3,558 research outputs found
Solar cycle variation in GONG and MDI data: 1995-2002
Both GONG and MDI projects have measured {\it p}-mode frequencies of the Sun
for more than 7 years. Here we review what we have learnt from the temporal
variation of the oscillation frequencies and splitting coefficients.Comment: 6 pages, Latex (requires Basi.sty), invited review presented in the
annual meeting of ASI (2003
Opacity effects on the solar interior. I. Solar structure
Despite recent major advances, the opacity remains a source of substantial
uncertainty in the calculation of solar models, and hence of solar oscillation
frequencies. Hence it is of substantial interest to investigate the sensitivity
of solar structure to changes in the opacity. Furthermore, we may hope from the
precise helioseismic inferences of solar structure to obtain information about
possible corrections to the opacities used in the model calculation. Here we
carry out detailed calculations of the influence on solar models of changes in
the opacity, including also evolutionary effects. We find that over the
relevant range the response of the model is approximately linear in the opacity
change, allowing the introduction of opacity kernels relating a general opacity
change to the corresponding model changes. Changes in the convection zone can
be characterized entirely by the change in the initial composition and mixing
length required to calibrate the model.Comment: 13 pages, 11 figures. Submitted to Astron. Astrophys., Sept. 19 1997
Uses A&A macro package aa.cls, version 4.01 for LaTeX2
Variation of Acoustic Power with Magnetic Field as seen in Gong+ Data
The acoustic spectra in sunspots are known to be richer in higher frequency
power. We have attempted a generalized study of the effect of magnetic fields
on the shape of the acoustic spectrum using GONG+ bread-board data (spatial
scale of ~ 2 arc-sec per pixel) of 11 May, 2000 and 12 June, 2000. The mean
power spectra of the velocity oscillations were obtained by averaging over
several spectra for different values of the magnetic field. With increasing
magnetic field, the acoustic power increases at higher frequencies and
decreases at lower frequencies with a transition at ~= 5 mHz. This behaviour is
slightly different from earlier results obtained from SOHO/MDI data.Comment: 6 pages, 3 figures, To appear in Solar Physic
Probing Subsurface Flows in Active Region NOAA 12192 - Comparison with NOAA 10486
Active Region (AR) 12192 is the biggest AR observed in solar cycle 24 so far.
This was a long-lived AR which survived for four Carrington rotations (CR) and
exhibited several unusual phenomena. We measure the horizontal subsurface flows
in this active region in multiple rotation using the ring-diagram technique of
local helioseismology and the Global Oscillation Network Group (GONG+)
Dopplergrams, and investigate how different was the plasma flow in AR 12192
from that in AR 10486. Both regions produced several high M- and X-class flares
but had different CME productivity. Our analysis suggests that these ARs had
unusually large horizontal flow amplitude with distinctly different directions.
While meridional flow in AR 12192 was poleward that supports the flux transport
to poles, it was equatorward in AR 10486. Furthermore, there was a sudden
increase in the magnitude of estimated zonal flow in shallow layers in AR 12192
during the X3.1 flare, however, it reversed direction in AR 10486 with X17.2
flare. These flow patterns produced strong twists in horizontal velocity with
depth in AR 10486 that persisted throughout the disk passage as opposed to AR
12192, which produced a twist only after the eruption of the X3.1 flare that
disappeared soon after. Our study indicates that the sunspot rotation combined
with the re-organization of magnetic field in AR 10486 was not sufficient to
decrease the flow energy even after several large flares that might have
triggered CMEs. Furthermore, in the absence of sunspot rotation in AR 12192,
this re-organization of magnetic field contributed significantly to the
substantial release of flow energy after the X3.1 flare.Comment: The Astrophysical Journal (in press), 39 pages including 15 figures
and 4 table
Divergent Horizontal Sub-surface Flows within Active Region 11158
We measure the horizontal subsurface flow in a fast emerging active region
(NOAA 11158) using the ring-diagram technique and the HMI high-spatial
resolution Dopplergrams. This active region had a complex magnetic structure
and displayed significant changes in the morphology during its disk passage.
Over the period of six days from 2011 February 11 to 16, the temporal variation
in the magnitude of total velocity is found to follow the trend of magnetic
field strength. We further analyze regions of individual magnetic polarity
within AR 11158 and find that the horizontal velocity components in these
sub-regions have significant variation with time and depth. The leading and
trailing polarity regions move faster than the mixed-polarity region. Further,
both zonal and meridional components have opposite signs for trailing and
leading polarity regions at all depths showing divergent flows within the
active region. We also find a sharp decrease in the magnitude of total
horizontal velocity in deeper layer around major flares. It is suggested that
the re-organization of magnetic fields during flares combined with the sunspot
rotation decreases the magnitude of horizontal flows or that the flow kinetic
energy has been converted into the energy released by flares. After the decline
in flare activity and the sunspot rotation, the flows tend to follow the
pattern of the magnetic activity.We also observe less variation in the velocity
components near the surface but these tend to increase with depth, further
demonstrating that the deeper layers are more affected by the topology of
active regions.Comment: 24 pages, 10 figure and 3 table
Variation of solar irradiance and mode frequencies during Maunder minimum
Using the sunspot numbers reported during the Maunder minimum and the
empirical relations between the mode frequencies and solar activity indices,
the variations in the total solar irradiance and 10.7 cm radio flux for the
period 1645 to 1715 is estimated. We find that the total solar irradiance and
radio flux during the Maunder minimum decreased by 0.19% and 52% respectively,
as compared to the values for solar cycle 22.Comment: To appear in Astrophysics and Space Science, Also available at
http://www.prl.res.in/~sushan
On the solar rotation rate in the upper convection zone
We present results on variation in rotation rate in the upper convection zone
using data from GONG and MDI/SOHO covering a period of more than four years. We
find that the first few odd-order splitting coefficients vary systematically
with the solar cycle. The rotation rate near the solar surface calculated from
analytical methods agrees well with that of inversion techniques. The residual
rotation rate in the outer layers seem to be correlated with the solar
activity.Comment: To appear in the proceedings of Helios- and Asteroseismology at the
Dawn of the New Millennium, ESA-SP 464, Ed. A. Wilson. Requires esapub.cls.
Also available at http://www.uso.ernet.in/~sushant/paper.htm
Acoustic Mode Frequencies of the Sun during the Minimum Phase between Solar Cycles 23 and 24
We investigate the spatial and temporal variations of the high-degree mode
frequencies calculated over localized regions of the Sun during the extended
minimum phase between solar cycles 23 and 24. The frequency shifts measured
relative to the spatial average over the solar disk indicate that the
correlation between the frequency shift and magnetic field strength during the
low-activity phase is weak. The disk-averaged frequency shifts computed
relative to a minimal activity period also reveal a moderate correlation with
different activity indices, with a maximum linear correlation of about 72%.
From the investigation of the frequency shifts at different latitudinal bands,
we do not find a consensus period for the onset of solar cycle 24. The
frequency shifts corresponding to most of the latitudes in the northern
hemisphere and 30 degree south of the equator indicate the minimum epoch to be
February 2008, which is earlier than inferred from solar activity indices.Comment: Accepted for publication in Solar Physic
Variations in High Degree Acoustic Mode Frequencies of the Sun during Solar Cycle 23 and 24
We examine continuous measurements of the high-degree acoustic mode
frequencies of the Sun covering the period from 2001 July to June 2014. These
are obtained through the ring-diagram technique applied to the full-disk
Doppler observations made by the Global Oscillation Network Group (GONG). The
frequency shifts in the degree range of 180-1200 are correlated with different
proxies of solar activity e.g. 10.7 cm radio flux, the International Sunspot
Number and the strength of the local magnetic field. In general, a good
agreement is found between the shifts and activity indices, and the correlation
coefficients are found to be comparable with intermediate degree mode
frequencies. Analyzing the frequency shifts separately for the two cycles, we
find that cycle 24 is weaker than cycle 23. Since the magnetic activity is
known to be different in the two hemisphere, for the first time, we compute the
frequency shifts over the two hemispheres separately and find that the shifts
also display hemispheric asymmetry; the amplitude of shifts in the northern
hemisphere peaked during late 2011, more than two years earlier than the south.
We further correlate the hemispheric frequency shifts with the hemispheric
sunspot number and mean magnetic activity index. Since the frequency shifts and
the hemispheric activity indices are found to be significantly correlated, we
suggest that the shifts be used as an indicator of hemispheric activity since
not many indices are measured over the two hemispheres separately. We also
investigate the variation at different latitudinal bands and conclude that the
shifts in active latitudes correlate well with the local magnetic activity
index.Comment: Accepted for publication in Ap
Inhomogeneous Power Distribution in Magnetic Oscillations
We apply ring-diagram analysis and spherical harmonic decomposition methods
to compute 3-dimensional power spectra of magnetograms obtained by the Global
Oscillation Network Group (GONG) during quiet periods of solar activity. This
allows us to investigate the power distribution in acoustic waves propagating
in localized directions on the solar disk. We find evidence of the presence of
five-minute oscillations in magnetic signals that suggests a non-homogeneous
distribution of acoustic power. In this paper, we present our results on the
asymmetry in oscillatory power and its behaviour as a function of frequency,
time and magnetic field strength. These characteristics are compared with
simultaneous velocity measurements.Comment: To appear in the Proceedings of "IV HELAS International Conference"
(Lanzarote, Feb 1-5, 2010), Astronomical Notes, 2010, Eds. T. Roca Cortes, P.
Palle and S. Jimenez Reye
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