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Author name code: nigam
ADS astronomy entries on 2022-09-14
author:"Nigam, Rakesh"
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Title: Note on Travel Time Shifts Due to Amplitude Modulation in
Time-Distance Helioseismology Measurements
Authors: Nigam, R.; Kosovichev, A. G.
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.
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Title: Analytical Models for Cross-Correlation Signal in Time-Distance
Helioseismology
Authors: Nigam, R.; Kosovichev, A. G.; Scherrer, P. H.
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.
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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.
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 δ<SUP>18</SUP>O 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.
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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
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.
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Title: Effect of phase speed filters on time-distance correlations
of acoustic waves on the Sun.
Authors: Nigam, R.; Rajaguru, P.; Kosovichev, A. G.
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.
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Title: The source of solar oscillations
Authors: Nigam, Rakesh
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.
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Title: Numerical Simulations of Oscillation Modes of the Solar
Convection Zone
Authors: Georgobiani, D.; Kosovichev, A. G.; Nigam, R.; Nordlund,
Å.; Stein, R. F.
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.
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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.
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.
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Title: The source of solar oscillations
Authors: Nigam, R.
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.
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Title: Source of Solar Acoustic Modes
Authors: Nigam, R.; Kosovichev, A. G.
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.
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Title: Phase and Amplitude Difference between Velocity and Intensity
Helioseismic Spectra
Authors: Nigam, R.; Kosovichev, A. G.
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.
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Title: Solar P-Mode Spectrum Asymmetries: Testing Theories With
Numerical Simulations
Authors: Georgobiani, Dali; Nigam, Rakesh; Kosovichev, Alexander G.;
Stein, Robert F.
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.
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Title: The Source of Solar Oscillations
Authors: Nigam, R.; Kosovichev, A. G.
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.
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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.
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.
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Title: Measuring the Sun's Eigenfrequencies from Velocity and
Intensity Helioseismic Spectra: Asymmetrical Line Profile-fitting
Formula
Authors: Nigam, R.; Kosovichev, A. G.
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.
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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.
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.
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Title: Asymmetry and Fitting of Velocity and Intensity Power Spectra
from SOHO/MDI
Authors: Nigam, R.; Kosovichev, A. G.
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.
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Title: Line asymmetry and excitation mechanism of solar oscillations
Authors: Nigam, R.; Kosovichev, A. G.; Scherrer, P. H.
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.
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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.
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.
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Title: Analysis of Velocity and Intensity Helioseismic Spectra
from SOHO/MDI
Authors: Nigam, R.; Kosovichev, A. G.; Scherrer, P. H.; Schou, J.
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.
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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.
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.
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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.
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.
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.
1996BASI...24..195N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Search for Sources of Acoustic Power Using Wavelet Analysis
Authors: Milford, P.; Nigam, R.
1995ASPC...76..504M Altcode: 1995gong.conf..504M
No abstract at ADS
---------------------------------------------------------
Title: Zeroth Order of "Observing Efficiency" of Space Telescope
Authors: Nigam, R. C.
1985BAAS...17..549N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stellar Magnitude Rectification of the SKYMAP Catalog
Authors: Nigam, R. C.
1984BAAS...16..477N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Effect of Lunar Inequality on the Anomalistic Year
Authors: Nigam, R. C.
1965JAnSc..12..100N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Secular Decrease in the Inclination of Artificial
Satellites
Authors: Nigam, R. C.
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.
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.
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.
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.
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.
