explanation blue bibcodes open ADS page with paths to full text
Author name code: rangarajan
ADS astronomy entries on 2022-09-14
=author:"Rangarajan, K.E."
---------------------------------------------------------
Title: Hα full line spectropolarimetry as diagnostics of
chromospheric magnetic field
Authors: Nagaraju, K.; Sankarasubramanian, K.; Rangarajan, K. E.
2020JApA...41...10N Altcode:
Analysis of spectropolarimetric observations of two circular sunspots
located close to disk centre in Hα (6562.8 Å) and Fe I (6569.22 Å)
is presented in this paper. The corresponding active region numbers are
NOAA 10940 and NOAA 10941 referred to as AR1 and AR2, respectively. The
vector magnetic field at the photosphere is derived through inversion
of Stokes profiles of Fe I under Milne-Eddington atmospheric model. The
chromospheric vector magnetic field is derived from Hα Stokes profiles
under weak-field approximation. Azimuthally averaged magnetic field
as a function of radial distance from the centre of sunspot at
the photosphere and chromosphere are studied. At the photosphere,
the radial variation shows a well known behaviour that the total
field and the line-of-sight (LOS) component monotonically decrease
from centre to the edge of the sunspot and the transverse component
initially increases, reaches a maximum close to half the sunspot
radius and then decreases. LOS and the transverse components become
equal close to half the sunspot radius consistent with the earlier
findings. At the chromosphere, all the components of the magnetic
field decrease with the sunspot radius. However, the LOS component
decreases monotonically whereas the transverse component decreases
monotonically up to about 0.6 times the sunspot radius after which it
reaches a constant value. Azimuthally averaged magnetic field gradient
from photosphere to chromosphere is also presented here.
---------------------------------------------------------
Title: Diagnosing chromospheric magnetic field through simultaneous
spectropolarimetry in Hα and Ca II 854.2 nm
Authors: Nagaraju, K.; Sankarasubramanian, K.; Rangarajan, K. E.
2020IAUS..354...46N Altcode:
Measurement of magnetic field in this layer is challenging both from
point of view of observations and interpretation of the data. We
present in this work about spectropolarimetric observations of a pore,
simultaneously in Ca ii (CaIR) at 854.2 nm (CaIR) and H α (656.28
nm). The observed region includes a small scale energetic event (SSEE)
taking place in the region between the pore and the region which show
opposite polarity to that of pore at the photosphere. The energetic
event appears to be a progressive reconnection event as shown by the
time evolution of the intensity profiles. Closer examination of the
intensity profiles from the downflow regions suggest that the height of
formation of CaIR is higher than that of Hi α, contrary to the current
understanding about their height of formation. Preliminary results on
the inversion of Stokes-I and V profiles of CaIR are also presented.
---------------------------------------------------------
Title: National Large Solar Telescope
Authors: Hasan, S. S.; Banerjee, D.; Ravindra, B.; Sankarasubramanian,
K.; Rangarajan, K. E.
2017CSci..113..696H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Astronomical site survey report on dust measurement, wind
profile, optical turbulence, and their correlation with seeing over
IAO-Hanle. Astronomical site survey report over IAO-Hanle
Authors: Ningombam, Shantikumar S.; Kathiravan, S.; Parihar, P. S.;
L. Larson, E. J.; Mohanan, Sharika; Angchuk, Dorje; Jorphel, Sonam;
Rangarajan, K. E.; Prabhu, K.
2017ExA....43..145N Altcode: 2017ExA...tmp....8N
The present work discusses astronomical site survey reports on dust
content, vertical distribution of atmospheric turbulence, precipitable
water vapor (PWV), surface and upper-air data, and their effects on
seeing over the Indian Astronomical Observatory (IAO) Hanle. Using
Laser Particulate Counter, ambient dust measurements at various sizes
(0.3 μm to 25 μm) were performed at various locations at the site
during November 2015. Estimated volume concentration for the particle
size at 0.5 μm was around 10,000 per cubic foot, which is equivalent
to ten thousand class of clean room standard protocol. During the
measurement, surface wind speed varied from 0-20 m s <SUP>-1</SUP>,
while estimated aerosol optical depth (AOD) using Sky radiometer
(Prede) varied from 0.02-0.04 at 500 nm, which indicates the site
is fairly clean. The two independent measurements of dust content
and aerosol concentrations at the site agreed well. The turbulence
or wind gust at the site was studied with wind profiles at three
different heights above the ground. The strength of the wind gust
varies with time and altitude. Nocturnal temperature across seasons
varied with a moderate at summer (6-8 <SUP>∘</SUP>C) and lower in
winter (4-5 <SUP>∘</SUP>C). However, the contrast between the two
is significantly small due to cold and extremely dry typical climatic
conditions of the site. The present study also examined the effects of
surface and upper-air data along with Planetary Boundary Layer (PBL)
dynamics with seeing measurement over the site. Further, a comparative
study of such observed parameters was conducted with other high altitude
astronomical observatories across the globe.
---------------------------------------------------------
Title: Solar Astronomy at High Altitude
Authors: Hasan, S. S.; Bagare, S. P.; Rangarajan, K. E.
2014PINSA..80..815H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Transit of Venus on 2012 June 06: stray light estimation
and restoration of Ca-K images of Twin Telescope from Kodaikanal
Observatory
Authors: Prabhu, K.; Rangarajan, K. E.; Ravindra, B.;
Sankarasubramanian, K.; Selvendran, R.; Kumaravel, P.
2013BASI...41..227P Altcode:
Observations of the transit of Venus were made from the Kodaikanal
Observatory on 2012 June 06 in Ca-K wavelengths. Only half of the event
was visible from India. We utilized this unique opportunity to compute
the contribution of the scattered light within the Twin Telescope
optics at Kodaikanal. The instrumental and atmospheric scattered
light are derived from the model point spread function which is a
combination of four Gaussians with different widths and weights. The
restoration procedure has significantly improved the rms contrast of
the images. The rms contrast of the stray light corrected images are
almost two fold larger than that those of the uncorrected images. The
derived network element sizes matches well with previous observations.
---------------------------------------------------------
Title: Sunspot Magnetometry from Kodaikanal
Authors: Nagaraju, K.; Sankarasubramanian, K.; Rangarajan, K. E.
2010ASSP...19..504N Altcode: 2010mcia.conf..504N
Spectroscopic magnetic field measurements in sunspots have been
carried out at Kodaikanal since such research was started by John
Evershed (Evershed 1944). Subsequently, a Stokes polarimeter was
built by Balasubramaniam and collaborators for the spectrograph at
the Kodaikanal Tower Telescope (KTT), with the goal of measuring
vector magnetic fields. Although the achieved accuracy is limited,
the telescope model developed by them is very accurate (Balasubramaniam
et al. 1985) and is still used even today to correct for instrumental
polarization. A similar polarimeter was developed by Sankarasubramanian
and collaborators, which is more modern in having motorized rotation
of the polarization optics and in using CCD detectors instead of
photographic registration on film (Sankarasubramanian et al. 2002). The
accuracy in magnetic field measurement achieved with this instrument
was quite high. Very recently, a dual-beam polarimeter has been
installed, which uses an efficient and well-balanced modulation
scheme. The calibration and characterization of this instrument are
presented in Nagaraju et al. (2008b); results on magnetic structuring
from the photosphere to the chromosphere are presented in Nagaraju et
al. (2008a, 2009).
---------------------------------------------------------
Title: Spectropolarimetry with the NLST
Authors: Sankarasubramanian, K.; Hasan, S. S.; Rangarajan, K. E.
2010ASSP...19..156S Altcode: 2010mcia.conf..156S
India's National Large Solar Telescope (NLST) will provide
opportunities to observe the Sun with high spatial, spectral, and
polarimetric resolution. The large aperture also enables high-cadence
spectropolarimetry with moderate spatial resolution. A multi-slit
spectropolarimeter is planned as one of the back-end instruments for
this powerful telescope, primarily to measure vector magnetic fields
in both active and quiet regions. An integral-field unit added with the
multi-slit spectropolarimeter will enable fast-cadence observation. Here
we discuss the scientific requirements for such an instrument, along
with advantages and limitations of the concept and preliminary design
details.
---------------------------------------------------------
Title: Spectropolarimetry of an Active Region at the Photosphere
and Chromosphere
Authors: Nagaraju, K.; Sankarasubramanian, K.; Rangarajan, K. E.
2009ASPC..405..251N Altcode:
Initial results on the simultaneous spectropolarimetric observations of
an active region at the photosphere and chromosphere are presented. For
this purpose, the Fe I line at λ6569 and the H I at λ6563 (H
α) are used. Stratification of the line-of-sight (LOS) velocity
and magnetic fields above an active region are discussed. The LOS
magnetic field strengths are derived using the center-of-gravity (COG)
method and the LOS velocity gradients are derived using the bisector
technique. From this analysis it is found that both the velocity and
magnetic gradients are larger in the umbral region above the sunspot
compared to the penumbral region. And the magnetic field strength
decreases much faster with height in the umbral region compared to
the penumbral region. Upflows with larger LOS velocity gradients are
located in the regions where stronger photospheric fields are observed.
---------------------------------------------------------
Title: Magnetic and Velocity Properties of G-Band Bright Points
Around a Pore
Authors: Rangarajan, K. E.; Sankarasubramanian, K.; Nagaraju, K.
2009ASPC..405..255R Altcode:
Physical properties of G-band bright points (GBPs) surrounding a
pore are studied using observations spanning from photosphere to
chromosphere. The magnetic field strengths obtained in these bright
points are of the order of 1.2~kG. These GBPs show very little linear
polarization and the magnetic field lines are vertical. The observed
GBPs show a fill-fraction between 20 to 40%, revealing that they
are smaller than the achieved spatial resolution of 0.2 arcsec. The
observed GBPs harbor accelerated downflows.
---------------------------------------------------------
Title: On the performance of a dual-beam polarimeter at Kodaikanal
Tower Telescope
Authors: Nagaraju, K.; Sankarasubramanian, K.; Rangarajan, K. E.;
Ramesh, K. B.; Singh, Jagdev; Devendran, P.; Hariharan
2008BASI...36...99N Altcode:
Calibration and characterization of a dual-beam polarimeter installed
at Kodaikanal Tower Telescope (KTT) for spectropolarimetric observations
are presented in this paper. It was found that a slit width of 48 μ is
optimum for the spectrograph setup at KTT and corresponding spectral
resolution is 32.47 mÅ. It was demonstrated that the precision in
polarization measurement can be achieved better than 0.1 % by increasing
the exposure time. However, the polarimetric calibration accuracy is
limited to 0.35 % for Stokes Q and U parameters and 0.2 % for Stokes
V parameter, mainly due to the uncertainty in the retardance of the
calibration retarder. A comparison of the magnetic field measurement
between the Solar Optical Telescope (SOT) onboard Hinode and KTT for
an active region was made and a good match was found after spatial
smearing of Hinode/SOT measurements by 5.12”.
---------------------------------------------------------
Title: On the Weakening of the Chromospheric Magnetic Field in
Active Regions
Authors: Nagaraju, K.; Sankarasubramanian, K.; Rangarajan, K. E.
2008ApJ...678..531N Altcode: 2008arXiv0801.3128N
Simultaneous measurement of line-of-sight (LOS) magnetic and velocity
fields at the photosphere and chromosphere are presented. The Fe I
line at λ6569 and Hα at λ6563 are used for deriving the physical
parameters at photospheric and chromospheric heights, respectively. The
LOS magnetic field obtained through the center-of-gravity method shows
a linear relation between the photospheric and chromospheric fields for
field strengths less than 700 G. But in strong field regions, the LOS
magnetic field values derived from Hα are much weaker than what one
gets from the linear relationship, and also from those expected from
the extrapolation of the photospheric magnetic field. We discuss in
detail the properties of the magnetic field observed in Hα from the
point of view of observed velocity gradients. The bisector analysis
of Hα Stokes I profiles shows larger velocity gradients in those
places where strong photospheric magnetic fields are observed. These
observations may support the view that the stronger fields diverge
faster with height compared to weaker fields.
---------------------------------------------------------
Title: An efficient modulation scheme for dual beam polarimetry
Authors: Nagaraju, K.; Ramesh, K. B.; Sankarasubramanian, K.;
Rangarajan, K. E.
2007BASI...35..307N Altcode: 2007arXiv0710.1694N
An eight stage balanced modulation scheme for dual beam polarimetry
is presented in this paper. The four Stokes parameters are weighted
equally in all the eight stages of modulation resulting in total
polarimetric efficiency of unity. The gain table error inherent in
dual beam system is reduced by using the well known beam swapping
technique. The wavelength dependent polarimetric efficiencies of
Stokes parameters due to the chromatic nature of the waveplates are
presented. The proposed modulation scheme produces better Stokes Q
and V efficiencies for wavelengths larger than the design wavelength
whereas Stokes U has better efficiency in the shorter wavelength
region. Calibration of the polarimeter installed as a backend instrument
of the Kodaikanal Tower Telescope is presented. It is found through
computer simulation that a 14 % sky transparency variation during
calibration of the polarimeter can introduce 1.8 % uncertainty in the
determination of its response matrix.
---------------------------------------------------------
Title: Contribution Functions for Stokes Vector Profiles
Authors: Nagaraju, K.; Rangarajan, K. E.
2005BASI...33..357N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Study of Modulation and Demodulation Schemes for a Two Beam
Polarimeter
Authors: Nagaraju, K.; Sankarasubramanian, K.; Ramesh, K. B.;
Rangarajan, K. E.
2005BASI...33..410N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Two-Beam Spectropolarimeter for Kodaikanal Tower Telscope
Authors: Ramesh, K. B.; Nagaraju, K.; Rangarajan, K. E.;
Sankarasubramanian, K.
2005BASI...33..410R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Digitization of Kodaikanal Data
Authors: Singh, Jagdev; Muneer, S.; Bagare, S. P.; Rangarajan, K. E.;
Ramesh, K. B.; Varghese, B. A.; Samson, J. P. A.; Devendran, P.;
Hariharan, G.
2003BASI...31..111S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Ground Based Microwave Radiometric Technique in Remote Sensing
Authors: Rangarajan, K. E.; Varghese, B. A.
2003BASI...31..223R Altcode:
As an important tool of atmospheric remote sensing, microwave
radiometers are used for temperature profiling and vapor and liquid
column measurements. In addition, ice water path can be retrieved using
radiometer observations. In this review, the potential of radiometers is
demonstrated by comparison with radiosonde data and observations using
Global Positioning Systems (GPS). Performance of various mathematical
retrieval methods for water vapor and cloud liquid water profiles
using microwave radiometer measurements are compared. These include
regression methods. Newton iteration and neural networking. A specific
case of temperature inversion near the ground and its retrieval by
Philips-Twomay method is discussed.
---------------------------------------------------------
Title: Measurement of solar vector magnetic fields using Kodaikanal
tower telescope
Authors: Sankarasubramanian, R.; Rangarajan, K. E.; Ramesh, K. B.
2002BASI...30..473S Altcode:
A Stokes polarimeter was built at the Kodaikanal Tower Telescope to
study the vector field map of active regions like sunspot using the
well known Fe I lines 6301.5 Å and 6302.5 Å lines. The Mueller matrix
of the polarimeter was obtained using simple laboratory and field
tests. The instrumental polarisation due to the oblique reflections
are removed before the observed Stokes profiles were analysed. The
advanced Stokes Polarimeter data analysis program (ASP code) was used to
invert the corrected Stokes profiles to calculate the vector magnetic
field parameters of NOAA 8951. The maximum field strength obtained at
the umbra of the sunspot was 2500 Gauss. The variation of the field
strength, the line-of-sight angle and the azimuthal angle along a
cut which passes through the umbra resembles that of a simple round
sunspot. The accuracy in the calculation of the vector field is high
in the umbra and low in the outer edge of the penumbra of the observed
sunspot. The range of errors in the calculation of the field strength,
line-of-sight inclination angle, and the azimuthal angle of the magnetic
vector are 20-100 Gauss, 1-5O, and 12-20O respectively. We also find a
good fit for the observed Stokes profiles of the Ti I line at 6303.78
Å with a synthetic profile produced using the vector field parameters
derived from the Fe I 6301.5 Å and 6302.5 Å lines.
---------------------------------------------------------
Title: Effects of `Seeing' on Vector Magnetograph Measurements
Authors: Leka, K. D.; Rangarajan, K. E.
2001SoPh..203..239L Altcode:
We present a study of the effects of atmospheric seeing on quantities
derived from observations of solar polarized light - specifically,
the vector magnetic flux and quantities derived from its magnitude and
direction. Data from the Imaging Vector Magnetograph (`IVM') at the
U. Hawaii/Mees Solar Observatory, are degraded by various degrees by
applying a blur function to the `incoming light', simulating a range
of seeing conditions. A quantitative study of the resulting effects
on derived quantities including total magnetic flux, vertical electric
current density and magnetic shear angles, are discussed as a function
of the imposed degradation. The generality of the seeing effects is
explored by comparing the results from two different active regions;
we find that the results are comparable for those quantities directly
computed from the magnetic flux vector (e.g., summed, as in total flux)
but less so for those quantities involving higher-order calculations
(e.g., derivatives, as in vertical currents). We suggest that for
temporal series data from any instrument, a method such as that which
we outline here, be applied in order to model the uncertainties imposed
on the data (in addition to instrumental uncertainties, etc.) due to
seeing variations.
---------------------------------------------------------
Title: The Effect of “Seeing” on Imaging Vector Magnetograph
Measurements of Solar Active Regions
Authors: Leka, K. D.; Rangarajan, K. E.
2001AGUSM..SP41B06L Altcode:
All ground-based instruments are subject to atmospheric seeing;
here we attempt to quantify the effects of atmospheric seeing on
vector magnetograph observations and the parameters derived from
them. We present the results of our study on the effect of seeing
using Imaging Vector Magnetograph (IVM) raw data and subsequent
data reduction. The blurring due to seeing is modeled by convolving
a Gaussian function with various widths with raw polarization data
of good-seeing quality to simulate different seeing conditions. The
IVM data reduction procedure is carried out on all data to arrive
at vector magnetic flux and velocity images; from these are derived
commonly used parameters such as vertical current density, shear angle
and magnetic free energy. We find that poorer seeing conditions during
observations reduce the derived magnetic field strength, as expected,
and influence the other parameters in generally expected ways. Our
study results in defining the limits on uncertainties due to seeing
in parameters derived from vector magnetic flux observations.
---------------------------------------------------------
Title: Parametric Study of Molecular Line Polarization in the Solar
Atmosphere
Authors: Rangarajan, K. E.; Mohan Rao, D.
2000JApA...21..269R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The effect of angle-dependent non-coherent electron scattering
and partial redistribution on the polarization of resonance lines
Authors: Rangarajan, K. E.
1999MNRAS.308.1053R Altcode:
The polarized radiative transfer equation is solved when angle-dependent
partial redistribution and non-coherent electron scattering are
included as line-scattering mechanisms. A static atmosphere with
plane parallel symmetry is assumed. Test calculations are used to
illustrate the effects of the electron-scattering coefficient, the
thermalization parameter and the continuous absorption coefficient on
the line polarization. Results of angle-averaged and angle-dependent
redistribution functions are compared and it is shown that
angle-dependent functions should be used to model the wing polarization
of optically thin lines. The lower the continuous absorption compared
with the electron scattering, the higher the wing polarization.
---------------------------------------------------------
Title: Molecular Line Polarization in the Solar Atmosphere: MGH Lines
Authors: Mohan Rao, D.; Rangarajan, K. E.
1999ApJ...524L.139M Altcode:
The polarimetric observations of the quiet Sun show linear polarization
in molecular lines of C<SUB>2</SUB>, MgH, etc. The molecular lines
are very faint in the intensity spectrum. We propose that the
polarization is caused by coherent scattering processes in the
solar atmosphere as in the case of atomic lines. The intensity
and polarization profiles of MgH lines are calculated using the
quiet-Sun model by Vernazza, Avrett, & Loeser. This procedure
allows us to estimate the oscillator strengths (f), the inelastic
collisional rates (Γ<SUB>I</SUB>), and the depolarizing elastic
collision rates (D<SUP>(2)</SUP>) for the molecular line transitions
in the solar atmosphere. It is found that for the line λ5165.933,
f=0.04, Γ<SUB>I</SUB>=1.85×10<SUP>7</SUP> s<SUP>-1</SUP>, and
D<SUP>(2)</SUP>=9.52×10<SUP>6</SUP> s<SUP>-1</SUP>. The line λ5168.147
fits the observations for f=0.05, Γ<SUB>I</SUB>=2.91×10<SUP>7</SUP>
s<SUP>-1</SUP>, and D<SUP>(2)</SUP>=3.69×10<SUP>7</SUP> s<SUP>-1</SUP>.
---------------------------------------------------------
Title: Effect of waves on resonance scattering polarization in
solar atmosphere
Authors: Rangarajan, K. E.
1999ASSL..243..179R Altcode: 1999sopo.conf..179R
No abstract at ADS
---------------------------------------------------------
Title: Resonance line polarization in the presence of wave motion.
Authors: Rangarajan, K. E.
1997A&A...320..265R Altcode:
The effect of wave velocity on resonance line polarization is
studied. We assumed a photosphere - chromosphere type of temperature
structure for the atmosphere with plane parallel symmetry. The
"Discrete Space Theory technique" to solve the polarized radiative
transfer in the presence of velocity fields and under the assumption
of Complete Redistribution mechanism is briefly described along
with the numerical checks performed. We considered both sinusoidal
and sawtooth wave functions for the velocity in the medium. We find
the linear polarization in the resonance lines to be increased when
averaged over the whole period of the wave compared to the static
medium case. The sawtooth wave shows a different average polarization
profile compared to the sinusoidal wave. Line center intensity as
well as the polarization vary with a period which is half of that of
the wave. The variation amplitude for polarization is higher compared
to the intensity towards the limb. Hence polarization in the limb may
serve as an additional useful information to characterize the wave.
---------------------------------------------------------
Title: Non-conservative Rayleigh scattering in a finite
atmosphere-II. Polarization in telluric lines
Authors: Mohan Rao, D.; Rangarajan, K. E.; Abhyankar, K. D.
1995BASI...23..446M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Non-conservative Rayleigh scattering in a finite atmosphere -
II. Polarization in telluric lines
Authors: Mohan Rao, D.; Rangarajan, K. E.; Abhyankar, K. D.
1995BASI...23..211M Altcode:
We consider non-conservative Rayleigh scattering in finite
atmospheres and study the polarization in the molecular lines both
in the transmitted and reflected sunlight. We first investigate the
parameterized models to estimate the effects of optical thickness, line
strength, continuous absorption etc. on line polarization. This study
helped us to analyse some typical O<SUB>2</SUB> and H<SUB>2</SUB>O
band lines in the earth's atmosphere. For the chosen parameters,
we find that the polarization is more sensitive to optical thickness
than the line strength. The continuous absorption affects the wing
polarization selectively.
---------------------------------------------------------
Title: Non-conservative Rayleigh scattering in a finite
atmosphere. I. Polarization of skylight
Authors: Rangarajan, K. E.; Mohan Rao, D.; Abhyankar, K. D.
1994BASI...22..465R Altcode:
Non-conservative Rayleigh scattering in finite atmospheres is
studied for various values of ω where ω is the albedo for single
scattering. The atmosphere is stratified into plane-parallel homogeneous
layers. The sky polarization is plotted for certain typical positions
of the sun. <P />We find that for the transmitted light the Babinet and
Brewster neutral points in the principal meridian are brought closer
to the sun for the lesser value of ω, while the Arago neutral point
moves towards the anti-solar point with decreasing ω. As the albedo
ω decreases, we get higher amount of polarization in the region where
I<SUB>l</SUB> < I<SUB>r</SUB> but lower amount of polarization where
I<SUB>l</SUB> > I<SUB>r</SUB>. Here I<SUB>l</SUB> and I<SUB>r</SUB>
---------------------------------------------------------
Title: Polarized resonance line transfer with collisional
redistribution
Authors: Mohan Rao, D.; Rangarajan, K. E.
1993A&A...274..993M Altcode:
In this paper we study the effect of depolarizing collisions on
resonance line polarization in the absence of magnetic fields. The
role of γ, the coherence parameter is investigated in determining the
percentage of polarization across the spectral line profiles. We also
try to find out whether the partial frequency redistribution function
R<SUB>III</SUB> (which describes the frequency redistribution due to
collisions) can be replaced by the complete redistribution (CRD, i.e
φ(χ)φ(x‧)) in resonance line polarization calculations. We have
chosen idealized models to illustrate the differences in polarization
when CRD, R<SUB>III</SUB>, and R<SUB>II</SUB> functions are used
in the radiative transfer calculations. We considered isothermal,
effectively thin and semi-infinite atmospheres. R<SUB>III</SUB>
function gives different polarization in the Doppler core compared to
CRD when depolarizing collisions are neglected, but it gives the same
polarization as CRD in the wings in all circumstances. It is proposed
that any redistribution function which is different from φ(χ)φ(x‧)
at certain frequencies is likely to show different polarization compared
to CRD at those frequencies even though the total specific intensities
may not differ. <P />We find that the percentage of polarization at
the line centre to be a monotonic function of the parameter γ when
depolarizing collisions are included in the calculations. This important
aspect is absent if the depolarizing collisions are neglected. Therefore
one may use this property for diagnostic purposes. The character of
the wing polarization is altered when γ = 0.9 compared to γ = 1.0
(R<SUB>II</SUB> redistribution). Therefore γ is an important parameter
while evaluating the wing polarization. We find that CRD may be a
good approximation to R<SUB>III</SUB> when depolarizing collisions
are included in the approximation.
---------------------------------------------------------
Title: The combined effect of partial redistribution and non-coherent
electron scattering on polarized resonance line transfer
Authors: Nagendra, K. N.; Rangarajan, K. E.; Rao, D. M.
1993MNRAS.262..855N Altcode:
The combined effect of partial frequency redistribution by atoms
(PRD) and noncoherent electron scattering (NCES) on line formation is
studied, taking account of the polarization state of the radiation
field in resonance line scattering. For the purpose of comparison,
a study is also made of redistribution by other mechanisms, namely
complete redistribution and coherent scattering (CS) combined with
NCES. A static medium stratified into plane-parallel homogeneous
layers is considered. A conventional two-level atom approximation is
employed as the basic model. The emergent linear polarization profile
exhibits interesting characteristics in its variation across the line
profile. Atomic redistribution plays an important role at the line
core and the near wings of the resonance line. Electron scattering,
however, makes significant contributions in the far wings (x is greater
than 10). For resonance lines with high optical thickness, NCES leads
to measurable changes of polarization in the far wings irrespective
of the atomic scattering mechanism employed.
---------------------------------------------------------
Title: The effects of non-coherent electron scattering on spectral
line formation
Authors: Rangarajan, K. E.; Mohan Rao, D.; Peraiah, A.
1991MNRAS.250..633R Altcode:
Spectral line profiles from a plane-parallel homogeneous atmosphere have
been calculated for the complete and partial redistribution mechanisms
(CRD and PRD) including coherent and noncoherent electron scattering. It
is found that in all the cases where the electron scattering coefficient
is more than the continuous absorption coefficient, there are measurable
changes in the spectral line profiles. In such situations, the PRD
profiles have more emission hump compared to CRD profiles in the
intermediate frequency range. Extended wings and additional broadening
of the line profiles are obtained for the case of noncoherent electron
scattering compared to coherent electron scattering. These results have
implications in the derivation of physical properties (for example,
microturbulent velocities) of stellar atmospheres from spectral lines.
---------------------------------------------------------
Title: The Effects of Electron Scattering on the Si II 1816 Line in
the Solar Chromosphere (With 1 Figure)
Authors: Rangarajan, K. E.; Rao, D. M.
1991mcch.conf...63R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Effects of stimulated emission on radiative transfer with
partial redistribution.
Authors: Rangarajan, K. E.; Mohan Rao, D.; Peraiah, A.
1990A&A...235..305R Altcode:
The authors study the non-LTE line transfer with stimulated
emission. Stimulated emission is important for red transitions
in hot stars and infrared transitions in cool stars. The authors
investigate the deviation of the absorption and emission profiles
from each other for a two-level atomic model with the line scattering
described by the angle-averaged redistribution functions. The partial
redistribution formalism has been used while solving the radiative
transfer equation. The correct expression for the source function
derived by Baschek, Mihalas and Oxenius (1981) has been employed to
obtain the emission profile and the radiation field. From this study,
the authors have obtained the following results: the redistribution
function R<SUB>III</SUB> gives the emission profile same as that of
the absorption profile (like complete redistribution) in the core and
also in the wings but with a small enhancement at the intermediate
frequency points whether stimulated emission is present or not. The
emergent emission profile is different from the absorption profile by
several factors in the wings for R<SUB>II</SUB>.
---------------------------------------------------------
Title: Effect of the Time Spent by the Photon in the Absorbed State
on the Time-dependent Transfer of Radiation
Authors: Rao, D. Mohan; Rangarajan, K. E.; Peraiah, A.
1990ApJ...358..622R Altcode:
The time-dependent transfer equation is derived for a two-level atomic
model which takes both bound-bound and bound-free transitions into
account. A numerical scheme is proposed for solving the monochromatic
time-dependent transfer equation when the time spent by the photon in
the absorbed state is significant. The method can be easily extended
to solve the problem of time-dependent line formation of the bound-free
continuum. It is used here to study three types of boundary conditions
of the incident radiation incident on a scattering atmosphere. The
quantitative results show that the relaxation of the radiation field
depends on the optical depth of the medium and on the ray's angle
of emergence.
---------------------------------------------------------
Title: Simulation of multiple aperture synthesis.
Authors: Saha, S. K.; Jayarajan, A. P.; Rangarajan, K. E.; Chatterjee,
S.
1988ESOC...29..661S Altcode: 1988hrii.conf..661S
In view of the importance of multiple aperture synthesis in optical
astronomy, a computer simulation of the same is demonstrated. The fringe
patterns obtained through the aperture mask of six and nine holes
arranged non-redundantly along a circle, are reported. The intensity
distribution formula at the focal plane, for interference of n beams
is presented. The effect of phase randomness on the focal intensity
distribution is also experimentally studied.
---------------------------------------------------------
Title: VRI Photometry of LMC SN 1987A
Authors: Rangarajan, K. E.; Jayakumar, K.; Appakutty, M.; Sheriff,
H. D.
1987IBVS.3051....1R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Preliminary Analysis of the Broad he i Emission Lines in R CrB
Authors: Surendiranath, R.; Rangarajan, K. E.; Kameswara Rao, N.
1986hdsr.proc..199S Altcode: 1986IAUCo..87..199S
No abstract at ADS
---------------------------------------------------------
Title: Effects of partial frequency redistribution functions R(II),
R(III) and R(V) on source functions
Authors: Mohan Rao, D.; Rangarajan, K. E.; Peraiah, A.
1984JApA....5..169M Altcode:
The effects of partial frequency redistribution on the formation of
spectral lines have been studied. The angle-averaged R(II), R(III)
and R(V) types of redistribution with isotropic phase function have
been considered. Transfer equation with plane-parallel geometry is
solved in isothermal atmospheres. For an atmosphere with constant
thermal sources, the frequency-dependent source function S/L/(R/V/)
lies below S/L/(R/III/) but above S/L/(R/II/) in the line wings.
---------------------------------------------------------
Title: Effects of redistribution with dipole scattering on line
source functions
Authors: Peraiah, A.; Rangarajan, K. E.
1981JApA....2..245P Altcode:
The partial frequency redistribution function for zero natural line
width and dipole scattering is used to obtain a simultaneous solution
of the statistical equilibrium and line transfer equations in the
comoving frame of an expanding gas. A non-LTE, two-level atom in an
expanding spherical medium is considered, and the population ratio of
the two levels, N2/N1, is calculated. The upper level population (N2),
initially set equal to zero, is found to be enhanced significantly
from its initial value. Variation in the velocity gradient had little
effect on the N2/N1 ratio when the geometrical thickness of the medium
was three or ten times the stellar radius; when the thickness was
increased to twenty times the central radius, however, the velocity
gradients changed this ratio considerably in the region where log T
is less than or equal to two, T being the total optical depth.
---------------------------------------------------------
Title: Photon escape probabilities in expanding atmospheres
Authors: Peraiah, A.; Rangarajan, K. E.; Rao, D. M.
1981JApA....2...81P Altcode:
A comparison of mean number of scatterings and escape probabilities
has been made in isotropic scattering and dipole scattering by using
the angle-averaged partial frequency redistribution function. The
equations of radiative transfer and statistical equilibrium have
been solved simultaneously in a spherically symmetric expanding
atmosphere. Two cases of atmospheric extension where the ratios of
outer to inner atmospheric radii are 3 and 10 have been treated. It
is found that the partial frequency redistribution gives a larger
mean number of scatterings compared to that given by complete
redistribution. Velocities tend to reduce the mean number of scatterings
and increase the mean escape probabilities.
---------------------------------------------------------
Title: Effects of velocities on Ca II H and K lines.
Authors: Rangarajan, K. E.; Mohan Rao, D.; Peraiah, A.
1981KodOB...3...75R Altcode:
With a chosen model atmosphere, the authors find double peaked emission
for H and K lines in the static medium. The results show that the
systematic velocity fields do play a significant role in determining
the shape of the H and K profiles.
---------------------------------------------------------
Title: Coronal photography at H alpha.
Authors: Rangarajan, K. E.; Giridhar, S.
1981otse.conf...25R Altcode:
No abstract at ADS
