45 research outputs found
A new code for Fourier-Legendre analysis of large datasets: first results and a comparison with ring-diagram analysis
Fourier-Legendre decomposition (FLD) of solar Doppler imaging data is a
promising method to estimate the sub-surface solar meridional flow. FLD is
sensible to low-degree oscillation modes and thus has the potential to probe
the deep meridional flow. We present a newly developed code to be used for
large scale FLD analysis of helioseismic data as provided by the Global
Oscillation Network Group (GONG), the Michelson Doppler Imager (MDI)
instrument, and the upcoming Helioseismic and Magnetic Imager (HMI) instrument.
First results obtained with the new code are qualitatively comparable to those
obtained from ring-diagram analyis of the same time series.Comment: 4 pages, 2 figures, 4th HELAS International Conference "Seismological
Challenges for Stellar Structure", 1-5 February 2010, Arrecife, Lanzarote
(Canary Islands
Comparison of the sidereal angular velocity of subphotospheric layers and small bright coronal structures during the declining phase of solar cycle 23
Context. We compare solar differential rotation of subphotospheric layers
derived from local helioseismology analysis of GONG++ dopplergrams and the one
derived from tracing small bright coronal structures (SBCS) using EIT/SOHO
images for the period August 2001 - December 2006, which correspond to the
declining phase of solar cycle 23. Aims. The study aims to find a relationship
between the rotation of the SBCS and the subphotospheric angular velocity. The
northsouth asymmetries of both rotation velocity measurements are also
investigated. Methods. Subphotospheric differential rotation was derived using
ring-diagram analysis of GONG++ full-disk dopplergrams of 1 min cadence. The
coronal rotation was derived by using an automatic method to identify and track
the small bright coronal structures in EIT full-disk images of 6 hours cadence.
Results. We find that the SBCS rotate faster than the considered upper
subphotospheric layer (3Mm) by about 0.5 deg/day at the equator. This result
joins the results of several other magnetic features (sunspots, plages,
faculae, etc.) with a higher rotation than the solar plasma. The rotation rate
latitudinal gradients of the SBCS and the subphotospheric layers are very
similar. The SBCS motion shows an acceleration of about 0.005 deg/day/month
during the declining phase of solar cycle 23, whereas the angular velocity of
subsurface layers does not display any evident variation with time, except for
the well known torsional oscillation pattern. Finally, both subphotospheric and
coronal rotations of the southern hemisphere are predominantly larger than
those of the northern hemisphere. At latitudes where the north-south asymmetry
of the angular velocity increases (decreases) with activity for the SBCS, it
decreases (increases) for subphotospheric layers.Comment: 6pages, 8 figures, Accepted for publication in Astronomy and
Astrophysic
Differential coronal rotation using radio images at 17 GHz
In the present work, we perform time-series analysis on the latitude bins of
the solar full disk (SFD) images of Nobeyama Radioheliograph (NoRH) at 17 GHz.
The flux modulation method traces the passage of radio features over the solar
disc and the autocorrelation analysis of the time-series data of SFD images
(one per day) for the period 1999-2001 gives the rotation period as a function
of latitude extending from 60 degree S to 60 degree N. The results show that
the solar corona rotates less differentially than the photosphere and
chromosphere, i.e., it has smaller gradient in the rotation rate.Comment: 5 pages, 5 figures, Accepted for publication in MNRAS letter
Sensitivity of the g-mode frequencies to pulsation codes and their parameters
From the recent work of the Evolution and Seismic Tools Activity (ESTA,
Lebreton et al. 2006; Monteiro et al. 2008), whose Task 2 is devoted to compare
pulsational frequencies computed using most of the pulsational codes available
in the asteroseismic community, the dependence of the theoretical frequencies
with non-physical choices is now quite well fixed. To ensure that the accuracy
of the computed frequencies is of the same order of magnitude or better than
the observational errors, some requirements in the equilibrium models and the
numerical resolutions of the pulsational equations must be followed. In
particular, we have verified the numerical accuracy obtained with the Saclay
seismic model, which is used to study the solar g-mode region (60 to
140Hz). We have compared the results coming from the Aarhus adiabatic
pulsation code (ADIPLS), with the frequencies computed with the Granada Code
(GraCo) taking into account several possible choices. We have concluded that
the present equilibrium models and the use of the Richardson extrapolation
ensure an accuracy of the order of in the determination of the
frequencies, which is quite enough for our purposes.Comment: 10 pages, 5 figures, accepted in Solar Physic
Low-Degree High-Frequency p and g Modes in the Solar Core
Solar gravity (g) modes propagate within the radiative part of the solar
interior and are highly sensitive to the physical conditions of the solar core.
They would represent the best tool to infer the structure and dynamics of the
radiative interior, in particular the core, if they were properly detected and
characterized. Although individual rotational splittings for g modes have not
yet been calculated, we have to understand the effect of these modes, and also
low-degree high-frequency p modes, on the inversion of the solar rotation rate
between 0.1 and 0.2 Rs. In this work, we follow the methodology developed in
Mathur et al. (2008) and Garcia et al. (2008), adding g modes and low-degree
high-frequency p modes to artificial inversion data sets, in order to study how
they convey information on the solar core rotation.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
Meridional Circulation and Global Solar Oscillations
We investigate the influence of large-scale meridional circulation on solar
p-modes by quasi-degenerate perturbation theory, as proposed by
\cite{lavely92}. As an input flow we use various models of stationary
meridional circulation obeying the continuity equation. This flow perturbs the
eigenmodes of an equilibrium model of the Sun. We derive the signatures of the
meridional circulation in the frequency multiplets of solar p-modes. In most
cases the meridional circulation leads to negative average frequency shifts of
the multiplets. Further possible observable effects are briefly discussed.Comment: 14 pages, 5 figures, submittted to Solar Physics Topical Issue
"HELAS
Predicting the Amplitude of a Solar Cycle Using the North-South Asymmetry in the Previous Cycle: II. An Improved Prediction for Solar Cycle~24
Recently, using Greenwich and Solar Optical Observing Network sunspot group
data during the period 1874-2006, (Javaraiah, MNRAS, 377, L34, 2007: Paper I),
has found that: (1) the sum of the areas of the sunspot groups in 0-10 deg
latitude interval of the Sun's northern hemisphere and in the time-interval of
-1.35 year to +2.15 year from the time of the preceding minimum of a solar
cycle n correlates well (corr. coeff. r=0.947) with the amplitude (maximum of
the smoothed monthly sunspot number) of the next cycle n+1. (2) The sum of the
areas of the spot groups in 0-10 deg latitude interval of the southern
hemisphere and in the time-interval of 1.0 year to 1.75 year just after the
time of the maximum of the cycle n correlates very well (r=0.966) with the
amplitude of cycle n+1. Using these relations, (1) and (2), the values 112 + or
- 13 and 74 + or -10, respectively, were predicted in Paper I for the amplitude
of the upcoming cycle 24. Here we found that in case of (1), the north-south
asymmetry in the area sum of a cycle n also has a relationship, say (3), with
the amplitude of cycle n+1, which is similar to (1) but more statistically
significant (r=0.968) like (2). By using (3) it is possible to predict the
amplitude of a cycle with a better accuracy by about 13 years in advance, and
we get 103 + or -10 for the amplitude of the upcoming cycle 24. However, we
found a similar but a more statistically significant (r=0.983) relationship,
say (4), by using the sum of the area sum used in (2) and the north-south
difference used in (3). By using (4) it is possible to predict the amplitude of
a cycle by about 9 years in advance with a high accuracy and we get 87 + or - 7
for the amplitude of cycle 24.Comment: 21 pages, 7 figures, Published in Solar Physics 252, 419-439 (2008
The quest for the solar g modes
Solar gravity modes (or g modes) -- oscillations of the solar interior for
which buoyancy acts as the restoring force -- have the potential to provide
unprecedented inference on the structure and dynamics of the solar core,
inference that is not possible with the well observed acoustic modes (or p
modes). The high amplitude of the g-mode eigenfunctions in the core and the
evanesence of the modes in the convection zone make the modes particularly
sensitive to the physical and dynamical conditions in the core. Owing to the
existence of the convection zone, the g modes have very low amplitudes at
photospheric levels, which makes the modes extremely hard to detect. In this
paper, we review the current state of play regarding attempts to detect g
modes. We review the theory of g modes, including theoretical estimation of the
g-mode frequencies, amplitudes and damping rates. Then we go on to discuss the
techniques that have been used to try to detect g modes. We review results in
the literature, and finish by looking to the future, and the potential advances
that can be made -- from both data and data-analysis perspectives -- to give
unambiguous detections of individual g modes. The review ends by concluding
that, at the time of writing, there is indeed a consensus amongst the authors
that there is currently no undisputed detection of solar g modes.Comment: 71 pages, 18 figures, accepted by Astronomy and Astrophysics Revie
Subsurface Meridional Circulation in the Active Belts
Temporal variations of the subsurface meridional flow with the solar cycle
have been reported by several authors. The measurements are typically averaged
over periods of time during which surface magnetic activity existed in the
regions were the velocities are calculated. The present work examines the
possible contamination of these measurements due to the extra velocity fields
associated with active regions plus the uncertainties in the data obtained
where strong magnetic fields are present. We perform a systematic analysis of
more than five years of GONG data and compare meridional flows obtained by
ring-diagram analysis before and after removing the areas of strong magnetic
field. The overall trend of increased amplitude of the meridional flow towards
solar minimum remains after removal of large areas associated with surface
activity. We also find residual circulation toward the active belts that
persist even after the removal of the surface magnetic activity, suggesting the
existence of a global pattern or longitudinally-located organized flows.Comment: 12 pages, 6 figures, Submitted to Solar Physics. Accepted
(08/25/2008