2,056 research outputs found
Solar Rotation Rate During the Cycle 24 Minimum in Activity
The minimum of solar cycle 24 is significantly different from most other
minima in terms of its duration as well as its abnormally low levels of
activity. Using available helioseismic data that cover epochs from the minimum
of cycle 23 to now, we study the differences in the nature of the solar
rotation between the minima of cycles 23 and 24. We find that there are
significant differences between the rotation rates during the two minima. There
are differences in the zonal-flow pattern too. We find that the band of fast
rotating region close to the equator bifurcated around 2005 and recombined by
2008. This behavior is different from that during the cycle 23 minimum. By
auto-correlating the zonal-flow pattern with a time shift, we find that in
terms of solar dynamics, solar cycle 23 lasted for a period of 11.7 years,
consistent with the result of Howe et al. (2009). The autocorrelation
coefficient also confirms that the zonal-flow pattern penetrates through the
convection zone.Comment: Accepted for publication in Ap
Rotational splitting as a function of mode frequency for six Sun-like stars
Asteroseismology offers the prospect of constraining differential rotation in
Sun-like stars. Here we have identified six high signal-to-noise main-sequence
Sun-like stars in the Kepler field, which all have visible signs of rotational
splitting of their p-mode frequencies. For each star, we extract the rotational
frequency splitting and inclination angle from separate mode sets (adjacent
modes with l=2, 0, and 1) spanning the p-mode envelope. We use a Markov chain
Monte Carlo method to obtain the best fit and errors associated with each
parameter. We are able to make independent measurements of rotational
splittings of ~8 radial orders for each star. For all six stars, the measured
splittings are consistent with uniform rotation, allowing us to exclude large
radial differential rotation. This work opens the possibility of constraining
internal rotation of Sun-like stars.Comment: Published in Astronomy and Astrophysics. 4 pages, 3 figure
Characterization of solar-cycle induced frequency shift of medium- and high-degree acoustic modes
Although it is well known that the solar acoustic mode frequency increases as
the solar activity increases, the mechanism behind it is still unknown. Mode
frequencies with 20 < l < 900 obtained by applying spherical harmonic
decomposition to MDI full-disk observations were used. First, the dependence of
solar acoustic mode frequency with solar activity was examined and evidence of
a quadratic relation was found indicating a saturation effect at high solar
activity. Then, the frequency dependence of frequency differences between the
activity minimum and maximum was analyzed. The frequency shift scaled by the
normalized mode inertia follows a simple power law where the exponent for the p
modes decreases by 37% for modes with frequency larger than 2.5 mHz.Comment: Proceedings of GONG-SoHO 24: A new era of seismology of the sun and
solar-like star
Solar Dynamics, Rotation, Convection and Overshoot
We discuss recent observational, theoretical and modeling progress made in
understanding the Sun's internal dynamics, including its rotation, meridional
flow, convection and overshoot. Over the past few decades, substantial
theoretical and observational effort has gone into appreciating these aspects
of solar dynamics. A review of these observations, related helioseismic
methodology and inference and computational results in relation to these
problems is undertaken here.Comment: 31 pages, 10 figures, Space Science Review
A study of possible temporal and latitudinal variations in the properties of the solar tachocline
Temporal variations of the structure and the rotation rate of the solar
tachocline region are studied using helioseismic data from the Global
Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI)
obtained during the period 1995--2000. We do not find any significant temporal
variation in the depth of the convection zone, the position of the tachocline
or the extent of overshoot below the convection zone. No systematic variation
in any other properties of the tachocline, like width, etc., is found either.
Possibility of periodic variations in these properties is also investigated.
Time-averaged results show that the tachocline is prolate with a variation by
about 0.02R_sun in its position. The depth of the convection zone or the extent
of overshoot does not show any significant variation with latitude.Comment: To appear in MNRA
Solar-cycle variation of the sound-speed asphericity from GONG and MDI data 1995-2000
We study the variation of the frequency splitting coefficients describing the
solar asphericity in both GONG and MDI data, and use these data to investigate
temporal sound-speed variations as a function of both depth and latitude during
the period from 1995-2000 and a little beyond. The temporal variations in even
splitting coefficients are found to be correlated to the corresponding
component of magnetic flux at the solar surface. We confirm that the
sound-speed variations associated with the surface magnetic field are
superficial. Temporally averaged results show a significant excess in sound
speed around 0.92 solar radii and latitude of 60 degrees.Comment: To be published in MNRAS, accepted July 200
On The Determination of MDI High-Degree Mode Frequencies
The characteristic of the solar acoustic spectrum is such that mode lifetimes
get shorter and spatial leaks get closer in frequency as the degree of a mode
increases for a given order. A direct consequence of this property is that
individual p-modes are only resolved at low and intermediate degrees, and that
at high degrees, individual modes blend into ridges. Once modes have blended
into ridges, the power distribution of the ridge defines the ridge central
frequency and it will mask the true underlying mode frequency. An accurate
model of the amplitude of the peaks that contribute to the ridge power
distribution is needed to recover the underlying mode frequency from fitting
the ridge.
We present the results of fitting high degree power ridges (up to l = 900)
computed from several two to three-month-long time-series of full-disk
observations taken with the Michelson Doppler Imager (MDI) on-board the Solar
and Heliospheric Observatory between 1996 and 1999.
We also present a detailed discussion of the modeling of the ridge power
distribution, and the contribution of the various observational and
instrumental effects on the spatial leakage, in the context of the MDI
instrument. We have constructed a physically motivated model (rather than some
ad hoc correction scheme) resulting in a methodology that can produce an
unbiased determination of high-degree modes, once the instrumental
characteristics are well understood.
Finally, we present changes in high degree mode parameters with epoch and
thus solar activity level and discuss their significance.Comment: 59 pages, 38 figures -- High-resolution version at
http://www-sgk.harvard.edu:1080/~sylvain/preprints/ -- Manuscript submitted
to Ap
What Is The Neon Abundance Of The Sun?
We have evolved a series of thirteen complete solar models that utilize
different assumed heavy element compositions. Models that are based upon the
heavy element abundances recently determined by Asplund, Grevesse, and Sauval
(2005) are inconsistent with helioseismological measurements. However, models
in which the neon abundance is increased by 0.4-0.5 dex to log N(Ne) = 8.29 +-
0.05 (on the scale in which log N(H) = 12) are consistent with the
helioseismological measurements even though the other heavy element abundances
are in agreement with the determinations of Asplund et al. (2005). These
results sharpen and strengthen an earlier study by Antia and Basu (2005). The
predicted solar neutrino fluxes are affected by the uncertainties in the
composition by less than their 1sigma theoretical uncertainties.Comment: Accepted for publication by ApJ. Minor editorial change
Generation of artificial helioseismic time-series
We present an outline of an algorithm to generate artificial helioseismic time-series, taking into account as much as possible of the knowledge we have on solar oscillations. The hope is that it will be possible to find the causes of some of the systematic errors in analysis algorithms by testing them with such artificial time-series
- …