CHAPTER 1
Introduction
1.1 Objective
The Earth’s atmosphere can be considered as a great natural laboratory for
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studying a huge number of physical, chemical and dynamical processes, which have
mighty impact on the living creatures’ existence. Because of this, atmospheric research
has always been a prime field of active cogitations. For the last few centuries, significant
investigations have been carried out by several theoretical and observational studies
related to various regions of the atmosphere. The troposphere, the lowest part of the
earth’s atmosphere has been always an interest and hence extensive experimentations
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have been performed. The stratosphere, immediate layer above the troposphere, has also
drawn significant attention for studying most important species, i.e. ozone and also for
chemical and dynamical processes. Recent development of the rocket and satellite
technologies for the past few decades has facilitated upper atmospheric studies.
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The mesosphere region (often called as mesosphere and lower thermosphere
(MLT) region) is the least explored one of the earth’s atmosphere because of instrumental
limitations to probe it. This region is extremely important for attaining notions of gravity
wave, planetary wave and tidal phenomena in the atmosphere. These waves are mostly
generated from the lower atmosphere and propagate to the upper atmosphere. In course of
its propagation, these waves carry energy and momentum and finally they deposit this
energy in the mesosphere and lower thermosphere region. Thus the dynamical condition
of this region is strongly controlled and modified by these wave features and the
mesopause region (80 - 100 km) is very important for controlling wave dynamics. The
mesopause layer acts as a filter for allowing/not-allowing several kinds of waves created
in the lower atmosphere and pass through it to the thermosphere. So it plays a significant
role in middle and upper atmospheric coupling processes.
Equatorial region is substantially effected by a number of wave activities due to
maximum incident solar incident radiation and resulting large scale convection activities
1 compared to the mid and high latitudes. So investigation of equatorial dynamics urges an
extra importance for global scale atmospheric circulation. Also this region is not studied
extensively so far. Hence my prime objective of the present research work is to study the
equatorial mesosphere region and the associated coupling processes with the lower
thermosphere and the stratosphere regions.
1.2 Evolution of the Earth’s Atmosphere
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There is very little evidence so far regarding the origin of the universe and hence
the solar system and our planet earth. Cosmologists say that the universe was created 15 20 billion years ago, when an immense explosion of highly dense center matter occurred
which is normally called as Big Bang. The Earth was created 4.6 billion years ago from
the sun followed by a cosmic collision and started rotating around the sun due to
gravitational force. At the initial stage after the birth of the earth it was like a fiery ball,
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surrounded by the extremely hot gases envelope, which was called our primordial
atmosphere. The gases formed at the birth of the earth dissipated very rapidly before they
could be held by the strong gravity of the earth. After that a secondary atmosphere was
created by the emission of the earth’s interior molten elements. At that time there was
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large amount of hydrogen and very little or no amount of oxygen which could not sustain
living beings like present days. The hot gases cooled and condensed to form the solid
earth crust and released a large amount of volatile gases and vapors from the hot matter.
Part of the gases formed water to make oceans and the residual part remained as
atmosphere. The secondary atmosphere used to contain large amount of carbon dioxide
(CO2), water vapor (H2O), nitrogen (N2) and also other trace species. As the earth cooled
down further, the interactions between the earth crust and the existing atmosphere at that
time and also several complex chemical reactions led to the formation of the evolved
atmosphere, so that living creatures can sustain. First living creature was uni-cellular
microbes which needed a very less amount of oxygen for their survival. These microbes
most probably existed in the oceans and absorbed CO2 and released oxygen in the
presence of water and sunlight as part of a biological process called photosynthesis.
These microbes later gave rise to multi-cellular creatures due to transformation by the
complex evolution and adaptation processes. It is believed that this evolution developed
2 in first billion years and atmosphere gradually stabilized to the present stratified state in
the rest billions of years.
1.3 Composition of the Atmosphere
There is no sharp boundary between the earth’s atmosphere and the space. The
concentration of the atmospheric gases or species decreases with the altitude. Heavy
constituents are present at the lower part and the lighter species are dominant in the upper
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atmosphere due to the gravity of the earth. Hence it is a mixture of several gases which
form the atmosphere. The composition of the present atmosphere has very little
resemblance with the early atmosphere. The details of the present atmospheric
constituents and their abundance have been show in a tabular form (Table 1.1)
Atmospheric compositions with their abundance.
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Table 1.1
From the above table it is conspicuous that the nitrogen and oxygen are the most
abundant gases in the atmosphere. Together nitrogen and oxygen constitute almost 99%
of the total species by mass as well as volume. Their amount also remains almost
constant with the time, so they are considered as permanent species. Other gases form
very less ~ 1% of the total constituents. Though CO2 abundance is very less in the
atmosphere, it is estimated that for each CO2 molecule in the atmosphere, there are 105
more CO2 molecules stored as carbonate compound in the sedimentary rock. As nitrogen
is inert and non-soluble in water and non-condensable, it dominates in gaseous form.
Also initial atmosphere was not so rich with oxygen. Recent geochemical evidence
3 (Kasting, 2001) has discovered that before 2300 million years, oxygen went through a
dramatic increase, but what actually triggered this unusual augmentation is still an open
question. The water vapor is another important constituent of the atmosphere, although it
varies with time. It can be maximum of ~ 3% (by volume). Water can exist in three
forms, i.e. solid, liquid and gas. Constant precipitation and evaporation changes its
abundance in the atmosphere. The remaining gases (< 1%) are called as trace gases
because of their less abundance, but these gases are very important for studying chemical
the last two centuries.
1.4
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properties and radiative balance. The trace gases have changed by significant amount for
Thermal structure of the Atmosphere
The earth’s atmosphere is stratified in several spheres according to the
temperature structure. These are troposphere (0 - 15 km), stratosphere (15 - 50 km),
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mesosphere (50 - 90 km) and thermosphere (90 - 500 km). Figure 1.1 shows the thermal
structure of the atmosphere. The density of the air decreases exponentially with the
altitude.
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1.4.1 Troposphere
This is the lowest region of the atmosphere which starts from the ground level and
extends up to 15 km. The higher limit of the troposphere is called tropopause. The
temperature in this region decreases as altitude increases because of the reduction of the
outgoing long-wave radiation emitting from the earth’s surface and adiabatic cooling.
This region is strongly effected by the convection related processes. The troposphere
contains more than 75% of the total atmospheric gases. The temperature decreases at a
rate of 6.5 K/km. The tropopause height changes with latitude and season. It can vary
from 9 km at polar region to 18 km in equatorial region. Being the closest part of the
atmosphere to the surface it is studied extensively in last couple of decades. The
temperature across tropopause can be as low as 210 K.
4 1.4.2 Stratosphere
The next layer above the troposphere is called stratosphere. It can extend up to 50
km. The uppermost level of the stratosphere is called stratopause which is a transition
zone between stratosphere and mesosphere. The temperature in stratosphere increases
with the altitude because of the presence of ozone which absorbs the ultra violet radiation
coming from the sun. The ozone in the stratosphere plays a very vital role by preventing
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the harmful ultra-violet radiation from the sun and maintaining the heating of the
Figure 1.1
Vertical temperature structure of the several spheres of the earth’s
atmosphere.
5 stratosphere. The maximum temperature of the stratosphere can reach up to 265 K.
Radiative processes are dominant in the stratosphere.
1.4.3
Mesosphere
Mesosphere is the last region of the middle atmosphere, after that thermosphere
starts. Temperature decreases here with altitude because of non-existence of absorbing
species. Also several radiative cooling processes reduce the temperature there. Rapid
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vertical mixing takes place here which maintains the thermal budget. This region is
extremely important for studying dynamical phenomena, e.g. gravity wave and tidal
characteristics. Mesopause (transition region of mesosphere and thermosphere) region is
the coldest part of the earth’s atmosphere. The temperature of the mesopause can be as
low as 170 K.
Thermosphere
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1.4.4
This region is the last part of the atmosphere. After thermosphere, the region is
called exosphere, where interplanetary space starts. The temperature in the thermosphere
increases with the altitude because of high absorption of the solar short wave radiation by
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the atmospheric species, e.g. molecular and atomic oxygen and nitrogen present there.
During solar quite condition maximum temperature becomes ~ 500 K and it can reach up
to ~ 2000 K during active solar condition.
1.4.5
Exosphere
The outermost layer of the atmosphere is called exosphere. The transition region
between thermosphere and exosphere is called thermopause or exobase. Here density is
very low, as it connects the earth’s atmosphere with the space. There is no sharp
transition region at its end which is called exopause (theoretically ~ 180,000 km).
Molecules in this region have very high kinetic energy and can easily escape from the
earth’s gravitational field to the space. The most light gases are present here, e.g.
hydrogen, helium etc.
6 1.4.6
Ionosphere
The region from 60 to 400 km, actually a part of mesosphere and thermosphere, is
called as ionosphere. The gas molecules here are ionized by the incoming solar radiation.
The electrical properties are different at different regions, so they are classified as D, E
and F region. D region contains lowest electron concentration and the F region contains
maximum. As solar radiation is the main causative factor for the generation of these
regions, the concentration of the charge ions and electrons decreases at night. Radio
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communication is strongly controlled by the characteristics of this region.
The atmospheric region, 0 - 110 km is also called as homosphere, as constituents
are more or less uniform (except water vapor) and radiative equilibrium persists here.
Turbulent mixing, eddie diffusion etc. processes dominate here. The upper boundary of
this region is called turbopause above which species are not uniformly distributed. The
region above the homosphere is called as heterosphere, where molecular diffusion
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dominates. The concentrations of the heavier molecules decrease very rapidly with the
altitude in comparison with the lighter ones. The temperature of the heterosphere varies
significantly with the altitude.
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1.5 Dynamics of the Atmosphere: Atmospheric waves
Wave is the most important parameter to express dynamics of the atmosphere.
Waves are mostly created by disturbance, turbulence etc. The most interesting feature of
the wave is that, it can transport energy and momentum from one region to other region
and thus causes global scale circulation and dynamical perturbation. A stably stratified
fluid possesses higher density as the depth increases. As our atmosphere is stably
stratified, it can support and propagate spontaneous wave motions. If it were possible to
visualize the waves in the atmosphere, we could see spectacular wave features existing in
our surrounding in all the directions. Hines, 1974 first proposed a “surrealistic”
representation of the waves in the atmosphere which is shown in Figure 1.2. In the
normal case, most of the waves propagate diagonally upward or downward and few
components progress along horizontal direction with various amplitudes due to the
generation mechanisms and ambient conditions. It is interesting to see some waves
propagate upward with increasing frequency and amplitude and finally they reflect back
7 at some layers of the atmosphere. Few waves don’t reflect, rather they break into several
smaller scale waves, giving rise to secondary wave generation phenomena. These
secondary waves can create turbulence in the ambient in some cases. Because of that we
observe a spectrum of various wave frequencies and also sometimes we see higher
frequency waves superimpose on the lower frequency components. The dominance and
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scale of variability of different waves also vary with time and space. Some waves last for
Figure 1.2
A surrealistic representation of the atmospheric waves after Hines, 1974.
few minutes and some waves exist for more than few hours or even few days. Some
waves ascend or descend with time, while some others appear or disappear with time.
Some waves have significantly high amplitude, at the same time other waves are hardly
noticeable because of less amplitude.
Waves produced in the atmosphere, can be classified into several types according
to their forcing (creation) and driving (propagation) mechanisms. The waves created by
8 small random forcing are referred to as free modes (e.g. resonant modes). Observed 2, 6,
15 etc. days large-scale atmospheric waves belong to this category. The waves excited at
the earth’s surface, by convective processes or airflow over non-uniform topography or
land-sea temperature contrast are said to be forced modes. Some waves can propagate to
higher altitude and dissipate their energy and momentum, carried from the lower
atmosphere and modify the atmospheric circulation processes. Other than normal
propagation, waves also refract through the atmospheric molecules. Large scale waves,
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propagate in both the directions (horizontally and vertically) with time are called as
global modes. The waves propagate in only zonal (East ↔ West) and vertical directions
are called as equatorial modes, because they are trapped in the equatorial atmospheric
region. If the waves propagate over several scale heights in the vertical direction, then
these are said to be internal modes. If the waves attenuate fast with the vertical
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propagation, then it is called as external modes.
1.5.1 Gravity waves
Gravity wave, the most significant atmospheric wave, is a disturbance with
relatively shorter horizontal extension (10 - 1000 km), arises due to vertical displacement
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of the air parcels in stable stratified fluid. These waves are created by 1) convection in the
troposphere, 2) wind flow over mountain (orographic effect), 3) strong wind shear in the
atmospheric layers, 4) sudden disturbances (e.g. thunderstorms, jets, large explosion etc.).
Although the characteristics of waves in the stratified fluid had been known for a long
time in the past, they remained veiled until Hines, 1960 applied gravity wave theory to
explain the origin of turbulence in the ionosphere. Application of the gravity wave theory
in the meteorology emerged a number of interesting atmospheric processes. Today it is
believed that gravity waves play an important role in the field of dynamic meteorology in
all scales. These waves have profound effect on all the regions of the atmosphere, starting
from the troposphere to the thermosphere. Effect of the gravity wave on upper
atmosphere and general circulation was studied by Lindzen, 1981; Holton, 1982. Effect
of gravity wave on mesoscale processes, was studied in the past by several investigators
(e.g. Uccellini, 1975; Chimonas and Nappo, 1987) in terms of interaction between
gravity waves and thunderstorm. On microscale, the role of gravity wave in the
9 interaction with turbulence in the stable planetary boundary layer was shown by Einaudi
and Finnigan, 1981 and Fua et al., 1982.
Almost all the theoretical studies have been carried out by assuming linear theory
to explain several gravity wave characteristics in the atmosphere. One advantage of the
linear theory is its simplicity for explanation. The linearization assumes that there is no
wave-wave interaction and corresponding energy transfer between propagating waves. It
considers the meteorological variables as slowly varying or stationary background and
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first order perturbation due to the gravity waves. In the middle atmosphere (stratosphere
and mesosphere) it often satisfies this condition, but in the troposphere, especially in the
stable planetary boundary layer, this assumption is not obeyed strictly. In the troposphere,
a number of wave frequencies are present and hence wave-wave interaction is very
important as far as theoretical estimates are concerned. But in the mesosphere, waves are
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considered to be monochromatic i.e. single frequency propagation and negligible
Figure 1.3
Typical gravity wave characteristics observed from ground based OH (6-
2) airglow emission intensity (left panel) and temperature (right panel) perturbation on
July-29, 1995 at 8 cardinal points of the sky at 60o zenith angle at Shigaraki, Japan after
Takahashi et al., 1999.
10 interaction with other waves. In spite of this limitation, linear wave theory is very useful
for first order wave analysis of observations.
As it is already mentioned previously that waves distribute energy from one
region to other region, as a result of that several atmospheric regions get coupled
dynamically. This distribution is done more rapidly by the waves than the mean flow. It
is now discovered that turbulence in the night time planetary boundary layer and clear air
turbulence (CAT) is caused by the gravity waves. A typical gravity wave activity in the
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nocturnal OH airglow emission has been shown in Figure 1.3. The plot shows actually a
superposition of number of waves with different scales.
The propagation of the gravity waves in the atmosphere depends upon the wind
and thermal structure of the ambient and varies with season and static stability. In the
propagation process, when the phase speed (c) of the gravity wave becomes equal to the
mean background wind speed (ū), the wave is absorbed and that layer is said to be critical
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layer for that particular wave component. Due to strong filtering effect in the stratosphere
and mesosphere, the gravity waves normally propagate westward in the winter and
eastward in the summer. As the atmospheric density decreases exponentially with the
altitude, it can be shown from the conservation of energy that amplitude of the gravity
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wave increases exponentially with altitude in the dissipation less atmosphere. At some
altitude it can reach to such a high value that its temperature perturbation causes superadiabatic lapse rate and becomes convectively unstable. As a result, the wave breaks
(Fritts et al., 1997) at that region and gives birth to smaller scale waves. Mostly the
mesospheric region is supportive for such kind of wave phenomena. Study of gravity
waves in the mesosphere have been done for the last few decades by several investigators
(e.g., Walterscheid, 1987; Swenson and Gardner, 1998; Guharay et al., 2008).
1.5.2
Atmospheric Tides
Atmospheric tides are special type of atmospheric waves which possess
periodicities of sub-harmonic of solar day i.e. 24, 12, 8 etc. and created by absorption of
the solar radiation during daytime. The 24 hour component is called diurnal tide, 12 hour
component is called semi-diurnal tide and the 8 hour component is called terdiurnal tide.
The periodic absorption of solar ultra-violet radiation by the ozone in the stratosphere and
11 the infrared radiation by the water vapor mostly in the troposphere causes the generation
of tides. In this context, it should be mentioned that absorption of the solar UV radiation
in the Schumann Runge bands and continuum (135 - 175 nm) play an important role in
the tidal forcing. An additional thermal driver is latent heat which is stored in the water
vapor and transported in the troposphere and released during precipitation.
Atmospheric tides are mainly classified in two types, migrating and nonmigrating tides. The migrating tides propagate from east to west following the solar
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motion relative to the earth. They have significant diurnal and semi-diurnal components.
The non-migrating tides can be stationary or propagate eastward or westward. Also the
moon’s attraction can cause a small semidiurnal tide. The diurnal tides dominate at higher
latitude and the semi-diurnal tides are prominent at low latitude regions (Manson, 1999;
Pancheva et al., 2002). There are two types of migrating diurnal tides. One is upward
propagating which is created in the troposphere and propagates upward, another is in situ
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thermospheric diurnal tide which is created at altitude above 100 km by in situ heating.
Tidal oscillation in the vertical wind causes significant change in the
concentration of the chemical species which exhibit large vertical gradient (e.g. atomic
oxygen, nitric oxide). Ground-based, satellite and theoretical model show that tides often
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govern the dynamics of the middle atmosphere. Tidal wind can have large effect on
gravity wave breaking and local heating or cooling (Liu and Hagan, 1998). The tidal
dampening above 80 km can provide the source of momentum and resultant turbulence in
that region. A typical diurnal tidal variation at the mesopause region long term (Oct, 1992
– Apr, 1995) meridional (south ↔ north) wind data at 95 km obtained from High
Resolution Doppler Imager (HRDI) instrument, onboard Upper Atmospheric Research
Satellite (UARS) by Burrage et al., 1995 has been shown in Figure 1.4.
There have been a number of observations carried out from several latitudes of
the globe to characterize the various types of tidal variability from ground based and
satellite based instruments for the last few decades (e.g., Mathews, 1976; Forbes, 1982;
Burrage et al., 1996; Hagan et al., 1996; Wu et al., 2005a). Although our understanding
regarding the tidal characteristics in the MLT region has been improved significantly,
still there are lots of unanswered questions to be addressed in the future e.g., variation of
tidal structure, season to season variability, global and local variations etc.
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Figure 1.4
Diurnal tide (a) amplitude and (b) phase (calculated from maximum
northward amplitude) observed from HRDI instrument onboard UARS at 95 km from an
equatorial site (20oN). The solid line is 10 day running average to represent the long
term variations (taken from Burrage et al., 1995).
13 1.6 Airglow : A useful tool to probe the upper atmospheric
dynamics
It is known that upper atmosphere is a very low dense medium whose constituents
are basically atomic oxygen, nitrogen along with hydrogen and helium gases. Also less
dominant constituents, e.g. nitric oxide (NO), carbon dioxide (CO2), carbon monoxide
(CO), nitrous oxide (N2O), water vapor (H2O), ozone (O3) etc. and metastable atoms and
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molecules are important for photo-chemistry, energetics and emissions of the upper
atmosphere. The incident ultra-violet (UV) solar radiation on the atmospheric gases
present there, causes them to be ionized and excited. When the excited molecules deexcite, radiations come out from them, which is known as Airglow. Airglow persists in
both day and night. On the basis of available literatures, airglow has been used as an
indispensable tool in remote sensing, not only for our planet earth, but also to study other
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planets’ atmosphere. At altitude above 80 km, the density is very low and that medium is
optically thin for the wavelengths greater than 350 nm. The only exceptions are the
infrared emission bands of the molecules over 2 μm wavelength for which radiative
processes are insignificant.
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The airglow in the upper atmosphere occurs mostly in the band 0.02 to 63 μm.
Almost more than 90% airglow lines are within the band 0.1 to 20 μm. It is mainly
produced by 25 species. Out of 25 species, 11 are atom and 14 are molecules (8 of them
are diatomic molecules). The excitation processes in nocturnal and diurnal atmosphere,
during geomagnetic quiet condition, give rise to 25 significant bands and 130 atomic
emissions. A large number of molecular bands and atomic emissions with high excitation
energy, manifest during intense geomagnetic storm or aurora in the high latitude because
of entrance of high solar energetic charge particles in the upper atmosphere. On the
whole, the airglow spectrum contains more than 1000 molecular bands and each of them
consists of several hundreds or thousands of spectral lines. The region 0.02 - 1 μm range
contains mainly ionization emissions from oxygen, nitrogen helium etc. This ionization
is mainly caused by the solar radiation and resonance scattering. The region 0.1 – 0.35
μm (UV), contains around ~ 570 bands and ~ 35 atomic lines. The region 0.35 – 20 μm,
consists of ~ 360 bands and 52 lines. The energetics of the longwave region is dominant.
14 Molecular bands and atomic lines observed from the earth’s surface.
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Table 1.2
15 With the available geophysical data of the airglow, only the visible and the near infrared
regions (0.35 – 2.0 μm) are studied extensively from the ground based instruments. The
shorter and longer wavelength regions other than these are studied by rocket and satellite
based instruments. So systematic simultaneous measurement using the all regions of the
spectrum, is a difficult task in the reality. The airglow has some altitudinal properties. It
is mainly a feature of the upper atmosphere (80 - 300 km) because their ionization
increases rapidly above 100 km and density decreases sharply with the altitude. Only the
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emissions from the oxygen, helium and hydrogen persist up to the altitude ~ 1000 km.
The details of the emitting species, nature of transition, wavelength and altitude region
and associated intensity above 70 km in the unit of Rayleigh (106 photons/sec/cm2) are
given in tabular format (Table 1.2). Nightglow spectrum of the upper atmosphere in the
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range 820-1020 nm after Semenov et al., 2002 has been shown in Figure 1.5.
Figure 1.5
Nightglow spectrum of dominant airglow lines by the atmospheric species
(OH and O2) after Semenov et al., 2002. The terms in the bracket represent the
vibrational level of transition.
Two extremely important geophysical parameters can be derived from the airglow
emissions. Those are intensity and rotational-vibrational temperature. Intensity of
individual band can give information about the chemical reaction processes associated
with the emission as well as dynamical condition of the emitting region. Its obtained
values and space-time variations provide the information regarding energy removal and
various processes for the production of excited molecules. On the other hand, rotational
and vibrational temperatures give information about the state of the medium, rates of
16 deactivation and establishment of equilibrium. It also provides a key to find the
probability of transition that eventually determines all observed parameters from the
emissions. Also temperature is one of the most important parameters, which can give true
information of the wave dynamical activities in the atmosphere. Few important airglow
emissions of our interest are described below.
1.6.1
Hydroxyl Airglow
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The emission spectrum of the hydroxyl is the most dominant one in the
atmosphere. Slipher, 1929 photographed a series of emission peaks at 653, 687 and 727
nm with a small dispersion in the range 580 - 770 nm. Later on, Slipher, 1933 published
emission spectra with 4 emission peaks up to 860 nm wavelength, although those peaks
were hardly detectable because of very poor resolution. The photometric observation was
also started in the early 1940’s (Herman, 1942) after the spectrographic studies as
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mentioned above. After few years, Meinel, 1948 took photos of spectra in 700 – 800 nm
range and Krassovsky, 1949 obtained spectra in the range 700 - 1100 nm with a
dispersion of about 25 nm/mm and for the first time rotational branches of P, Q and R are
observed clearly. After that numerous observations have been carried out with improved
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instruments and techniques till today.
For OH, transitions among various vibrational energy levels have been observed
maximum up to 9th level (Meinel, 1950). The most dominant mechanism regarding the
excited OH molecule for significant emission was first proposed by Bates and Nicolet,
1950, which was further modified by Bates and Moiseiwitsch, 1956.
H + O3 → OH(ν’)* + O2 + 3.34 eV ν’ ≤ 9
(1.1)
This is actually an exothermic reaction and the energy is sufficient to excite the OH
molecule up to 9th level. This mechanism explains the maximum possible energy level
for OH and it is the primary source of OH excitation (Takahashi and Batista, 1981). The
dependence of this reaction on O3 and therefore atomic oxygen means that the
unperturbed intensity of the emission is influenced by vertical transport within the
atmosphere, although with the time constant being large the intensity variation resulting
17 from these changes over a single night is negligible. In addition to that, intensity variation
takes place up to a factor of two. In the initial hours of the nightglow after sunset, the
intensity increases due to increase in ozone and in the late hours it decreases due to less
atomic oxygen.
After more than a decade, Krassovsky, 1971 put an objection in explaining the
OH airglow as described before and he proposed an alternative mechanism related to
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vibrational excited state of O2.
H + O2 → OH* + O
(1.2)
However, this reaction is unlikely as it will give energy level higher than 9, which is not
observed in the reality according to Bates and Moiseiwitsch, 1956. Other than these, there
are also three more possible reactions are present in the mesosphere to emit airglow. Two
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of them are photo-dissociation reactions.
H2O2 + hν → 2OH
(1.3)
H2O + hν → H + OH
(1.4)
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and the third one is dissociative recombination.
H2 + O → OH + H
(1.5)
To obtain useful results from the airglow observation, it is important to know the actual
height of the airglow emitting layer. Earlier observations from rocket borne (Rogers et
al., 1973; Barker & Waddoups, 1967; Takahashi et al., 1996) and satellite borne (Evans
& Shepherd, 1996; Zaragoza et al. 1998) instruments have concluded that the centre of
the OH emissions is around 85 - 87 km with full width half maxima (FWHM) of
approximately 10 km (Rogers et al., 1973; Hecht et al., 1995). Recently it is discovered
that the height of the OH layer varies significantly (Plagmann et al., 1998).
1.6.2
Molecular Oxygen emission bands
The molecular oxygen is also a very significant contributor to the atmospheric
airglow spectra. It not only absorbs the solar UV radiation of wavelength shorter than 300
18 nm, but also participates in numerous chemically active reactions. The oxygen molecule
has only one electronic state from which normal transition can take place. The other six
electronic states are metastable and transitions among them create nine systems of bands.
Out of nine, six are in UV region and rest three transitions are in infrared region.
Investigations of the molecular oxygen emissions in the ultraviolet spectral region
have demonstrated that they occur as a result of recombination of atomic oxygen (Barth,
1964).
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O + O + M → O2* + M
(1.6)
The lifetime of the excited O2 molecules in the metastable state are determined by the
collisions (Bates, 1988; Johnston and Broadfoot, 1993). The emission from the
atmospheric O2 molecule was identified in the nightglow simultaneously with the
hydroxyl emission (Meinel, 1950). The most intense transition occurs in 761.9 nm (0, 0),
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band. The next dominant emissions come from 864.5 nm (0, 1), band. Other bands are
not practically observed because the population of the level ν’ > 0 are almost two order of
magnitude less than ν’ = 0, hence their intensity is small (Slanger et al., 2000). The
height of the maximum emission of the molecular O2 is ~ 94 km.
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At altitude above 80 km, excited O2 molecules can be produced in the day time by
photolysis of the ozone, giving rise to metastable oxygen atom.
O3 + hν → O2 + O
λ ≤ 310 nm
(1.7)
At lower altitude some other reaction can contribute to some extent
O3 + O → O2 + O2
(1.8)
Systematic ground measurements of the nightglow characteristics, such as the
intensity and rotational temperature of the 864.5-nm band (0, 1), have revealed their
seasonal variations. Maxima are observed in winter and minima in summer. Nevertheless,
like many other emissions, the yearly average intensity shows long-term variation.
19 1.7
Outline of the Thesis
The current chapter has given a brief introduction of different atmospheric
regions, dominant processes therein and wave dynamical condition. In the next all
chapters, detailed studies of the atmospheric wave activities have been incorporated.
Chapter 2 illustrates the required instrumentation and methodology for carrying
out observations. Detailed description of the Mesosphere Lower Thermosphere
Photometer (MLTP) (designed and built in ARIES) has been given. Also other available
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instruments, e.g. Mesospheric Temperature Mapper (MTM), LIght Detection And
Ranging (LIDAR), Sounding of the Atmosphere using Broadband Emission Radiometry
(SABER), Microwave Limb Sounder (MLS) as utilized in the present thesis work, are
also depicted briefly. A part of this chapter is published in Guharay et al., 2009b.
Chapter 3 elaborates characterization of the gravity wave activities in terms of
several wave parameters (Krassovsky’s ratio, wave growth, vertical wavelength etc.). The
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second part of the chapter deals with the long term characteristics of the wave generated
processes (e.g. dissipation) from the ground based airglow observation. The results of this
chapter are published in Taori et al., 2007, Guharay et al., 2008 and Guharay et al.,
2009b.
Es
Chapter 4 depicts the multi-pattern nocturnal wave variation. Various wave
features, e.g. evident phase progression, no evident phase progression, anti-phase
variation, uni-active variation, duo-active variation in airglow emissions are explained in
the light of wave dynamics.
Chapter 5 discusses about the tidal activity in the mesosphere region. Observed
results show terdiurnal tide characteristics. The tidal component exhibits significant
variability in the amplitude, phase, vertical wavelength and wave growth factor.
Chapter 6 focuses on the planetary scale wave features in the low latitude
mesosphere in the light of ultra fast Kelvin (UFK) wave. The prominence of the planetary
wave signatures is observed from both ground based lidar and space based MLS
temperature data. The results of this chapter are communicated to International Journal
of Remote Sensing.
Chapter 7 concentrates on the thermal and dynamical coupling of the
stratosphere, mesosphere and lower thermosphere regions. Thermal coupling is
20 represented by the vertical temperature structure and static stability profile. Stratospheremesosphere coupling is shown by stratopause variability characteristics. Dynamical
coupling is delineated by the long period oscillations (semi-annual and annual
oscillations) in the middle atmospheric region. The results of this chapter are published in
Guharay et al., 2009a and Guharay et al., 2009c.
Chapter 8 or last one of the thesis describes the summary of all the works carried
out and the possible future plans in continuation of the present work in the field of wave
la
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dynamics.
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te
****End****
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