the reflecting layers can be
calculated. The DPS-4 is located at Eusébio (3.9∘ S,
38.4∘ W; geomagnetic coordinates -7.31∘ S,
32.40∘ E for 2011) (Fig. ), and it was used to
provide the F region critical frequency (foF2), peak height of the
F2 region (hmF2), and its minimum virtual height (h′F)
(bottom-side F region).
Map showing the geographic locations of the RENOIR instruments at
São João do Cariri (green dot) and Cajazeiras (red box). The Digisonde is
located at Eusébio (blue triangle). All instruments are in the Brazilian
northeast. The blue solid lines denote the geomagnetic equator (0∘)
and -10∘ for 2011.
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Table summarizes the conditions and MTM calculation
used in this work. A total of 257 nights was included in the database for 11
months of observation, from February to December 2011.
Table also shows the averaged F10.7 solar flux
index and number of observed nights (temperature or wind) which were used in
the monthly averages. The FPI measurements shown in
Table were obtained during the period from low to
moderate solar activity based on the solar flux index, which ranged from
∼ 83 SFU, in February, to ∼ 141 SFU in December
(1 SFU = 10-22 W m-2 Hz-1).
Table shows the local time of the MTM peak and the
respective amplitude relative to the shifted average International Reference
Ionosphere 2012 (IRI-2012) temperature computed for each month (details on
the next section).
Summary of observations made with the FPIs. NA means not available.
Month
Number of nights
Averages of F10.7 (SFU)
MTM time (LT)
MTM amplitude (K)
IRI shift (K)
January 2011
NA
83.4
NA
NA
NA
February 2011
23
94.5
23:47
77 ± 32
0
March 2011
22
155.8
24:10
69 ± 31
0
April 2011
29
112.5
23:47
64 ± 46
58
May 2011
22
95.8
NA
NA
-33
June 2011
14
95.8
NA
NA
-78
July 2011
21
94.2
21:40
65 ± 34
-90
August 2011
29
101.7
NA
NA
-90
September 2011
28
134.5
23:45
65 ± 38
60
October 2011
25
149.37
23:30
144 ± 48
-87
November 2011
20
153.46
23:00
83 ± 27
80
December 2011
24
141.2
21:50
72 ± 57
60
The neutral temperature in all five directions (zenith, north, south, east
and west) was averaged into half-hour bins for each month. The winds in the
plots were classified into meridional and zonal components. Furthermore,
there were observations in cardinal and common volume (CV) modes. These
operational modes were presented and discussed by .
cardinal mode sets the FPI to look in the cardinal directions and measures
the neutral wind and temperature. On the other hand, the CV mode sets both
FPIs to look at the same point in the sky in three different views: north CV,
inline CV, and south CV. It also measures the components of the wind and
temperature as well. In order to increase the amount of data and, therefore,
calculate the monthly average, the meridional and zonal wind components are
defined as the zonal component (cardinal east and west and CV zonal
component) and the meridional component (cardinal north and south and CV
meridional component). It was assumed that the variation in wind and
temperatures regarding the observation modes and distances between stations
(∼ 230 km) does not interfere with the calculation of the averages.
The weighted standard deviation was calculated for each half-hour bin, and
they were associated with the statistical uncertainties of the method of
analysis . These uncertainties were inserted as
error bars in the figures below; further details can be found in
.
The Digisonde continuously operated during 365 days in 2011. The temporal
resolution of the measurements was 10 min. Half-hour bins were used to
average the measurements of the ionospheric parameters. The standard
deviation of the averaged data in each half-hour bin was used as the
uncertainty of the averages.
Results and discussion
Identification of the MTM in the neutral and ionospheric parameters
In order to clarify the signature of the MTM in the ionospheric and neutral
parameters, Fig. shows FPI and ionosonde measurements
for two days: 26 June 2011 (without MTM) and 10 February 2011 (with MTM). On
10 February 2011 it was observed that the MTM causes a decrease in the
velocities of the meridional and zonal wind component
(Fig. a and b, right-hand panels) that propagate
northward and eastward, respectively. Then, the F region started to decrease
and reach the minimum height at about 200 km when the MTM reaches the
maximum peak (Fig. c). Lastly, an increase in the
relative intensity of the OI6300 (Fig. d) and in the
foF2 (Fig. c) was observed. On the other
hand, on 26 June 2011, the MTM was not observed and the dynamic of the wind
is totally different: the meridional wind flows poleward and the zonal wind
velocity is not fast when compared to the day with MTM (10 February 2011).
The behavior of the wind components did not cause the collapse of the F
region and did not alter the relative intensity and the critical frequency of
the F2 region.
Therefore, changes in the thermospheric meridional and zonal wind components
affect the height of the F region and these changes affect the intensity of
OI6300 associated with MTM. In sequence, we will describe and discuss these
effects and the interplay of MTM for each neutral and ionospheric parameter
through monthly averages.
The meridional (a) and zonal (b) component of
thermospheric neutral winds, foF2 (c), h′F (c),
hmF2 (c), relative intensity of OI6300 (d); the
bottom panel (e) shows thermospheric neutral temperature with the
averaged IRI-2012 curves, shifted to match the observed temperatures in the
early evening. All these parameters were plotted for days without
(26 June 2011) and with (10 February 2011) MTM. Positive values are northward
and eastward. LT is UT - 3.
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Neutral temperature and wind
It is well known that the thermospheric neutral temperature decreases over
night, as demonstrated in the neutral temperature model, IRI-2012
, shown in Fig.
(red line). However, during some months of the year, the observed neutral
temperature (Fig. , black line) increases near
midnight when compared to the empirical IRI-2012 model (e.g., February). This
anomalous behavior in the neutral temperature is known as MTM.
The IRI-2012 model does not reproduce the observed behavior of the MTM in
temperature. This suggests that the model does not use temperature data
observed from the equatorial region in its calculation.
The methodology applied to identify the MTM consists in comparing the
observed temperature to the results modeled using IRI-2012. Initially, the
modeled temperature was matched in the early evening (19:00–21:00 LT) with
the observed temperature by applying a constant offset, as described by
. The magnitudes of these shifts are given
in Table .
The MTM features, like shape, amplitude, and time of occurrence, are highly
variable over a short period (day to day) as well as a long period (year to
year, season to season, etc.) . The MTM peak is clearly observed
during the whole year, except in May, June, and August. The amplitude of the
MTM varies from 64 ± 46 K in April up to 144 ± 48 K in October.
The monthly temperature averages show a phase shift in the MTM peak of
approximately 0.25 h in September to 2 h in December, before midnight. On
the other hand, in February, March, and April the MTM peak occurs around
midnight. The present results agree with the work done by
, which analyzed neutral temperature data
for conditions of minimum solar activity during 2009 at Cajazeiras.
The present and previous studies carried
out in the Brazilian northeast presents MTM amplitude between 40 and 144 K,
values that corroborate measurements made in the Peruvian
and equatorial East African longitude sectors, which
present values between 50 and 200 K and between 30 and 110 K, respectively.
Studies made in the Indian sector by and
found that the MTM amplitudes between 80 and
570 K, which are higher than the Peruvian, equatorial East African
longitude, and Brazilian sectors.
Monthly averaged temperature observed from February to December 2011
(black triangles). The averaged IRI curves, shifted to match the observed
temperatures in the early evening (red squares).
![]()
The comparison of the temperature and the neutral meridional wind during the
year (Figs. and )
reveals that the peak velocity flowing to the equator, approximately precedes
the occurrence of MTM, or, in other words, the maximum in temperature occurs
just after an equatorward peak in the meridional wind component, as can be
seen in February to April and October to December. Otherwise, in winter
months, the meridional wind flows to the pole and the peak of MTM is not well
pronounced. showed, by using the National Center
for Atmospheric Research (NCAR) Thermosphere Ionosphere Electrodynamics
General Circulation Model (TIEGCM), that the vertical propagation of the
semidiurnal tide modes (2, 2) and (2, 3) interact with each other in summer
(MTM is well pronounced) but not in winter (MTM is not well pronounced). It
is well known that the meridional wind is very important to understand the
equatorial thermosphere–ionosphere dynamic. For example, the meridional wind
flowing to the equator is one of the mechanisms for the generation of MTM
.
One can note that the minimum in meridional wind speed is around the MTM
peak. The fact that the meridional wind flows equatorward is attributed to
the combination of semidiurnal tides with higher-order harmonics
. suggests that
the location of the pressure bulge in the summer (from November to February)
is due to the damping in the meridional wind before midnight. The pressure
bulge is theoretically defined as the result of a nonlinear interaction in
situ between the modulation of the diurnal electron density forced by EUV
radiation absorption and higher tidal modes of the thermospheric winds
.
In the winter solstice (from May to August), the pressure bulge is shifted to
the north in subsolar latitude, and a decrease in the trans-hemispheric
meridional wind around occurs midnight, as can be observed in
Fig. . However, during the winter months we
cannot observe the MTM peaks when compared with summer months. This is likely
due to the poleward meridional wind propagation.
Monthly average of the meridional component of thermospheric neutral
wind. Positive values are northward. LT is UT - 3.
![]()
In general, the zonal wind (Fig. ) flows eastward
during the whole night, regardless of the month, as already observed by
. However, the zonal wind leaves a signature
around midnight characterized by the occurrence of a minimum speed. According
to and , this minimum
speed is associated with the passage of the pressure bulge from the equator
to poles.
Monthly average of the zonal component of thermospheric neutral
wind. Positive values are eastward. LT is UT - 3.
![]()
compared neutral temperature measurements
observed by FPI to the Whole Atmosphere Model (WAM)
. They observed that the WAM model predicts MTM
peaks from 50 to 100 K in agreement with the observed FPI data. However, the
model shows MTM peaks during the winter that were not evident in the neutral
temperature. The WAM simulations have confirmed that the terdiurnal tide is a
fundamental parameter to realistically describe the MTM .
Height of the F region and <italic>fo</italic>F2
Monthly averages of ionosonde data from a low-latitude station were also used
to study the vertical movement of the F region with values of the ionospheric
parameters such as h′F, hmF2, and foF2, which are shown
in Fig. , in order to show the signature of the MTM peak
in the ionospheric parameters. Note that the data are plotted between 12:00
and 04:00 LT, to show the behavior of the F region during the transition
between day and night, e.g., the pre-reversal enhancement (PRE), and to
verify whether it is related to the MTM phenomenon. The PRE is basically
defined as an enhancement of the F region zonal electric field that occurs
after the sunset due to the development of the F region dynamo. The enhanced
eastward electric field is responsible for the enhancement in F region
vertical plasma drift observed at equatorial and low-latitude stations; more
details are found in . However, when the PRE
was compared to the MTM peaks, no clear correlation was observed between them
and they will not be discussed further.
From February to April and from September to December, the h′F and the
hmF2 shows an increase at about 18:00–20:00 LT within a range
between 300 and 550 km; it reaches its minimal height at about 200–300 km
close to midnight; then the layer rises again, by about 40 km, or sometimes
remains at constant height. It is important to note that the minimum height
starts at 23:00 LT in February and it happened later, around 24:00 LT, in
April. On the other hand, in September the minimum height occurred around
24:00 LT, and in December it was around 22:45 LT. These characteristics
showing seasonality on the minimum height are in agreement with the
seasonality of the MTM described by . The
authors attributed this variation on time to semidiurnal and high-order tides
that are enhanced in summer; thus, these tides make the nighttime
temperature oscillates more rapidly in summer than in winter.
Furthermore, from May to August, the h′F and hmF2 showed a
different behavior; the signature of the PRE in the h′F and the
hmF2 heights did not appear as in other months, and the height does
not exceed 260 and 350 km. showed that, during
the winter, the signature of the PRE in the F region height occurs later,
around 20:00 LT, and presents a small vertical drift displacement for this
period (around 20 m s-1). The behavior of the F region observed in the
Brazilian sector during the winter corroborates studies using data obtained
in the Indian sector e.g.,.
Monthly average of the minimum virtual height of the F region (h′F
– orange square), the critical frequency of F2 region (foF2 –
black circle) and the height maximum of the F2 region (hmF2 – blue
triangle). LT is UT - 3.
![]()
The midnight collapse of the F region is an MTM consequence in the meridional
wind that is reflected in the height of the F region. It is often observed at
low latitudes, such as in Arecibo and the
Indian sector . Similar
characteristics were also observed in the present study.
and demonstrated that
the cause of the decrease in the F region was the propagation of the pressure
bulge that produced the reversal in the thermospheric meridional wind
component. perceived that just a simple
meridional wind abatement is necessary for the collapse to occur. Therefore,
the variation in the meridional wind influences the morphology of the F
region due to latitudinal and longitudinal displacement of the bulge
pressure, corroborating our data.
Monthly average of the OI630 nm relative intensity. LT is
UT - 3.
![]()
Furthermore, the zonal wind is an important contributor for the height of the
F region due to the large value of the magnetic declination at São João
do Cariri and Cajazeiras (∼ 22∘ W for 2011). Thus, the zonal
wind can play a leading role in the understanding of ionospheric equatorial
processes that influences the ion drag and vertical plasma motion
. This is because the zonal
wind acts in the F region dynamo and may drive Pedersen currents favoring the
appearance of a vertical polarization electric field that forces the plasma
flows with the same velocity of the neutral wind
.
Regarding the critical frequency (also shown in Fig. ),
from February to April and from September to December, there were small
increments during the afternoon before the signature of the PRE in the F
region heights. During the signature of the PRE in the F region heights, the
foF2 diminished a little (as expected from the equatorial fountain
effect, which transports ionization from equatorial to low latitudes);
afterwards, it increased again with the maximum of one or 2 h before
midnight. Sometimes this second maximum is bigger than the first one. Lastly,
the foF2 decreases after the second maximum and reaches the minimum
values of about 4–6 MHz. From May to August, the foF2 shows a
small increment reaching the maximum around 16:00–18:00 LT; then, it
decreases to the minimum values (2–4 MHz) at dusk.
From May to August, it was observed that foF2 decreases after
18:00 LT, and the parameters h′F and HmF2 were between 200 and
300 km of altitude. This behavior is to be expected because the rate of loss
of electronic density is high for altitudes between 200 and 300 km
. On the other hand, from September to December and in
February there was a small decrease in foF2 in the PRE period,
which, for a short period of time, returned to growth. As suggested by
, this behavior during the PRE period is due to the upward
vertical drift that pushes the plasma to high altitudes where the
recombination rate is insignificant, whereas in low altitudes the electronic
density decreases significantly.
During March and April the foF2 decreased after the PRE period and
near midnight a small enhancement appeared, i.e., a combination of the
characteristics presented from May to August with the characteristics
presented from September to December and February, suggesting that there is a
precondition required for the enhancement of the foF2 after the PRE
period. associated the increase in foF2 after
the PRE period to the westward electric field inducing vertical drift,
concluding that downward drift is essential for the increase in the second
peak of foF2. This increase occurs after midnight at Sanya, China
(18.24∘ N; 109.50∘ E), and at Eusébio it occurs before
midnight.
Relative intensity
Figure shows the relative intensity of
atomic oxygen emission (OI6300) observed in 2011. From February to April and
from September to December, the behavior shows an increase in intensity
around midnight or 1 h before, whereas, from May to August, the relative
intensity is at a maximum in the early evening and decays during the night.
The dominant ion in the F region is the O+, and its recombination rate is
low during the night, whereas the height of the F region is kept constant
. When the meridional and zonal wind push down the
F region in subtropical latitudes due to the pressure bulge associated with
MTM for regions with high recombination (h≤300 km), the airglow
emission will increase .
It is possible to observe this behavior in the airglow associated with the
midnight collapse of the F region in the summer and some equinox months
observed in Fig. . The low values of relative intensity
of OI6300 at the beginning and the end of the night are associated with the
well-known increase in the F region vertical drift (PRE) and with low
density, respectively. During the winter, the relative intensity decreases
throughout the night because of little variation in height as explained in
the previous section.
Summary
In summary, we have presented a comprehensive data set of measurements from
February to December 2011 of the thermospheric neutral winds and temperatures
using two Fabry–Pérot interferometers and h′F, foF2, and
hmF2 measured by a Digisonde, which operated in the Brazilian
northeast during the increasing phase of the solar cycle.
The present study showed an MTM amplitude between 40 and 144 K; these values
were similar to other observations in the South American sector and differ
from the Indian Sector.
The MTM also brought a specific signature in the neutral wind, h′F,
foF2, hmF2, and relative intensity of OI6300 to the
equinoxes and summer solstice months as follows. The meridional and zonal
neutral wind velocities decrease, arising from pressure bulge. Then the F
region reaches the minimum height, and the relative intensity and
foF2 increases. In contrast, during the winter, the MTM peak is not
clearly evident. This may be due to the dynamics of the wind; the meridional
wind does not flow toward the equator and the speed of the zonal wind is
smaller than in the summer months. As a result the midnight collapse of the F
region does not occur and the relative intensity of OI6300 decreases during
the night.
The measurements of thermospheric neutral wind and temperature and
ionospheric parameters observed in this study showed the importance of the
midnight pressure bulge on the modification of the nighttime equatorial
thermosphere and support the MTM. Moreover, this work shows that the IRI-2012
model does not reproduce the MTM peaks in the equatorial region.