14,134 research outputs found
The discrepancy between solar abundances and helioseismology
There have been recent downward revisions of the solar photospheric
abundances of Oxygen and other heavy elements. These revised abundances along
with OPAL opacities are not consistent with seismic constraints. In this work
we show that the recently released OP opacity tables cannot resolve this
discrepancy either. While the revision in opacities does not seem to resolve
this conflict, an upward revision of Neon abundance in solar photosphere offers
a possible solution to this problem.Comment: To appear in ApJ Letter
Temporal variations of the rotation rate in the solar interior
The temporal variations of the rotation rate in the solar interior are
studied using frequency splittings from Global Oscillations Network Group
(GONG) data obtained during the period 1995-99. We find alternating latitudinal
bands of faster and slower rotation which appear to move towards the equator
with time - similar to the torsional oscillations seen at the solar surface.
This flow pattern appears to persist to a depth of about 0.1R_sun and in this
region its magnitude is well correlated with solar activity indices. We do not
find any periodic or systematic changes in the rotation rate near the base of
the convection zone.Comment: To appear in Ap
Determining solar abundances using helioseismology
The recent downward revision of solar photospheric abundances of Oxygen and
other heavy elements has resulted in serious discrepancies between solar models
and solar structure as determined through helioseismology. In this work we
investigate the possibility of determining the solar heavy-element abundance
without reference to spectroscopy by using helioseismic data. Using the
dimensionless sound-speed derivative in the solar convection zone, we find that
the heavy element abundance, Z, of 0.0172 +/- 0.002, which is closer to the
older, higher value of the abundances.Comment: To appear in Ap
Solar internal rotation rate and the latitudinal variation of the tachocline
A new set of accurately measured frequencies of solar oscillations are used
to infer the rotation rate inside the Sun, as a function of radial distance as
well as latitude. We have adopted a regularized least squares technique with
iterative refinement for both 1.5D inversion using the splitting coefficients
and 2D inversion using individual m splittings. The inferred rotation rate
agrees well with earlier estimates showing a shear layer just below the surface
and another one around the base of the convection zone. The tachocline or the
transition layer where the rotation rate changes from differential rotation in
the convection zone to almost latitudinally independent rotation rate in the
radiative interior is studied in detail. No compelling evidence for any
latitudinal variation in position and width of tachocline is found though it
appears that the tachocline probably shifts to slightly larger radial distance
at higher latitudes and possibly becomes thicker also. However, these
variations are within the estimated errors and more accurate data would be
needed to make a definitive statement about latitudinal variations.Comment: 15 pages, MNRAS-TeX, 15 figure
Ring diagram analysis of near-surface flows in the Sun
Ring diagram analysis of solar oscillation power spectra obtained from MDI
data is carried out to study the velocity fields in the outer part of the solar
convection zone. The three dimensional power spectra are fitted to a model
which has a Lorentzian profile in frequency and which includes the advection of
the wave front by horizontal flows, to obtain the two components of the
sub-surface flows as a function of the horizontal wave number and radial order
of the oscillation modes. This information is then inverted using OLA and RLS
methods to infer the variation in horizontal flow velocity with depth. The
average rotation velocity at different latitudes obtained by this technique
agrees reasonably with helioseismic estimates made using frequency splitting
data. The shear layer just below the solar surface appears to consist of two
parts with the outer part up to a depth of 4 Mm, where the velocity gradient
does not show any reversal up to a latitude of 60 degrees. In the deeper part
the velocity gradient shows reversal in sign around a latitude of 55 degrees.
The zonal flow velocities inferred in the outermost layers appears to be
similar to those obtained by other measurements. A meridional flow from equator
polewards is found. It has a maximum amplitude of about 30 m/s near the surface
and the amplitude is nearly constant in the outer shear layer.Comment: aastex, 12 figures, to appear in Ap.
Helioseismic analysis of the hydrogen partition function in the solar interior
The difference in the adiabatic gradient gamma_1 between inverted solar data
and solar models is analyzed. To obtain deeper insight into the issues of
plasma physics, the so-called ``intrinsic'' difference in gamma_1 is extracted,
that is, the difference due to the change in the equation of state alone. Our
method uses reference models based on two equations of state currently used in
solar modeling, the Mihalas-Hummer-Dappen (MHD) equation of state, and the OPAL
equation of state (developed at Livermore). Solar oscillation frequencies from
the SOI/MDI instrument on board the SOHO spacecraft during its first 144 days
in operation are used. Our results confirm the existence of a subtle effect of
the excited states in hydrogen that was previously studied only theoretically
(Nayfonov & Dappen 1998). The effect stems from internal partition function of
hydrogen, as used in the MHD equation of state. Although it is a pure-hydrogen
effect, it takes place in somewhat deeper layers of the Sun, where more than
90% of hydrogen is ionized, and where the second ionization zone of helium is
located. Therefore, the effect will have to be taken into account in reliable
helioseismic determinations of the astrophysically relevant helium-abundance of
the solar convection zone.Comment: 30 pages, 4 figures, 1 table. Revised version submitted to Ap
How much do helioseismological inferences depend upon the assumed reference model?
We investigate systematic uncertainties in determining the profiles of the
solar sound speed, density, and adiabatic index by helioseismological
techniques. We find that rms uncertainties-averaged over the sun of ~ 0.2%-0.4%
are contributed to the sound speed profile by each of three sources: 1)the
choice of assumed reference model, 2) the width of the inversion kernel, and 3)
the measurements errors. The density profile is about an order of magnitude
less well determined by the helioseismological measurements. The profile of the
adiabatic index is determined to an accuracy of about 0.2% . We find that even
relatively crude reference models yield reasonably accurate solar parameters.Comment: Accepted for publication in ApJ . Related material at
http://www.sns.ias.edu/~jn
Comparison of High-degree Solar Acoustic Frequencies and Asymmetry between Velocity and Intensity Data
Using the local helioseismic technique of ring diagram we analyze the
frequencies of high--degree f- and p-modes derived from both velocity and
continuum intensity data observed by MDI. Fitting the spectra with asymmetric
peak profiles, we find that the asymmetry associated with velocity line
profiles is negative for all frequency ranges agreeing with previous
observations while the asymmetry of the intensity profiles shows a complex and
frequency dependent behavior. We also observe systematic frequency differences
between intensity and velocity spectra at the high end of the frequency range,
mostly above 4 mHz. We infer that this difference arises from the fitting of
the intensity rather than the velocity spectra. We also show that the frequency
differences between intensity and velocity do not vary significantly from the
disk center to the limb when the spectra are fitted with the asymmetric profile
and conclude that only a part of the background is correlated with the
intensity oscillations.Comment: Accepted for publication in Astrophysical Journa
Charge and Statistics of Quasiparticles in Fractional Quantum Hall Effec
We have studied here the charge and statistics of quasiparticle excitations
in FQH states on the basis of the Berry phase approach incorporating the fact
that even number of flux quanta can be gauged away when the Berry phase is
removed to the dynamical phase. It is observed that the charge and
statistical parameter of a quasiparticle at filling factor
are given by and
, with the fact that the charge of the quasihole is
opposite to that of the quasielectron. Using Laughlin wave function for
quasiparticles, numerical studies have been done following the work of
Kj{\o}nsberg and Myrheim \cite{KM} for FQH states at and it is
pointed out that as in case of quasiholes, the statistics parameter can be well
defined for quasielectrons having the value .Comment: 12 pages, 4 figure
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