503 research outputs found
Structure of the near-surface layers of the Sun: asphericity and time variation
We present results on the structure of the near-surface layers of the Sun
obtained by inverting frequencies of high-degree solar modes from "ring
diagrams". We have results for eight epochs between June 1996 and October 2003.
The frequencies for each epoch were obtained from ring diagrams constructed
from MDI Dopplergrams spanning complete Carrington rotations. We find that
there is a substantial latitudinal variation of both sound speed and the
adiabatic index Gamma_1 in the outer 2% of the Sun. We find that both the
sound-speed and Gamma_1 profiles change with changes in the level of solar
activity. In addition, we also study differences between the northern and
southern hemispheres of the Sun and find a small asymmetry that appears to
reflect the difference in magnetic activity between the two hemispheres.Comment: To appear in ApJ (January 2007
The Sub-Surface Structure of a Large Sample of Active Regions
We employ ring-diagram analysis to study the sub-surface thermal structure of
active regions. We present results using a large number of active regions over
the course of Solar Cycle 23. We present both traditional inversions of
ring-diagram frequency differences, with a total sample size of 264, and a
statistical study using Principal Component Analysis. We confirm earlier
results on smaller samples that sound speed and adiabatic index are changed
below regions of strong magnetic field. We find that sound speed is decreased
in the region between approximately r=0.99R_sun and r=0.995R_sun (depths of 3Mm
to 7Mm), and increased in the region between r=0.97R_sun and r=0.985R_sun
(depths of 11Mm to 21Mm). The adiabatic index is enhanced in the same deeper
layers that sound-speed enhancement is seen. A weak decrease in adiabatic index
is seen in the shallower layers in many active regions. We find that the
magnitudes of these perturbations depend on the strength of the surface
magnetic field, but we find a great deal of scatter in this relation, implying
other factors may be relevant.Comment: 16 pages, 11 figures, accepted for publication in Solar Physic
A Detailed Analysis of the Dust Formation Zone of IRC+10216 Derived from Mid-IR Bands of C2H2 and HCN
A spectral survey of IRC+10216 has been carried out in the range 11 to 14 um
with a spectral resolution of about 4 km s^-1. We have identified a forest of
lines in six bands of C2H2 involving the vibrational states from the ground to
3nu5 and in two bands of HCN, involving the vibrational states from the ground
up to 2nu2. Some of these transitions are observed also in H13CCH and H13CN. We
have estimated the kinetic, vibrational, and rotational temperatures, and the
abundances and column densities of C2H2 and HCN between 1 and 300 R* (1.5E16
cm) by fitting about 300 of these ro-vibrational lines. The envelope can be
divided into three regions with approximate boundaries at 0.019 arcsec (the
stellar photosphere), 0.1 arcsec (the inner dust formation zone), and 0.4
arcsec (outer dust formation zone). Most of the lines might require a large
microturbulence broadening. The derived abundances of C2H2 and HCN increase by
factors of 10 and 4, respectively, from the innermost envelope outwards. The
derived column densities for both C2H2 and HCN are 1.6E19 cm^-2. Vibrational
states up to 3000 K above ground are populated, suggesting pumping by
near-infrared radiation from the star and innermost envelope. Low rotational
levels can be considered under LTE while those with J>20-30 are not
thermalized. A few lines require special analysis to deal with effects like
overlap with lines of other molecules.Comment: 8 pages, 16 figures, 2 machine-readable tables, accepted in the
Astrophysical Journa
Helioseismological Implications of Recent Solar Abundance Determinations
We show that standard solar models are in good agreement with the
helioseismologically determined sound speed and density as a function of solar
radius, the depth of the convective zone, and the surface helium abundance, as
long as those models do not incorporate the most recent heavy element abundance
determinations. However, sophisticated new analyses of the solar atmosphere
infer lower abundances of the lighter metals (like C, N, O, Ne, and Ar) than
the previously widely used surface abundances. We show that solar models that
include the lower heavy element abundances disagree with the solar profiles of
sound speed and density as well as the depth of the convective zone and the
helium abundance. The disagreements for models with the new abundances range
from factors of several to many times the quoted uncertainties in the
helioseismological measurements. The disagreements are at temperatures below
what is required for solar interior fusion reactions and therefore do not
significantly affect solar neutrino emission. If errors in thecalculated OPAL
opacities are solely responsible for the disagreements, then the corrections in
the opacity must extend from 2 times 10^6 K (R = 0.7R_Sun)to 5 times 10^6 K (R
= 0.4 R_Sun), with opacity increases of order 10%.Comment: ApJ in press; clarified Figure
An optical time-delay estimate for the double gravitational lens system B1600+434
We present optical I-band light curves of the gravitationally lensed double
QSO B1600+434 from observations obtained at the Nordic Optical Telescope (NOT)
between April 1998 and November 1999. The photometry has been performed by
simultaneous deconvolution of all the data frames, involving a numerical lens
galaxy model. Four methods have been applied to determine the time delay
between the two QSO components, giving a mean estimate of \Delta_t = 51+/-4
days (95% confidence level). This is the fourth optical time delay ever
measured. Adopting a Omega=0.3, Lambda=0 Universe and using the mass model of
Maller et al. (2000), this time-delay estimate yields a Hubble parameter of
H_0=52 (+14, -8) km s^-1 Mpc^-1 (95% confidence level) where the errors include
time-delay as well as model uncertainties. There are time-dependent offsets
between the two (appropriately shifted) light curves that indicate the presence
of external variations due to microlensing.Comment: 15 pages, 4 figures, accepted for publication in 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
Interpreting Helioseismic Structure Inversion Results of Solar Active Regions
Helioseismic techniques such as ring-diagram analysis have often been used to
determine the subsurface structural differences between solar active and quiet
regions. Results obtained by inverting the frequency differences between the
regions are usually interpreted as the sound-speed differences between them.
These in turn are used as a measure of temperature and magnetic-field strength
differences between the two regions. In this paper we first show that the
"sound-speed" difference obtained from inversions is actually a combination of
sound-speed difference and a magnetic component. Hence, the inversion result is
not directly related to the thermal structure. Next, using solar models that
include magnetic fields, we develop a formulation to use the inversion results
to infer the differences in the magnetic and thermal structures between active
and quiet regions. We then apply our technique to existing structure inversion
results for different pairs of active and quiet regions. We find that the
effect of magnetic fields is strongest in a shallow region above 0.985R_sun and
that the strengths of magnetic-field effects at the surface and in the deeper
(r < 0.98R_sun) layers are inversely related, i.e., the stronger the surface
magnetic field the smaller the magnetic effects in the deeper layers, and vice
versa. We also find that the magnetic effects in the deeper layers are the
strongest in the quiet regions, consistent with the fact that these are
basically regions with weakest magnetic fields at the surface. Because the
quiet regions were selected to precede or follow their companion active
regions, the results could have implications about the evolution of magnetic
fields under active regions.Comment: Accepted for publication in Solar Physic
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