Author name code: nigam ADS astronomy entries on 2022-09-14 author:"Nigam, Rakesh" ------------------------------------------------------------------------ Title: Note on Travel Time Shifts Due to Amplitude Modulation in Time-Distance Helioseismology Measurements Authors: Nigam, R.; Kosovichev, A. G. Bibcode: 2010ApJ...708.1475N Altcode: 2009arXiv0911.4295N Correct interpretation of acoustic travel times measured by time-distance helioseismology is essential to get an accurate understanding of the solar properties that are inferred from them. It has long been observed that sunspots suppress p-mode amplitude, but its implications on travel times have not been fully investigated so far. It has been found in test measurements using a "masking" procedure, in which the solar Doppler signal in a localized quiet region of the Sun is artificially suppressed by a spatial function, and using numerical simulations that the amplitude modulations in combination with the phase-speed filtering may cause systematic shifts of acoustic travel times. To understand the properties of this procedure, we derive an analytical expression for the cross-covariance of a signal that has been modulated locally by a spatial function that has azimuthal symmetry and then filtered by a phase-speed filter typically used in time-distance helioseismology. Comparing this expression to the Gabor wavelet fitting formula without this effect, we find that there is a shift in the travel times that is introduced by the amplitude modulation. The analytical model presented in this paper can be useful also for interpretation of travel time measurements for the non-uniform distribution of oscillation amplitude due to observational effects. Title: Analytical Models for Cross-Correlation Signal in Time-Distance Helioseismology Authors: Nigam, R.; Kosovichev, A. G.; Scherrer, P. H. Bibcode: 2007ApJ...659.1736N Altcode: 2007astro.ph..2499N In time-distance helioseismology, the time signals (Doppler shifts) at two points on the solar surface separated by a fixed angular distance are cross-correlated, and this leads to a wave packet signal. Accurately measuring the travel times of these wave packets is crucial for inferring the subsurface properties in the Sun. The observed signal is quite noisy, and to improve the signal-to-noise ratio and make the cross-correlation more robust, the temporal oscillation signal is phase-speed filtered at the two points in order to select waves that travel a fixed horizontal distance. Hence a new formula to estimate the travel times is derived in the presence of a phase-speed filter, and it includes both the radial and horizontal component of the oscillation displacement signal. It generalizes the previously used Gabor wavelet that was derived without a phase-speed filter and included only the radial component of the displacement. This is important since it will be consistent with the observed cross-correlation that is computed using a phase-speed filter, and it also accounts for both the components of the displacement. The new formula depends on the location of the two points on the solar surface that are being cross-correlated and accounts for the travel time shifts at different locations on the solar surface. Title: A first look at past sea surface temperatures in the equatorial Indian Ocean from Mg/Ca in foraminifera Authors: Saraswat, R.; Nigam, R.; Weldeab, S.; Mackensen, A.; Naidu, P. D. Bibcode: 2005GeoRL..3224605S Altcode: Sea surface temperature (SST) for the central equatorial Indian Ocean, has been reconstructed over the last ~137 kyr, from Mg/Ca of the planktonic foraminiferal species Globigerinoides ruber. According to our record the equatorial Indian Ocean SST was ~2.1°C colder during the last glacial maximum as compared to present times. The data further shows that the surface equatorial Indian Ocean was comparatively warmer during isotopic stage 5e than at present (~29.9 vs ~28.5°C). Comparison of the equatorial Indian Ocean SST with the Antarctic δD and Greenland δ18O records, shows that the major high-latitude cooling/warming events are also present in the equatorial Indian Ocean SST variation record. Similarity between the equatorial Indian Ocean SST and the equatorial Pacific SST suggests the possibility of a common mechanism controlling the SSTs in both the equatorial Indian Ocean and the Pacific Ocean. Title: Palaeoceanographic implications of abundance and mean proloculus diameter of benthic foraminiferal species Epistominella exigua in sub-surface sediments from distal Bay of Bengal fan Authors: Saraswat, R.; Nigam, R.; Barreto, Lea Bibcode: 2005JESS..114..453S Altcode: Temporal variation in abundance and mean proloculus diameter of the benthic foraminiferal species Epistominella exigua has been reconstructed over the last ∼ 50,000 yr BP, from a core collected from the distal Bay of Bengal fan, to assess its potential application in palaeoceanographic reconstruction studies. The down-core variation shows significant change in abundance of E. exigua during the last ∼ 50,000 yr BP. In view of the present day abundance of this species from areas with strong seasonal organic matter supply, we conclude that at ∼ 7, ∼ 22, ∼ 33 and ∼ 46kyr BP, strong seasonality prevailed in the distal Bay of Bengal fan, probably indicating either strong or prolonged north-east monsoon or weakened south-west monsoon. For the first time, a strong correlation is observed in abundance and mean proloculus diameter of E. exigua. Based on coherent variation in mean proloculus diameter and abundance, it is postulated that mean proloculus diameter can also be used to infer increased seasonality in organic matter production, thus variation in strength or duration of monsoon. Thus, this study establishes that the down-core variation in the abundance and mean proloculus diameter of Epistominella exigua can be used to infer past climatic variations from the distal Bay of Bengal fan. Title: Effect of phase speed filters on time-distance correlations of acoustic waves on the Sun. Authors: Nigam, R.; Rajaguru, P.; Kosovichev, A. G. Bibcode: 2005AGUSMSP11B..02N Altcode: Use of phase-speed filters in time-distance helioseismic measurements is crucial to obtain spatially resolved information about localised sub-surface structures. These filters have to be chosen such that the travel times of the waves that are filtered in are themselves not affected by the filtering process. Here we derive analytically the cross-correlation signal that results from phase-speed filtered signals, assuming plane wave conditions. The resulting wavelet explicitly depends on the parameters of the filters, such as the phase-speed and its dispersion, in contrast to the currently used Gabor wavelet, and hence accounts for any filter induced changes in travel times. Alternatively, this new wavelet allows the determination of optimum parameters for the filters. Title: The source of solar oscillations Authors: Nigam, Rakesh Bibcode: 2000PhDT.........7N Altcode: The Sun is permeated by acoustic oscillations. The findings in this dissertation address the characteristics of the source exciting these waves and is consistent with the following proposed excitation mechanism: blobs of hot gas continually rise in the outer layer of the convection zone where they are cooled and collapse. This volume change results in monopolar emission of sound. Cool, dense parcels of gas then accelerate downward into the intergranular lanes and lead to dipolar acoustic emission due to the monopole source. Finally, the void left behind by the downflow is filled by horizontal flow resulting in Reynolds stresses which produce quadrupolar emission. During this process of acoustic excitation by turbulent convection there is photospheric darkening seen in the intensity observations. Power spectra of these oscillations obtained with the Michelson Doppler Imager instrument on-board the Solar and Heliospheric Observatory are asymmetric about their central peaks. At frequencies above the acoustic cutoff frequency, the asymmetry is reduced. Surprisingly, a reversal in asymmetry is seen, along with a high frequency shift between velocity and intensity; where the velocity power drops off rapidly compared to the intensity power. The observed phase difference between velocity and intensity jumps in the vicinity of an eigenfrequency and is not 90° as predicted by adiabatic theory of oscillations below the acoustic cutoff frequency. The granulation signal is partially correlated with the oscillations, observed as photospheric darkening, and is related to the strength of the acoustic source. A model to explain the observed power spectra and the phase difference shows that the correlated signal is higher in intensity than in velocity. A novel asymmetric formula is derived and used to fit the power spectra, thus allowing accurate determination of the eigenfrequencies, resulting in more precise information about the solar interior and rotation. Finally, different types of excitation sources at various depths are studied, and a best match with observations occur when monopole and quadrupole acoustic sources are placed in the superadiabatic layer at a depth of 75 km below the photosphere where the turbulence is most intense and consistent with the proposed excitation mechanism. Title: Numerical Simulations of Oscillation Modes of the Solar Convection Zone Authors: Georgobiani, D.; Kosovichev, A. G.; Nigam, R.; Nordlund, Å.; Stein, R. F. Bibcode: 2000ApJ...530L.139G Altcode: 1999astro.ph.12485G We use the three-dimensional hydrodynamic code of Stein & Nordlund to realistically simulate the upper layers of the solar convection zone in order to study physical characteristics of solar oscillations. Our first result is that the properties of oscillation modes in the simulation closely match the observed properties. Recent observations from the Solar and Heliospheric Observatory (SOHO)/Michelson Doppler Imager (MDI) and Global Oscillations Network Group have confirmed the asymmetry of solar oscillation line profiles, initially discovered by Duvall et al. In this Letter, we compare the line profiles in the power spectra of the Doppler velocity and continuum intensity oscillations from the SOHO/MDI observations with the simulation. We also compare the phase differences between the velocity and intensity data. We have found that the simulated line profiles are asymmetric and have the same asymmetry reversal between velocity and intensity as observed. The phase difference between the velocity and intensity signals is negative at low frequencies, and phase jumps in the vicinity of modes are also observed. Thus, our numerical model reproduces the basic observed properties of solar oscillations and allows us to study the physical properties which are not observed. Title: Three-dimensional simulations of solar oscillations: line profiles and asymmetries Authors: Georgobiani, D. G.; Nigam, R.; Kosovichev, A. G.; Stein, R. F.; Nordlund, A. Bibcode: 1999AAS...194.5605G Altcode: 1999BAAS...31..912G In order to study spectral characteristics of the solar oscillations, we use the Stein-Nordlund 3d hydrodynamic code to generate lond temporal sequencies of realistically simulated upper layers of the solar convective zone. The simulation domain ranges from 0.5 Mm above the surface of tau =1 to 2.5 Mm below this surface, and is 6 Mm by 6 Mm wide. We have generated 24 hours of solar time. We calculate power spectra of the vertical velocity and temperature at different heights and the emergent intensity at the surface. Here, we present the profiles of velocity, intensity and temperature for both radial (l = 0) and first nonradial (l = 700) mode. We compare line profiles from the simulation with the power spectra of the Doppler velocity and continuum intensity from the SOHO/MDI observations. Both simulated and observed profiles demonstrate similar types of asymmetry, and the asymmetry reversal between the local quantities like velocity and temperature, and emergent intensity profiles is also present in the simulated data. The preliminary results are promising as they allow us to establish a connection between the observational data and realistic simulations, and enable us to understand better the physics of solar oscillations. Title: The source of solar oscillations Authors: Nigam, R. Bibcode: 1999AAS...194.2101N Altcode: 1999BAAS...31..857N In this study the role of line asymmetry and phase difference between velocity and intensity helioseismic spectra for understanding the excitation of solar oscillations is discussed. The solar intensity and velocity oscillations are usually observed from variations in an absorption line. These variations consist of two parts: solar oscillation modes and granulation noise. Because the oscillation modes are excited by granulation, we argue that the granulation signal (noise) is partially correlated with the oscillations. The data from the Michelson Doppler Imager (MDI) instrument on board the Solar and Heliospheric Observatory (SOHO) have clearly revealed a reversal of asymmetry between velocity and intensity power spectra. We have shown that the cause of reversal in asymmetry between velocity and intensity power spectra is due to the presence of the correlated noise in the intensity data. This noise is also responsible for the high-frequency shift in the two spectra at and above the acoustic cutoff frequency. Our theory also explains the deviation of the observed phase difference between velocity and intensity from that predicted by simple adiabatic theory of solar oscillations. The observed phase, jumps in the vicinity of an eigenfrequency, but theory does not explain such jumps. We studied different types of excitation sources at various depths and found that monopole and quadrupole acoustic sources when placed in the superadiabatic layer (at a depth of 75 km below the photosphere) match the observations. For these source types, the sign of the correlation is negative corresponding to photospheric darkening. Finally, an asymmetric fitting formula is used to determine the eigenfrequencies of solar oscillations by fitting both the velocity and intensity power spectra. Title: Source of Solar Acoustic Modes Authors: Nigam, R.; Kosovichev, A. G. Bibcode: 1999ApJ...514L..53N Altcode: Solar acoustic modes are found to be excited in a thin superadiabatic layer of turbulent convection (about 75+/-50 km below the photosphere) beneath the Sun's surface. Comparing the theoretical power spectra of both velocity and pressure oscillations of medium angular degree with that obtained from the Michelson Doppler Imager instrument on board the Solar and Heliospheric Observatory, we find that a composite source consisting of a monopole, which corresponds to mass or entropy fluctuations, and a quadrupole, which consists of the Reynolds stress, excites these oscillations. The dominant source is of a monopole type since it provides the best match to the observed velocity and intensity oscillation power spectra. For the above source to match the observed asymmetry in intensity, a part of the background is found to be correlated with the pressure perturbation. The sign of the correlation is found to be negative, which suggests that there is photospheric darkening prior to the occurrence of the localized acoustic event, in agreement with the previous finding of P. R. Goode and coworkers. Title: Phase and Amplitude Difference between Velocity and Intensity Helioseismic Spectra Authors: Nigam, R.; Kosovichev, A. G. Bibcode: 1999ApJ...510L.149N Altcode: An explanation for the phase and amplitude difference between velocity and intensity oscillations of the Sun is provided. The phase difference along the modal lines in the power spectra was originally observed by Deubner and coworkers in 1989. From a simple adiabatic theory of solar oscillations, one expects this phase difference to be 90° for modes below the acoustic cutoff frequency (bound states) and zero for modes above the acoustic cutoff frequency (scattered states). But, surprisingly, from observations, the bound states show a phase difference that is below 90° along modal lines, and the scattered states also show a nonzero phase difference. We compute the phase difference between the velocity and intensity oscillations using medium angular degree data obtained from the Michelson Doppler Imager instrument on board the Solar and Heliospheric Observatory and confirm Deubner's result. We conclude that the unusual phase characteristics of the solar oscillations can be attributed to the fact that a part of the background is correlated to the source responsible for exciting the waves. The idea of the correlated background also explains why the high-frequency modes above the acoustic cutoff frequency are stronger in intensity than in the velocity power spectrum relative to the uncorrelated background, while at frequencies below the acoustic cutoff the velocity power relative to the uncorrelated background is stronger compared to the intensity. In addition, this explains the relative shift of the maxima in the velocity and intensity high-frequency power spectra. Title: Solar P-Mode Spectrum Asymmetries: Testing Theories With Numerical Simulations Authors: Georgobiani, Dali; Nigam, Rakesh; Kosovichev, Alexander G.; Stein, Robert F. Bibcode: 1999soho....9E..58G Altcode: We use a 36 hour sequence of 3-D hydrodynamic simulations of solar convection to study the line profiles of the acoustic modes and their asymmetries. We construct power spectra of the emergent intensity and the vertical velocity at a fixed height of 200 km above the t = 1 surface, as well as their phase differences. We compare the synthetic results with those obtained from the SOHO/MDI observations. The simulations and observations show similar direction of asymmetry and reversal of asymmetry between the velocity and intensity. Our preliminary results confirm the theoretical model of Nigam (Nigam et al. 1998). To make the simulation results more realistic, the intensity and velocity will in future be obtained from the synthetic NiI 6768 line used in the observations. Title: The Source of Solar Oscillations Authors: Nigam, R.; Kosovichev, A. G. Bibcode: 1998AAS...19310002N Altcode: 1998BAAS...30.1397N In this study the role of line asymmetry and phase difference between velocity and intensity helioseismic spectra for understanding the excitation of solar oscillations is discussed. The solar intensity and velocity oscillations are usually observed from variations in an absorption line. These variations consist of two parts: solar oscillation modes and granulation noise. Because the oscillation modes are excited by granulation, we argue that the granulation signal (noise) is partially correlated with the oscillations. The data from the Michelson Doppler Imager (MDI) instrument on board the Solar and Heliospheric Observatory (SOHO) have clearly revealed a reversal of asymmetry between velocity and intensity power spectra. We have shown that the cause of reversal in asymmetry between velocity and intensity power spectra is due to the presence of the correlated noise in the intensity data. This noise is also responsible for the high-frequency shift in the two spectra at and above the acoustic cutoff frequency. Our theory also explains the deviation of the observed phase difference between velocity and intensity from that predicted by simple adiabatic theory of solar oscillations. The observed phase, jumps in the vicinity of an eigenfrequency, but theory does not explain such jumps. We studied different types of excitation sources at various depths and found that monopole and quadrupole acoustic sources when placed in the superadiabatic layer (at a depth of 75 km below the photosphere) match the observations. For these source types, the sign of the correlation is negative corresponding to photospheric darkening. Finally, an asymmetric fitting formula is used to determine the eigenfrequencies of solar oscillations by fitting both the velocity and intensity power spectra. Title: Asymmetry and Frequencies of Low-Degree p-Modes and the Structure of the Sun's Core Authors: Toutain, T.; Appourchaux, T.; Fröhlich, C.; Kosovichev, A. G.; Nigam, R.; Scherrer, P. H. Bibcode: 1998ApJ...506L.147T Altcode: An accurate determination of the frequencies of low-degree solar p-modes is an important task of helioseismology. Using 679 days of solar oscillation data observed in Doppler velocity and continuum intensity from two Solar and Heliospheric Observatory instruments (the Michelson Doppler Imager and the SunPhotoMeter), we show that fitting the spectra with Lorentzian profiles leads to systematic differences between intensity and velocity frequencies as large as 0.1 μHz for angular degrees l=0, 1, and 2 because of the opposite asymmetry between intensity and velocity. We use a physics-based asymmetrical line shape to fit p-mode lines, and we demonstrate that their asymmetry is statistically significant and that frequency differences are considerably reduced. These measurements provide more accurate estimates of the solar eigenfrequencies. We discuss inferences of the structure of the solar core. Title: Measuring the Sun's Eigenfrequencies from Velocity and Intensity Helioseismic Spectra: Asymmetrical Line Profile-fitting Formula Authors: Nigam, R.; Kosovichev, A. G. Bibcode: 1998ApJ...505L..51N Altcode: Solar eigenfrequencies are generally determined by fitting a Lorentzian to the spectral lines in the power spectrum. This assumes that the spectral line is symmetric. Recent observations from the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory have indicated that the power spectra of p-modes show varying amounts of asymmetry. Line asymmetry is an intrinsic property of solar oscillations and depends on the properties of the excitation source and the background noise correlated with the oscillations. Neglecting asymmetry leads to systematic errors in the determination of frequencies and thus affects the results of inversions. In this Letter, we use a simple physical model to derive a new fitting formula that incorporates the effects of asymmetry. It is then tested on artificial and real solar MDI data. A comparison of the results of a symmetric fit with those of an asymmetric one shows that there is a systematic shift in the eigenfrequencies. Our formula will yield more accurate estimates of the solar eigenfrequencies, which is important for improving the accuracy of helioseismic inversions. Title: Asymmetry in Velocity and Intensity Helioseismic Spectra: A Solution to a Long-standing Puzzle Authors: Nigam, R.; Kosovichev, A. G.; Scherrer, P. H.; Schou, J. Bibcode: 1998ApJ...495L.115N Altcode: We give an explanation for the opposite sense of asymmetry of the solar acoustic mode lines in velocity and intensity oscillation power spectra, thereby solving the half-decade-old puzzle of Duvall and coworkers. The solution came after comparing the velocity and intensity oscillation data of medium angular degree l obtained from the Michelson Doppler Imager instrument on board the Solar and Heliospheric Observatory with the theoretical power spectra. We conclude that the solar noise in the velocity and intensity spectra is made up of two components: one is correlated to the source that is responsible for driving the solar p-modes, and the other is an additive uncorrelated background. The correlated component of the noise affects the line profiles. The asymmetry of the intensity spectrum is reversed because the correlated component is of a sufficiently large level, while the asymmetry of the velocity spectrum remains unreversed because the correlated component is smaller. This also explains the high-frequency shift between velocity and intensity at and above the acoustic cutoff frequency. A composite source consisting of a monopole term (mass term) and a dipole term (force due to Reynolds stress) is found to explain the observed spectra when it is located in the zone of superadiabatic convection at a depth of 75+/-50 km below the photosphere. Title: Asymmetry and Fitting of Velocity and Intensity Power Spectra from SOHO/MDI Authors: Nigam, R.; Kosovichev, A. G. Bibcode: 1998ESASP.418..945N Altcode: 1998soho....6..945N The line profiles of solar modes show marked asymmetry at frequencies less than the acoustic cut-off frequency. Observations from the Michelson Doppler Imager instrument on board the Solar and Heliospheric Observatory have revealed a reversal of asymmetry between velocity and intensity power spectra of medium angular degree. We have argued that the cause of reversal in asymmetry between velocity and intensity power spectra is due to the presence of correlated noise, whose level happens to be more in the intensity data, hence reverses its asymmetry (Nigam et al., 1998). The correlated noise is also responsible for the high-frequency shift in the two spectra at and above the acoustic cut-off frequency. It is found that the asymmetry depends on the type and depth of the source that excites the solar acoustic modes. By studying line asymmetry an insight into the physics of excitation of solar oscillations can be gained. Finally, a fitting formula incorporating line asymmetry is developed. This is used to simultaneously fit the two spectra. For the theoretical spectra, the fits yield the same fitted frequency, which is close to the eigenfrequency computed from the solar model. The frequency corrections will have an impact on the inversions. Title: Line asymmetry and excitation mechanism of solar oscillations Authors: Nigam, R.; Kosovichev, A. G.; Scherrer, P. H. Bibcode: 1998IAUS..185..195N Altcode: The width and asymmetry of lines in the power spectrum of solar oscillations, obtained from the Michelson Doppler Imager (MDI) data, on board the Solar and Heliospheric Observatory (SOHO), are used to study the physics of excitation and damping of the oscillations. A theoretical model for solar oscillations is developed. In this model, the asymmetry is an effect of interference between the trapped waves from the source that pass through the region of wave propagation in the Sun's interior. From this the power spectrum is computed for different values of the source location and for various values of the angular degree l. It is seen that there is marked line asymmetry below the acoustic cut-off frequency, which corresponds to the asymmetry of bound states in quantum mechanics. The asymmetry is reduced above the acoustic cut-off frequency, which corresponds to the asymmetry of scattered states, which is a result of interference between an outward direct wave from the source and corresponding inward untrapped waves. The asymmetry is found to depend strongly on the source location and on the value of l. We discuss the properties of the solar acoustic source inferred from the MDI data. Title: Probing the Internal Structure of the Sun with the SOHO Michelson Doppler Imager Authors: Kosovichev, A. G.; Nigam, R.; Scherrer, P. H.; Schou, J.; Reiter, J.; Rhodes, E. J., Jr.; Toutain, T. Bibcode: 1997AAS...191.7311K Altcode: 1997BAAS...29R1322K The inference of the thermodynamic structure of the Sun from the observed properties of the solar normal modes of oscillation is a principal goal of helioseismology. We report the results of the first year of continuous observations of the Sun's internal structure using data from the Medium-l Program of the Michelson Doppler Imager (MDI) on board ESA/NASA spacecraft SOHO. The data provide continuous coverage of the acoustic (p) modes of angular degree l from 0 to 250, and the fundamental (f) mode of the Sun from l=100 to 250. During two 2-month intervals, the high-degree modes, up to l=1000, have been observed. The great stability of solar Dopplergrams measured by MDI permits detection of lower amplitude oscillations, extending the range and precision of measured normal mode frequencies, and thus substantially increasing the resolution and precision of helioseismic inversions. We present new inversion results for the radial and latitudinal seismic solar structures with particular attention to the transition region between the radiative and convection zones and to the energy-generating core. We discuss evidence for convective overshoot at the base of the convection zone, and the significance of deviations in the core structure from the standard evolutionary model. Comparing the f-mode frequencies with the corresponding frequencies of the standard solar models, we argue that the apparent photospheric solar radius (695.99 Mm) used to calibrate the models should be reduced by approximately 0.3 Mm. The discrepancy between the `seismic' and apparent photospheric radii is not explained by the known systematic errors in the helioseismic and photospheric measurements. If confirmed, this discrepancy represents a new interesting challenge to theories of solar convection and solar modeling. Using f-mode frequency splitting we estimate the large-scale structure of the subsurface magnetic fields. The variations of the solar oscillation frequencies during the first year of MDI observations are also discussed. Title: Analysis of Velocity and Intensity Helioseismic Spectra from SOHO/MDI Authors: Nigam, R.; Kosovichev, A. G.; Scherrer, P. H.; Schou, J. Bibcode: 1997SPD....28.0904N Altcode: 1997BAAS...29..913N We give an explanation for the cause of the asymmetry of spectral lines of solar oscillation power spectrum. We also explain the cause of the opposite sense of asymmetry in velocity and intensity oscillation power spectra, thereby resolving a half-decade old puzzle. The motivation for the investigation came after comparing the velocity and intensity data obtained from the Michelson Doppler Imager (MDI) instrument on board the Solar and Heliospheric Observatory (SOHO). The analysis is based on a theoretical model of wave excitation with viscous damping in conjunction with a spherically symmetric solar model. Neglecting asymmetry can lead to systematic errors in the eigenfrequency measurements, which in turn leads to errors in inversion. This research was supported by NASA grant NAG5-3077 at Stanford University. Title: Structure and Rotation of the Solar Interior: Initial Results from the MDI Medium-L Program Authors: Kosovichev, A. G.; Schou, J.; Scherrer, P. H.; Bogart, R. S.; Bush, R. I.; Hoeksema, J. T.; Aloise, J.; Bacon, L.; Burnette, A.; de Forest, C.; Giles, P. M.; Leibrand, K.; Nigam, R.; Rubin, M.; Scott, K.; Williams, S. D.; Basu, Sarbani; Christensen-Dalsgaard, J.; Dappen, W.; Rhodes, E. J., Jr.; Duvall, T. L., Jr.; Howe, R.; Thompson, M. J.; Gough, D. O.; Sekii, T.; Toomre, J.; Tarbell, T. D.; Title, A. M.; Mathur, D.; Morrison, M.; Saba, J. L. R.; Wolfson, C. J.; Zayer, I.; Milford, P. N. Bibcode: 1997SoPh..170...43K Altcode: The medium-l program of the Michelson Doppler Imager instrument on board SOHO provides continuous observations of oscillation modes of angular degree, l, from 0 to ∽ 300. The data for the program are partly processed on board because only about 3% of MDI observations can be transmitted continuously to the ground. The on-board data processing, the main component of which is Gaussian-weighted binning, has been optimized to reduce the negative influence of spatial aliasing of the high-degree oscillation modes. The data processing is completed in a data analysis pipeline at the SOI Stanford Support Center to determine the mean multiplet frequencies and splitting coefficients. The initial results show that the noise in the medium-l oscillation power spectrum is substantially lower than in ground-based measurements. This enables us to detect lower amplitude modes and, thus, to extend the range of measured mode frequencies. This is important for inferring the Sun's internal structure and rotation. The MDI observations also reveal the asymmetry of oscillation spectral lines. The line asymmetries agree with the theory of mode excitation by acoustic sources localized in the upper convective boundary layer. The sound-speed profile inferred from the mean frequencies gives evidence for a sharp variation at the edge of the energy-generating core. The results also confirm the previous finding by the GONG (Gough et al., 1996) that, in a thin layer just beneath the convection zone, helium appears to be less abundant than predicted by theory. Inverting the multiplet frequency splittings from MDI, we detect significant rotational shear in this thin layer. This layer is likely to be the place where the solar dynamo operates. In order to understand how the Sun works, it is extremely important to observe the evolution of this transition layer throughout the 11-year activity cycle. Title: Internal structure and rotation of the Sun: First results from MDI data Authors: Kosovichev, A. G.; Schou, J.; Scherrer, P. H.; Bogart, R. S.; Bush, R. I.; Hoeksema, J. T.; Aloise, J.; Bacon, L.; Burnette, A.; De Forest, C.; Giles, P. M.; Leibrand, K.; Nigam, R.; Rubin, M.; Scott, K.; Williams, S. D.; Basu, Sarbani; Christensen-Dalsgaard, J.; Däppen, W.; Rhodes, E. J., Jr.; Duvall, T. L., Jr.; Howe, R.; Thompson, M. J.; Gough, D. O.; Sekii, T.; Toomre, J.; Tarbell, T. D.; Title, A. M.; Mathur, D.; Morrison, M.; Saba, J. L. R.; Wolfson, C. J.; Zayer, I.; Milford, P. N. Bibcode: 1997IAUS..181..203K Altcode: No abstract at ADS Title: New Views of the Sun's Interior from the SOHO/MDI Space Experiment Authors: Scherrer, P. H.; Bogart, R. S.; Bush, R. I.; Hoeksema, J. T.; Kosovichev, A. G.; Nigam, R.; Schou, J.; Duvall, T. L., Jr. Bibcode: 1996AAS...189.1803S Altcode: 1996BAAS...28.1298S The strking stability of solar Dopplergrams measured by the Michelson Doppler Imager (MDI) instrument on the SOHO spacecraft, without an intervening atmosphere, substantially decreases the noise in the solar oscillations power spectrum compared with groundbased observations. This permits detection of lower amplitude oscillations, extending the range of measured normal mode frequencies. This is important for improving resolution and precision of helioseismic inferences about the Sun's internal structure and dynamics. The MDI observations also reveal the asymmetries of oscillation spectral lines that until now have been largely hidden in noise. The line asymmetries agree with a theory of excitation of solar oscillations by acoustic sources localized in the upper convective boundary layer. High-resolution MDI images make it possible to measure the travel time of acoustic waves propagating inside the Sun by comparing points on the surface as close as 2.4 Mm. This is sufficient to detect supergranulation flows beneath the surface. Coupled with tomographic inversion techniques, we can now study the 3-dimensional evolution of the flows near the photosphere. The sound-speed profile inferred from normal modes frequencies shows a sharp variation at the edge of the energy-generating core, something not accounted for by the standard evolution theory. The analysis also confirms recent GONG results suggesting that helium is less abundant than theory predicts in a thin layer just beneath the convection zone. Inversion of the multiplet frequency splittings shows significant rotational shear in this thin layer. This shear flow probably generates turbulence that mixes the plasma in the upper radiative zone. This layer is likely to be the place where the solar dynamo operates. Continuous observation of the evolution of this transition layer during the entire 11-year activity cycle will be extremely important for understanding the mechanisms of solar activity. Title: Study of solar high-frequency modes near the acoustic cut-off frequency Authors: Nigam, R.; Kosovichev, A. G. Bibcode: 1996BASI...24..195N Altcode: No abstract at ADS Title: Search for Sources of Acoustic Power Using Wavelet Analysis Authors: Milford, P.; Nigam, R. Bibcode: 1995ASPC...76..504M Altcode: 1995gong.conf..504M No abstract at ADS Title: Zeroth Order of "Observing Efficiency" of Space Telescope Authors: Nigam, R. C. Bibcode: 1985BAAS...17..549N Altcode: No abstract at ADS Title: Stellar Magnitude Rectification of the SKYMAP Catalog Authors: Nigam, R. C. Bibcode: 1984BAAS...16..477N Altcode: No abstract at ADS Title: Effect of Lunar Inequality on the Anomalistic Year Authors: Nigam, R. C. Bibcode: 1965JAnSc..12..100N Altcode: No abstract at ADS Title: On the Secular Decrease in the Inclination of Artificial Satellites Authors: Nigam, R. C. Bibcode: 1963SAOSR.112.....N Altcode: Merson and King-Hele noticed a marked decrease in the inclination of Sputnik 2 and suggested among the possible causes that of the rotation of the atmosphere. Wildhack studied the effect of the transverse atmospheric drag on the inclination and showed a secular decrease caused by atmospheric rotation. In view of the smallnes of this effect, he was skeptical that it could be used to obtain any definite information on winds and tides in the upper atmosphere. Sterne, however, from his analysis of the inclination of Sputnik 2, suggested the probability of an atmospheric wind blowing from west to east at about 13 mph, at heights between 150 and 250 km. Utilizing the increased accuracy in the orbital elements that has become available in the past 4 years, the results of this Special Report appear to suggest winds moving at high speeds in the upper atmosphere. Title: The Orbits of the Satellites 1959 α1 and 1959 α2 and the Perturbations on the Perigee Distance of 1959 α1 Authors: Nigam, R. C. Bibcode: 1961SAOSR..81.....N Altcode: Orbital elements for the two satellites, 1959 α1 and α2, for the period April 2, 1960, through August 1, 1961, are tabulated. The various perturbations on the perigee distance of Satellite 1959 α1 have been determined from launch on February 18, 1959, through August 1, 1961; they show that the radiation pressure produces a variation in the perigee distance of this satellite with a period of 450 days and an amplitude of 1.5 km. Title: A Determination of the Atmospheric Oblateness from the Motion of Two Low Satellites. Authors: Nigam, R. C. Bibcode: 1961AJ.....66..292N Altcode: An appropriate theory has been developed to derive the effect of the atmospheric oblateness on the acceleration n, defined as the rate of change of mean motion n. By mean motion is meant the number of revolutions made by the satellite in one day from one perigee to another. This effect is a periodic one, as one would expect, with half the period of the argument of perigee. For small values of eccentricity (e <0.2), it is expressed by [n2a2(1;;{{;) sin2i1 3 15H 67H 1 2e -e2- - O(e3) cos2 , 2 8ae 8a where k (r) = 2' (CDA /m) p (r), = ac/H, H is the scale height, f the atmospheric oblateness, q the geocentric perigee distance, etc. In order to get reliable values for the atmospheric oblateness, one needs a few low satellites in polar orbit having a lifetime in which the perigee has made several revolutions of the earth. Further, if the satellites are not spherical, reliable information about the mode of tumbling should also be available from some independent source. As none of the above-mentioned characteristics could be met in the satellites available for this investigation, we chose satellites 1958 and 1958 , both of which had their perigee altitude less than 200 km, for a preliminary check of the derived theoretical expression for the effect of atmospheric oblateness on the accelerations, and to derive the value for the atmospheric oblateness. The atmospheric oblateness at the altitudes of 176 and 186 km, which are the mean altitudes of the perigee points of the satellites 1958 and 1958 , respectively, are obtained as 1/284 and 1/238, respectively. The value of the atmospheric oblateness computed theoretically assuming the solid-body rotation of the atmosphere for an altitude of 176 km is 1/291. The quantitative results on the atmospheric oblateness are, therefore, far from being exact. These, on taking into consideration the uncertainty inherent in the two determinations, are, however, in conformity with the theoretical atmospheric models near 200 km, as given by T. E. Sterne (Astron. J. 63, 81,1958) and F. S. Johnson (J. Geophys. Research 65, 2227,1960). The results therefore appear to suggest that the atmospheric oblateness increases with altitude. An exact determination of the atmospheric oblateness must however await more extensive data. Title: The Revised Orbit of Satellite 1958 Zeta Authors: Nigam, R. C. Bibcode: 1961SAOSR..64.....N Altcode: A preliminary orbit for Satellite 1958 Zeta, computed by Veis, was based on theoretical values of perigee distance as computed by the integration of equations by Sterne, utilizing the Smithsonian Model Atmosphere No. 2. The object of the present paper is to revise the orbit without making any assumptions and to determine the observed values of the perigee distance and accelerations. This is made possible by the Differential Orbit Improvement Program (DOI) of Veis and Moore, available only after the preliminary orbit given by Veis. Some of the data, including sun-perigee distance, differ from those he gave. Title: The Orbits and the Accelerations of Satellites 1959 α1 and 1959 α2 Authors: Nigam, R. C. Bibcode: 1960SAOSR..53.....N Altcode: This paper gives orbital information for Satellite 1959 α1 from the time it was launched on February 17, 1959, through March 31, 1960, and for Satellite 1959 a2 for the period from March 12, 1959, through March 31, 1960. An analysis of the acceleration of each satellite during the period indicated is included. Other parameters, such as the angle between the sun and the perigee ψ, the latitude of perigee Φ, are also given.