ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-34-91-2016Twin mesospheric bores observed over Brazilian equatorial regionMedeirosA. F.PaulinoI.igopaulino@gmail.comhttps://orcid.org/0000-0001-9560-1842TaylorM. J.https://orcid.org/0000-0002-3796-0836FechineJ.TakahashiH.BuritiR. A.LimaL. M.WrasseC. M.Universidade Federal de Campina Grande, Campina Grande/PB, BrazilUtah State University, Logan, UT 84322-4405, USAInstituto Nacional de Pesquisas Espaciais, São José dos Campos/SP, BrazilUniversidade Estadual da Paraíba, Campina Grande/PB, BrazilI. Paulino (igopaulino@gmail.com)22January2016341919626August201515December201522December2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://angeo.copernicus.org/articles/34/91/2016/angeo-34-91-2016.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/34/91/2016/angeo-34-91-2016.pdf
Two consecutive mesospheric bores were observed simultaneously by two all-sky
cameras on 19 December 2006. The observations were carried out in the
northeast of Brazil at two different stations: São João do Cariri
(36.5∘ W, 7.4∘ S) and Monteiro (37.1∘ W, 7.9∘ S), which are by about
85 km apart. The mesospheric bores were observed within an interval of
∼ 3 h in the NIR OH and OI557.7 nm airglow emissions. Both bores
propagated to the east and showed similar characteristics. However, the first
one exhibited a dark leading front with several trailing waves behind and
progressed into a brighter airglow region, while the second bore, observed
in the OH layer, was comprised of several bright waves propagating into a darker
airglow region. This is the first paper to report events like these, called
twin mesospheric bores. The background of the atmosphere during the
occurrence of these events was studied by considering the temperature
profiles from the TIMED/SABER satellite and wind from a meteor radar.
Atmospheric composition and structure (airglow and aurora) – meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides)Introduction
Since the discovery of mesospheric bores over the Hawaiian Islands in the
beginning of 1990s , prominent bore-like waves, which were
initially termed mesospheric frontal events, have been observed and
studied at equatorial and low latitudes e.g.,, mid-latitudes e.g., and high latitudes e.g.,.
These gravity waves are characterized by a leading front, usually followed by
trains of waves, when observed in the nightglow emission. They can often
appear in different emission, for instance, in the NIR OH and OI557.7 nm
(OI5577) emission, in which their emission peaks are separated by about 10 km
altitude e.g.,.
Theories and observations had shown that mesospheric bores propagate for long
distances trapped in ducts, which are regions between two evanescent levels
and occur due to the thermodynamic condition of the local atmosphere.
Depending on the position of the duct in the mesosphere and lower
thermosphere (MLT), mesospheric bores can be observed with different contrast
patterns in the airglow images (i.e., one can observe a dark bore in the
higher airglow emission and bright in the lower airglow emission and
vice versa). This was explained using the complementary effects
.
showed that these features are consistent with a ducted
perturbation propagating horizontally at mesospheric heights. Subsequent
lidar, imager and TIMED satellite measurements had demonstrated the existence
of a temperature inversion layer supporting the ducted wave hypothesis
.
The wind as well as the temperature structures seems to have an important
role in the formation and propagation of mesospheric bores into a duct.
Doppler duct occurs when the wind is the most important process to the
formation of it. Otherwise, if the temperature structure is predominant in
the generation of the duct, it is termed a thermal duct. Moreover, ducts can
arise to combine thermal and Doppler effects and are collectively termed
dual ducts .
In this paper, two consecutive mesospheric bores were observed in the airglow
images within a time interval of ∼ 3 h. These are termed “twin
mesospheric bores”. Coincident mesospheric wind profiles from the meteor
radar and temperature profiles from the TIMED/SABER satellite were used in
order to investigate the background propagation conditions for the bores.
Instrumentation and observation
From November 2005 to August 2008 coordinated airglow observations were made
in the northeast of Brazil using all-sky imagers at two different stations:
São João Cariri (36.5∘ W, 7.4∘ S) and Monteiro (37.1∘ W, 7.9∘ S),
which are separated by a distance of about 85 km.
Optical measurements at Monteiro used an all-sky imager system, which was
described in detail by . This charge-coupled device (CCD) has an area of
6.45 cm2, high resolution, 1024 × 1024 back-illuminated array with a pixel
depth of 14 bits. Moreover, this equipment has a high quantum efficiency, low
dark current (0.5 electron pixel-1 s-1), low readout noise (15 electrons rms) and
high linearity (0.05 %) that makes it possible to achieve quantitative
measurements of airglow emissions. The camera uses a fast (f/4) all-sky
telecentric lens system that enables high signal-to-noise (20:1) airglow
images. Sequentially, images of near-infrared (NIR) OH and OI5577 were made
using the exposed time of 15 and 90 s, respectively. Measurements at São
João Cariri were carried out using a similar optical device. The second imager was
produced by KEO consultants, and the same time intervals of exposition for
observing images of NIR OH and OI5577 were used.
Figure 1 shows the two mesospheric bores in the OH and OI5577 airglow images.
The first one is (a) at 05:16 UT in the OI5577 layer and (c) at 05:25 UT in
the OH layer. The second one is (b) at 07:00 in the OI5577 layer and (d) at 06:54 UT in
the OH layer. The first bore presented a dark front in the
airglow images of OH and OI5577. The second bore exhibited a bright front in
the OH images. The contrast associated with the second bore was significantly
low in the OI5577 images, but a dark leading front was recognized. These
mesospheric bores were observed simultaneously at both observation sites in
the airglow emissions. These bores were observed on 19 December 2006 and
there was a time interval of ∼ 3 h between the first and second
bores. These two bores exhibit similar properties and were all propagating
eastward. However, the first event exhibited a dark leading front with
several trailing waves behind and progressed into a brighter airglow region
in both airglow emission, while the second bore was comprised of several bright
waves propagating into a dark airglow region, as observed in the OH images.
In the OI5577 images, it was observed that a dark leading front progressed
into a brighter airglow region. (See the movie in the Supplement for further
details about the propagation of these bore, http://dx.doi.org/10.5446/17724.)
All-sky images of the twin mesospheric bores: (a) the first bore (front dark)
observed at 05:25 UT in the OH image at Monteiro; (b) the second bore (front bright)
observed at 06:54 UT in the OH image at Monteiro; (c) the first bore (front dark) observed
at 05:16 UT in the OI5577 image at São João do Cariri; (d) the second bore (front dark)
observed at 07:00 UT in the OI5577 image at Monteiro. Horizontal arrows indicate approximately
the propagation direction of the bores. (See the movie in the Supplement for further
details about the propagation of these bore, http://dx.doi.org/10.5446/17724.)
In the first case, the model predicts a duct below these
airglow emission layers, and in the second case the duct would be between OH
and OI5577 layers. Consequently, the ducting layer must be moved upward
during the course of the night according to the prediction.
Analysis and discussion
Table 1 shows the parameters of the twin mesospheric bores. It should be
noted that both bores have almost the same phase speed (observed and
intrinsic) and almost same propagation direction. The wavelength and observed
period are characteristics of their trailing waves. The intrinsic parameters
were calculated using the background winds observed by the meteor radar
deployed at São João do Cariri. These parameters were estimated using a
2-D fast Fourier transform spectrum analysis . The observed parameters of
these bores compare favorably to the previous observations made in different
places around the world e.g.,
An important factor used to characterize mesospheric bores is the presence of
ducting condition. Some previous works have suggested that the occurrence of
mesospheric bores could be related to the presence of thermal duct formed by
a temperature inversion layer in the MLT e.g.,. There are also reports on the
thermal–Doppler ducts e.g.,. To evaluate the
occurrence of such structures in the night of these events (19 December
2006), temperature profiles obtained by the TIMED/SABER over São João do
Cariri and Monteiro area were also analyzed.
Horizontal parameters of the twin bores derived from the images and meteor radar measurements.
Bore 1Bore 2Start (UT)03:1306:03End (UT)05:31–Propagation direction (degree from the north)97.3104.4Wavelength (km)17.033.2Observed period (min)5.310.3Observed phase speed (m s-1)53.053.7Zonal wind at 87 km height (m s-1)-11.4-7.4Meridional wind at 87 km height (m s-1)49.324.4Intrinsic period (min)4.08.2Intrinsic phase speed (m s-1)70.766.9
Figure 2 shows the map of the observation sites. The dashed circle represents
the field of view of the São João do Cariri (filled square) imager, and
dotted circle shows the field of view of Monteiro (filled black circle). Six
sets of soundings were made by the SABER instrument at geographical locations
closer to the two all-sky imagers, and these soundings were made almost
simultaneously to the second bore event (06:14 to 06:19 UT). Thus, the meteor
radar and SABER measurements were combined in order to investigate in
detail the nature of the second bore propagation condition in the MLT
region.
Observation sites (filled square São João do Cariri; filled circle Monteiro)
and the field of view of the imagers (dashed line circle São João do Cariri; dotted line
circle Monteiro). Six soundings of the TIMED/SABER satellite are shown as well (triangle at 06:14 UT,
diamond at 06:15, star at 06:16, asterisk at 06:17, big X at 06:18 and a plus at 06:19).
The six temperature profiles corresponding to the SABER soundings shown in Fig. 2.
(a) Vertical profiles of the buoyancy frequency. (b) Meridional (dotted line),
zonal (dashed line) and wind in the direction of the wave (solid line) at 06:15 obtained
from the meteor radar. Error bars represent the uncertainties in the determination of the
winds. (c) Square of the vertical wave number calculated using Taylor–Goldstein dispersion
relation. Error bars represent the uncertainties in the
estimation of the m2 from the second differentiation of the wind.
Figure 3 shows the six temperature profiles from the SABER measurements. The
symbol of each line is the same as the one used in Fig. 2 and is associated
with its position on the map. The first temperature profile (line + triangle
at 06:14 UT) shows an inversion layer with the peak at ∼ 81 km height.
The last temperature profile (line + plus), therefore, shows a more
pronounced peak at ∼ 90 km height. The temperature profiles observed at
06:16 (line + star) and 06:17 (line + asterisk) were within the field of view
of the imager, which presented inversion layers with peaks around 87 km
height.
found that, in the equatorial region, the daily mean
radiance profiles for ascending and descending portions of the TIMED/SABER
orbits differ by as much as 40 % between an altitude of 70 and 110 km, with
the sign changing in an oscillatory manner. The natural interpretation is
that the 15 µm emission rate directly reflects wave-like temperature and
minor species density oscillations in the atmosphere. These, in turn, are
basically manifestations of atmospheric tides, particularly the diurnal tide
that dominates at low latitudes and produces the ascending/descending
contrasts. Otherwise, the temperature oscillations arise from adiabatic tidal
vertical displacements, which also modulate the atmospheric total density and
the density of all minor constituents.
The present observations could suggest two possibilities: the inversion layer
moved upwards or had a gradient in the latitude (i.e., it was lower in the
low latitudes). also reported that the inversion layers are
most often at around ∼ 83 km at night and ∼ 95 km at daytime.
Figure 4a shows the buoyancy frequency profiles derived from SABER
temperature profiles. Figure 4b shows the observed wind profiles at 06:15 UT
derived from the meteor radar measurements. The blue dashed line
represents the meridional (north–south) wind, the red dashed line is to
the zonal (east–west) wind, and the black solid line represents the wind in
the direction of the wave, which is quite similar to the zonal wind because
the wave was propagating eastward.
The wave propagation condition can be verified from the square of the
vertical wave number, m2, given by the Taylor–Goldstein relation:
m2=N2(u‾-c)2-u‾zz(u‾-c)-kH2,
where u‾ is the background wind velocity in the direction of the
wave propagation, u‾zz is the second derivative of the wind
with respect to the altitude in the propagation direction of the wave, c is
the phase speed of the wave, kH=2π/λH is the horizontal wave
number, N2 is the buoyancy frequency, and λH is the horizontal
wavelength.
The results for the m2 using the six SABER soundings and the wind from the
meteor radar are shown in Fig. 4c. If the m2 is negative, it means
that the wave does not have conditions to propagate vertically. Thus, a duct
is formed whenever there is positive m2 between two regions of negative
m2.
In Fig. 4c, duct region appears from 06:16 UT between 84 and 90 km
height, and it is stronger in the other later profiles. It is important to keep in mind
that the temperature profiles for 06:16 and 06:17 were within the
field of view of the image (Fig. 2). This means that the duct, where the
second bore propagated, was exactly in the OH layer. These SABER soundings
were taken about 45 min earlier than the images shown in Fig. 1b. At
this time (not shown here), no substantial differences between the leading
front and the airglow environment could be observed, which suggest that the
duct could be over the OH layer. A few minutes later, the leading front started
to become brighter, as shown in Fig. 1b, indicating that the duct region
must be moved up to a level higher than the OH layer but lower than the
OI5577 layer.
If the right-side terms of the Eq. (1) were compared, one can see that
the first term (due to the inversion layer) was much stronger than the other
terms. It means that the inversion of the temperature in this region was
caused by the thermal effects. Furthermore, this shape of the duct region was
pronounced when the first and second terms of the Eq. (1) were
considered (i.e., the gradient of the wind contributes favorably to the
formation of this duct).
Summary
Coordinated observations were made simultaneously in the northeast of Brazil
using two all-sky CCD cameras, one located at São João do Cariri and the
other at Monteiro, ∼ 85 km apart from each other. On
the night of 18–19 December 2006, two consecutive bore events (twin
mesospheric bores) were observed in the OH and OI5577 airglow emissions (the
second one ∼ 3 h later than the first one). Both waves exhibited
similar spectral characteristics and propagated eastward, but they exhibited
different complementary patterns in the airglow images. Using supplementary
measurements from the TIMED/SABER satellite and a meteor radar, the duct
condition could be studied when the second bore started to be observed in the
airglow images.
Based on the theory for the mesospheric bores and the observations of this
work, the first bore probably propagated into a duct below the OH layer
(< 87 km height) and the second bore occurred into a thermal duct that
started overlapping to the OH layer and moved upward over time. However, at
the end of the observation, the duct for the second bore was still below the
OI5577 layer (< 96 km height).
Data availability
If someone wants access to these data, please contact either
A. F. Medeiros (afragoso@df.ufcg.edu.br) or H. Takahashi
(hisao.takahashi@inpe.br).
The Supplement related to this article is available online at doi:10.5194/angeo-34-91-2016-supplement.
Acknowledgements
Support for these measurements was provided by NSF grant ATM 0000959 and
NASA grant NNG04GA136 as part of coordinated measurements with the TIMED
satellite. A. F Medeiros acknowledges the CAPES grant for providing
financial support to visit USU during 2006–2007. This work has been also
financed by the CNPq (no. 473473/2013-5, no. 301078/2013-0, no. 478117/2013-2).
The images and wind measurements used to produce the results of this
paper were obtained from the Observatório de Luminescência Atmosférica da
Paraíba at São João do Cariri, which is supported by the
Universidade Federal de Campina Grande and Instituto Nacional de Pesquisas
Espaciais.
The topical
editor, C. Jacobi, thanks J. V. Bageston and P. R. Fagundes for help in evaluating this paper.
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