Summer internship report submitted in PRL during PRL SIP 2015

1. A
summer internship report
on
Instrumentation for dayglow photometry
Submitted by:
Praveen Kumar Singh
Department of Environmental Science
Central University of Rajasthan
Under guidance of:
Prof. Duggirala Pallamraju
Space and Atmospheric Science Division (SPA-SC),
Physical Research Laboratory
Physical Research Laboratory
Navrangpura, Ahmedabad-380009

2. Certificate
This is to certify that the report entitled “Instrumentation for dayglow
photometry” is a bonafied work carried out by Mr. Praveen Kumar Singh under
the guidance and supervision of Prof. Duggirala Pallamraju at Physical Research
Laboratory, Ahmedabad.
To the best of my knowledge and belief, this work embodies the work of candidate
himself, has duly been completed and fulfills the requirement of the summer
internship Program (SIP-2015).
Prof. Duggirala Pallamraju
Space and Atmospheric Science Division (SPA-SC),
Physical Research laboratory,
Ahmedabad
i

3. Acknowledgement
I acquire this opportunity with much pleasure to acknowledge the invaluable
assistance of Physical Research Laboratory and all the people who have helped me
through the course of my journey in successful completion of this summer
internship.
I wish to express my sincere gratitude to my Guide, Prof. Duggirala Pallamraju,
Professor, Space and Atmospheric Science Division for his guidance, help and
motivation. Apart from the subject of my study, I learnt a lot from him, which I
am sure, will be useful in different stages of my life. I would like to thank Mr.
Kedar A. Phadke, Scientist-SC for his help in understanding methodology as well
as various instruments I came across.
I would like to thank Dr. Bhushit G. Vaishnav, Head, Academic Services for
providing me with this wonderful opportunity to work at Physical Research
Laboratory. I express my thanks to Mr. Deepak Kumar Karan, Senior Research
Fellow for his kind cooperation during the period of my summer internship. I also
express my thanks to Dirgha and Sharanya, who shared work space with me.
I am grateful to my friends who gave me a pleasant environment to work for entire
duration of my internship. Last but not the least I would like to express my special
thanks to my family for their continuous motivation and support.
Regards,
Praveen Kumar Singh
Dated:
ii

4. Index
No. Contents Page no.
Certificate i
Acknowledgement ii
1 Airglow 1
2 Significance of airglow measurements 3
3 Challenges of dayglow measurements 4
4 PMT based dayglow photometer 7
5 CCD based dayglow photometer 8
6 Production Mechanisms of Dayglow 12
7 Summary 14
8 Bibliography 15

5. 1
1. Airglow
In earth’s upper atmosphere a wide variety of photochemical, chemical and dynamical
processes occur continuously. Due to these chemical reactions, the absorbed solar energy is
released back in the form of radiations that appears as a faint glow referred to as ‘airglow’.
The airglow emission is present at all times, both during night and day. The airglow during
daytime is called the ‘dayglow’ during twilighttime, the ‘twilightglow’ and as ‘nightglow’
during nighttime.
All the three species, i.e., ions, electrons and neutrals participate in these chemical
reactions. Due to these chemical reactions an atom, molecule, or their ionic species reach in
their excited states. The excited species then return to their ground states giving out photons
in the ultraviolet to infrared spectral region of the electromagnetic spectrum, known as
deexcitation process. Some of the excited species return to their respective ground states via
metastable states during deexcitation process. Different metastable states have different life-
times for deexcitation. Quenching occurs at lower altitudes due to higher collision rate and so
it does not contribute to air glow emissions process. The densities of the reactants vary with
the height. The intensity of the emission depends on the densities of the reactants, the larger
the reactants, greater the intensity. So, every emission corresponds to a particular height and
provides information on the behavior of the reactants at the altitude of their origin.
Airglow emissions are continuous and global in nature. There are large numbers of
photochemical and chemical processes by which the excited states are produced. They are;
hν + A A* + K. E. Photon impact excitation
hν + A A+
* + K. E. Photon impact ionization and excitation
hν + AB A* + B* + K. E. Photo dissociative excitation
e + AB+
A* + B Dissociative recombination
e* + A e + A* Photoelectron excitation
e* + A e + A+
* + e Photoelectron ionization excitation
e* + AB e + A* + B Photoelectron impact dissociative
excitation
e* + AB e + A+
* + B* + E Dissociative ionization excitation
Where, hν is the energy of the incident photon from the sun, A and B are atomic species, e*
can represent both a photoelectron and energetic electrons of solar wind origin.

6. 2
The excitation processes can be categorized as; (a) the excitation by external agencies
such as solar photons and particle precipitation, and, (b) the mutual interaction amongst the
atmospheric species. The process (b) does not require external source and so it can be
operative at all the times i.e. day and night, where-as processes of the first category, those are
due to solar photons they are operative only during the daytime.
Airglow studies carried out at different time periods give information about the
dynamics present at that time and about their various parameters. There are a number of
experiments performed successfully to measure the twilightglow and nightglow.
Measurement of nightglow is easy because background continuum is absent. The dayglow
emission intensity is very low in comparison to the background solar continuum and
measurement is not easy as compared to nightglow.
Airglow emissions occur in wavelengths ranging from ultraviolet to infrared but we
are interested here in 630.0 nm emission line only, which is also known as oxygen red line
and it occurs in the thermosphere.

7. 3
2. Significance of airglow measurements
In remote sensing technique we collect data from a distance without being in contact
with the object, when we illuminate the object by an artificial source then it is called active
remote sensing. When we collect photon of the object illuminated by natural source then it is
called passive remote sensing. Optical investigation of airglow is somewhat similar to passive
remote sensing of the upper atmosphere. Ground-based investigations by photometry and
spectrometry can provide us enough data to understand the behavior of the upper atmosphere
at different heights.
1. Airglow emission intensity has a direct relationship with the reacting species and
we can obtain information about the vertical distribution of species in ionosphere.
2. The optical techniques have their major importance in the study of neutral
dynamics of thermosphere because we cannot obtain data by using RADAR techniques.
3. Gravity waves influence the chemistry and composition of the thermosphere. The
airglow emission intensity also changes due to it. The spatial variation in emission intensity
reflects the characteristics of the gravity waves.
4. Various airglow emissions originate at different heights. By measuring the
intensities at different wavelengths nearly simultaneously one can infer the vertical
propagation of waves from one region to another thereby giving important clues on the mode
of coupling of these different regions.
In nighttime all the above mentioned studies have been carried out. Some of these
studies have also been centered at during twilighttime. However, very less number of studies
have been carried out in daytime.
Next section deals with the challenges of daytime measurement of emissions and
includes a brief note on experiments performed in past, around the globe.

8. 4
3. Challenges of dayglow measurements
The presence of solar scattered background continuum which is of the order of Mega
Rayleighs of magnitude is much greater than the dayglow emission intensity which of the
order of Kilo Rayleighs makes the measurement of dayglow emissions very challenging.
Therefore, innovative and efficient methods have to be employed to extract the dayglow
emissions from strong solar continuum. There had been few attempts in the past to separate
out the weak emission features from the strong continuum background and with limited
success.
Blamont and Donahue (1964) tried to measure the sodium D lines by scanning the
line at 0.2-Å resolution and subtracting the normalized solar spectrum to remove the
Fraunhofer structure. The output was unexpectedly of high intensities of ~ 30 KR and it could
be contribution of direct solar radiation along with the scattered part of the solar radiation.
This technique was used for those limited dayglow lines, which can be resonantly scattered
by a vapor cell.
Bens et al. (1965) employed a combination of high and low resolution Fabry-Perots
(F-P) in series and an interference filter acted as monochromator at fixed wavelength and
obtained signatures of dayglow intensities. They performed a sequence of experiments with
varied instrumentation and made a comparison between the sky spectrum with the solar
spectrum. They just detected the emission lines and no conclusions were obtained.
Jarrett and Hoey (1963) employed a single low resolution F-P etalon and an
interference filter to obtain a fringe system at 630.0 nm on photographic plate. The observed
fringe system was questionable because Fraunhofer lines could also produce such images.
Noxon and Goody (1962) used the polarization property of the background to reduce
it. They employed a chopping mechanism at the entrance slit of an Ebert Scanning
spectrometer. The one half of the slit was covered with a polarizer and the other half with an
optical attenuator. The signals through the two halves of the slit were made equal for the
strongly polarized, scattered, sunlight at right angles to the sun so that the resulting
cancellation is independent of the intensity i.e. independent of the presence of Fraunhofer
lines. Dayglow emission lines which were unpolarized were efficiently modulated and
detected. The spectral scanning polarimeter was insensitive to the Fraunhofer lines but
random noise and real fluctuations in polarization associated with these lines were
responsible for the failure of this technique.

9. 5
A photometer with a unique radial chopping mechanism was developed by Desai et al
(1979). They had used it for detection of rocket released lithium clouds during daytime. It
was constructed by using pressure tuned F-P, an interference filter of 7.5 Å bandwidth and a
mask that isolates two concentric zones of equal angular width. They had isolated two zones
of the same area, one containing the line of interest and the other just beyond this line, and
obtained the difference between these two which gave the contribution due to the signal
alone. The basic assumption is that the contribution due to the background at and
immediately away from the emission feature is identical.
Narayanan et al (1989) had developed a unique photometer for the measurement of
daytime OI 630.0 nm emission. The photometer employed a pressure tuned low resolution
(104
) F-P etalon, temperature tuned narrowband (3 Å) interference filter, radial chopper and
up/down counting system. The radial chopper had two masks one is fixed and other is
rotating. The fixed disk passed through a optocouplar and enabled the generation of reference
pulses. Due to rotation of rotating mask the light passed through the system alternatively,
once through the central zone (signal plus background) and second time through the annular
zone (background). Reference pulse had acted as control pulse for the up/down counter and
difference between the number of pulses corresponding to the inner and outer zones
measured. The difference in counts had added up for defined time duration, passed onto a
digital to analog converter and plotted in X-T recorder. Total number of counts had recorded
in the other channel of X-T recorder simultaneously. The basic problem with the radial
chopping mechanism in this system was that the zones were never completely isolated and
there had always been a finite contribution from one zone when the measurements are made
through the other. Proper and complete subtraction of the background light is thus not
possible.
Figure 1. Schematic diagram of the dayglow photometer developed by
Narayanan et al (1989)

10. 6
Rees et al (2000) developed an instrument and obtained a true image of daytime
aurora which consists an imaging optical spectrograph illuminated by a wide angle lens based
on two capacitance stabilized F-P etalon operated in series with each other and a narrow band
interference filter. The etalon plate gaps were 3.0 mm and 278 µm respectively. Interference
fringes formed by the etalons were imaged by a 300 mm lens onto a 1024×1024 pixel 16 bit
charge coupled device (CCD) chip. They captured two dimensional image of the sky in
which wavelength was a unique function of radius. They obtained image of 630.0 nm aurora
emission as a bright ring approximately 80% of the radial distance from the image center to
the edge. By varying the optical path differences of both etalons, a sequence of ten images, as
the 630.0 nm ring were captured at different radii and a complete two dimensional image of
sky at λ=630.0 nm was build. The procedure
followed in three steps; first, wavelength map was
generated for each individual sky image such that
each pixel represented the central wavelength of the
spectral distribution that illuminated the
corresponding pixel in the sky image. In second step,
the sky emission feature was extracted from each
image. The third analysis step was to construct the 2-
dimensional monochromatic image from ten different
image radii. Figure 2. The sky image captured
by Rees et al (2000)

11. 7
4. PMT based dayglow photometer
An instrument called dayglow photometer (DGP) was developed by Narayanan et al
(1989) to conduct daytime airglow studies. The photometer employed a pressure tuned low
resolution (104
) F-P etalon, temperature tuned narrowband (3 Å) interference filter centered
(tuned) at 630.0 nm, a mask system, photo-multiplier tube (PMT) and data acquisition
system. The combination of two masks, a stator and a rotor made up the mask system. The
stator was divided into zones of 12 sections of equal areas, six segments are opaque and six
are transparent. The triangular and 'trapezoidal' segments are radially separated and hence
form the 'inner' and 'outer' annular zones. The rotor mask also had sections divided in a
similar way, except that there were only three transparent triangular segments, the diameter
of the same being equal to the outer dimensions of the outer zone (Pallamraju, 1996). This
mask had overcome the limitation encountered in the earlier version employed by Narayanan
et al. (1989). Another advantage was the area of light collection was more now; the flux
gathered is increased by a factor of 2 to 3 for the same intensity level. This mask system in
conjunction with the F P etalon, and synchronous photon counting systems made it possible
to remove the background contribution. Further, the signal plus background and the
background channels were now clearly separated in time due to the presence of the null zone.
This version of dayglow photometer yielded several exciting results of low and equatorial
electrodynamics.

12. 8
5. CCD based dayglow photometer
In the present project multiple changes have been made in the old design of DGP. The
new instrument has many improvements over PMT based photometer. It uses CCD detector
instead of PMT. The use of CCD improved the resolution of the instrument significantly and
the chances of adverse effects due to the use of high voltage are now reduced to zero. The
instrument does not use any rotating parts because the mask system is removed. We are able
to capture two dimensional images which can be stored in the computers digitally and
processed by using digital techniques.
5.1. Technical details
This instrument makes use of a narrow bandwidth (3 Å) interference filter to isolate
the wavelength of interest and a F-P etalon of 500 µm air gap is used to produce a fringe
pattern and essentially work as a very high spectral resolution filter. The fringe pattern
obtained from F P etalon is imaged onto a CCD of 1391×1039 pixel size by using a lens of
100 mm focal length. The following section deals with the working of each of the above
mentioned components:
i. Interference filter:
The inference filter used in this instrument has a narrow bandwidth of 3 Å and
centered at the wavelength 630.0 nm.
ii. Fabry-Perot etalon:
The F-P used in this instrument has an air gap (optical thickness) of 500 µm and
reflectance of 0.85. The interference formula for this etalon can be written as,
where, n, λ, µ and t are the order of the wavelength interference, wavelength of the radiation,
refractive index of the medium between the etalon plates and optical thickness, respectively.
The spectral resolution of the etalon at any wavelength λ depends upon the distance between
the two optical plates i.e. optical thickness t.
If the medium between the plates is air, then µ = 1 and our operating wavelength λ = 630 nm,
the order of the wavelength n is given by,

13. 9
n = 1587
Inter fringe distance better known as free spectral range of an etalon at that wavelength, is
given as,
For etalon used in this instrument the FSR turns out to 0.3969 nm or 0.4 nm (for λ = 630 nm).
Resolving power is given as the product of the order n (for wavelength λ) and finesse
F, of an F P. Hence,
Finesse F of an F-P depends upon the reflectivity of the plates, here r = 0.85
F = 19.31 for r = 0.85
Figure 3. Schematic diagram of CCD based DGP

14. 10
Resolving power of a F-P is,
R = 0.31×105
If we examine the fringe pattern formed when a F-P etalon is illuminated by a
monochromatic light source, the maxima occurs when, satisfied, and
minima when .
iii. The Charge Coupled Device (CCD):
A Sony ICX285AL CCD sensor used in this instrument. It is a diagonal 11 mm (Type
2/3) interline CCD solid-state image sensor with a square pixel array. The CCD is
monochrome and has pixel resolution of 1391×1039, nearly 1.45 Mega Pixels. The size of
each pixel is 6.45µm×6.45µm. The peak value of quantum efficiency is 65% at 540 nm and
sensitivity range is 300 nm – 1050 nm. The value of dark current is 0.0005 eˉ/pixel/sec at
0°C. Readout noise is 3.7 eˉ and gain is 0.267 eˉ/ADU. Minimum exposure possible is 1/1000
seconds and maximum is 10000 seconds. Analog to digital converter (ADC) is of 16 bits and
data format is RAW flexible image transport system (FITS). The cooling system is thermo
electrical with one level peltier element and permanent ventilation. We operate it at 0°C or
below 0°C to improve signal to noise ratio. An optimum value of exposure is selected to
avoid saturation. The CCD operates at 12 V DC which is provided by an AC adapter. We
connect the CCD to computer with USB cable. A driver softer is used to control the CCD and
capture images.
iv. Lens:
A lens of 100 mm focal length used to focus fringes formed by the F-P etalon onto
CCD.
5.2. Functioning of the instrument:
The light enters in the instrument through the interference filter. The interference
filter is centered on 630.0 nm wavelength and isolates light in the wavelength interval of
interest within its bandwidth of 3 Å. The isolated light falls on the F-P etalon. The fringe
pattern formed by F-P etalon is focused on the CCD chip with a 100 mm lens. The CCD chip
creates a two dimensional image for a given exposure. The two dimensional image contains
data for both the dayglow emission and the background continuum.

15. 11
The image is analyzed by using digital techniques and softwares like IDL. We get
circular fringe pattern in which 630.0 spectrum from a ring like structure. The procedure is
done in few steps;
First, a circular area which contains both emission spectrum and background will be
integrated. The background includes mainly scattering due to atmosphere. In second step, the
area which contains only emission spectrum will be integrated. The two circles have a
difference of wavelength λ=± 0.05 nm. The assumption that is taken here that contribution of
background will be same in both the
circular areas. As shown in figure 4 there
are two circular areas A1 and A2. A1
contain contributions from both daytime
emissions and scattering whereas A2
contains only scattering. The integrated
values calculated in second step for area
A2 is normalized to the integrated values
obtained for area A1. In the last step,
subtraction of A1 from normalized values
of A2 gives the value of daytime
emission.
Figure 5. Fringe pattern obtained with Mercury-Neon Lamp (left) and sky image
captured during observations (right)
Figure 4. Schematic diagram of sky image in
which area A1 contains dayglow emission
along with solar background and area A2
contains only solar background

16. 12
6. Production mechanisms of dayglow
There are three different important emission lines recognized in which the emission line
at 630.0 nm is used extensively to study the dynamics of thermosphere worldwide. The
production mechanisms of these lines are presented here.
1. OI 557.7 nm or Oxygen Green Line
When transition of atomic oxygen from O(1
S) to O(1
D) state takes place then we obtain
OI 557.7 nm emission line. There are a number of sources from which the emission arises;
they include electron impact, dissociative recombination, photo-dissociation of O2, and some
other chemical reactions.
Production of O(1
S);
A. Photoelectrons (ep) which have sufficient energy are responsible for excitation O(1
S)
and it is known as P E excitation,
O + ep O(1
S) + ep
B. The N2 is excited due to impact of photoelectron and energy is transferred through
collision, known as Collisional deactivation of N2.
N2 + ep N2 (A3
Σu
+
)
N2 (A3
Σu
+
) + O N2 + O(1
S)
C. Photodissociation by the solar photons in the wavelength range of (90 nm - 120 nm),
O2 + hν O(1
S) + O
D. Dissociative recombination takes place when O+
2 recombine with et to produce O(1
D).
O+
2 + et O(1
S) + O
E. In three-body recombination process an oxygen atom is excited to the O(1
S) state by
the three body recombination with two other atoms.
O + O + O O2 + O(1
S)
Loss of O(1
S);
A. De-excitation by emitting photon known as radiative transition
O(1
S) O(1
D) + hν (557.7 nm)
B. Collisional deactivation,
O(1
D) + X O + X
2. OI 630.0 nm or Oxygen Red Line
The OI 630.0 nm emission is a result of transition from O(1
D) to O(3
P) state.

17. 13
Production of O(1
D);
A. Photoelectron impact (PE) on the ground state oxygen, O (3
P).
O(3
P) + ep O(1
D) + e
B. Photodissociation (PD) of molecular oxygen in the Schumann-Runge continuum
(135-175 nm) of the solar radiation.
O2 + hν O(1
D) + O(3
P)
C. Dissociative recombination (DR) of molecular oxygen ion.
O2
+
+ e O(1
D) + O(3
P)
Loss of O(1
D);
A. Collisional quenching,
O(1
D) + X O + X, where X may be N2, O2, O, or et
B. Radiative transition to red doublet:
O (1
D) O + hν (630.0 nm, 636.4 nm)
3. OI 777.4 nm Line
When transition of O(5
P) to O(5
S) state occur emission of this wavelength takes place.
Radiative recombination of O+
with electron gives rise to its excited state O(5
P),
O+
+ e O(5
P)
The loss of O(5
P) is through radiative transition,
O(5
P) O(5
S) + hν (777.4 nm)
From the techniques described above, all these emissions as mentioned here can be
measured. A filter wheel type arrangement will be required. Further, the fringe pattern will be
different for different wavelengths. All these can be changed in an automated mode.

18. 14
7. Summary
I worked in Space and Atmospheric Science Division of Physical Research Laboratory during
my internship. My work focused on both the theoretical and experimental aspects, to
understand the dynamics of thermosphere. In this project I studied about the different type of
techniques used to measure the dayglow emissions around the globe, problems associated
with them and dayglow photometer, an instrument which was developed at PRL.

19. 15
8. Bibliography
1. Spectrophysics, Anne P. Thorne
2. Physics of the Aurora and Airglow, Joseph W. Chamberlain
3. Dayglow photometry: a new approach, Narayanan et al. (1989)
4. The First Daytime Ground-Based Optical image of the Aurora, Rees et al.
(2000)
5. Studies of Daytime Upper Atmospheric Phenomena Using Ground-Based
Optical Techniques, Prof. D. Pallamraju (1996)
6. Effects of Lower Atmospheric and Solar Forcings on Daytime Upper
Atmospheric Dynamics, Fazlul Islam Laskar (2015)