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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 -1, 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 ∘C) and lower in winter (4-5 ∘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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2003BASI...31..111S Altcode: No abstract at ADS Title: Ground Based Microwave Radiometric Technique in Remote Sensing Authors: Rangarajan, K. E.; Varghese, B. A. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1999ApJ...524L.139M Altcode: The polarimetric observations of the quiet Sun show linear polarization in molecular lines of C2, 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 (ΓI), and the depolarizing elastic collision rates (D(2)) for the molecular line transitions in the solar atmosphere. It is found that for the line λ5165.933, f=0.04, ΓI=1.85×107 s-1, and D(2)=9.52×106 s-1. The line λ5168.147 fits the observations for f=0.05, ΓI=2.91×107 s-1, and D(2)=3.69×107 s-1. Title: Effect of waves on resonance scattering polarization in solar atmosphere Authors: Rangarajan, K. E. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 O2 and H2O 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. Bibcode: 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.

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 Il < Ir but lower amount of polarization where Il > Ir. Here Il and Ir Title: Polarized resonance line transfer with collisional redistribution Authors: Mohan Rao, D.; Rangarajan, K. E. Bibcode: 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 RIII (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, RIII, and RII functions are used in the radiative transfer calculations. We considered isothermal, effectively thin and semi-infinite atmospheres. RIII 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.

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 (RII redistribution). Therefore γ is an important parameter while evaluating the wing polarization. We find that CRD may be a good approximation to RIII 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 RIII 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 RII. 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1981otse.conf...25R Altcode: No abstract at ADS