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Featured researches published by R. Nigam.


Solar Physics | 1997

STRUCTURE AND ROTATION OF THE SOLAR INTERIOR: INITIAL RESULTS FROM THE MDI MEDIUM-L PROGRAM

Alexander G. Kosovichev; Jesper Schou; Philip H. Scherrer; R. S. Bogart; R. I. Bush; J. T. Hoeksema; J. Aloise; L. Bacon; A. Burnette; C. De Forest; Peter Mark Giles; K. Leibrand; R. Nigam; M. Rubin; K. Scott; S. D. Williams; Sarbani Basu; J. Christensen-Dalsgaard; Werner Dappen; Edward J. Rhodes; T. L. Duvall; Robert D. Howe; M. J. Thompson; D. O. Gough; T. Sekii; Juri Toomre; Theodore D. Tarbell; Alan M. Title; D. Mathur; M. Morrison

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 Astrophysical Journal | 1998

Measuring the Sun's Eigenfrequencies from Velocity and Intensity Helioseismic Spectra: Asymmetrical Line Profile-fitting Formula

R. Nigam; Alexander G. Kosovichev

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.


The Astrophysical Journal | 1998

Asymmetry in Velocity and Intensity Helioseismic Spectra: A Solution to a Long-standing Puzzle

R. Nigam; Alexander G. Kosovichev; Philip H. Scherrer; Jesper Schou

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.


The Astrophysical Journal | 1998

Asymmetry and Frequencies of Low-Degree p-Modes and the Structure of the Sun's Core

Thierry Toutain; T. Appourchaux; Claus Frohlich; Alexander G. Kosovichev; R. Nigam; Philip H. Scherrer

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.


The Astrophysical Journal | 1999

Source of Solar Acoustic Modes

R. Nigam; Alexander G. Kosovichev

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 Suns 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.


The Astrophysical Journal | 1999

Phase and Amplitude Difference between Velocity and Intensity Helioseismic Spectra

R. Nigam; Alexander G. Kosovichev

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 Deubners 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.


The Astrophysical Journal | 2000

Numerical Simulations of Oscillation Modes of the Solar Convection Zone.

Dali Georgobiani; Alexander G. Kosovichev; R. Nigam; Åke Nordlund; Robert F. Stein

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.


The Astrophysical Journal | 2007

Analytical Models for Cross-Correlation Signal in Time-Distance Helioseismology

R. Nigam; Alexander G. Kosovichev; Philip H. Scherrer

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.


Symposium - International Astronomical Union | 1997

Internal structure and rotation of the Sun: First results from the MDI data

Alexander G. Kosovichev; Jesper Schou; P.H. Scherrer; R. S. Bogart; R. I. Bush; J. T. Hoeksema; J. Aloise; L. Bacon; A. Burnette; C. De Forest; Peter Mark Giles; K. Leibrand; R. Nigam; M. Rubin; K. Scott; S. D. Williams; Sarbani Basu; J. Christensen-Dalsgaard; Werner Dappen; Edward J. Rhodes; T. L. Duvall; Robert D. Howe; M. J. Thompson; D. O. Gough; T. Sekii; Juri Toomre; Theodore D. Tarbell; Alan M. Title; D. Mathur; M. Morrison

The Medium- l Program of the Michelson Doppler Imager (MDI) instrument on board SOHO provides continuous observations of oscillation modes of angular degree, l , from 0 to ∼ 300. 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. 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. 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.


The Astrophysical Journal | 2010

NOTE ON TRAVEL TIME SHIFTS DUE TO AMPLITUDE MODULATION IN TIME-DISTANCE HELIOSEISMOLOGY MEASUREMENTS

R. Nigam; Alexander G. Kosovichev

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Edward J. Rhodes

University of Southern California

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Juri Toomre

University of Colorado Boulder

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