3,198 research outputs found
Subsurface magnetic fields from helioseismology
Using even-order frequency splitting coefficients of global p-modes it is
possible to infer the magnetic field in the solar interior as a function of
radial distance and latitude. Results obtained using GONG and MDI data are
discussed. While there is some signal of a possible magnetic field in the
convection zone, there is little evidence for any temporal variation of the
magnetic field in the solar interior. Limits on possible magnetic field in the
solar core are also discussed. It is generally believed that the solar dynamo
is located in the tachocline region. Seismic studies do not show any
significant temporal variation in the tachocline region, though a significant
latitudinal variation in the properties of the tachocline are found. There is
some evidence to suggest that the latitudinal variation is not continuous and
the tachocline may consist of two parts.Comment: 8 pages, to appear in proceedings of IAU Coll. 188, on Magnetic
Coupling of the Solar Atmospher
Estimate of solar radius from f-mode frequencies
Frequency and rotational splittings of the solar f-modes are estimated from
the GONG data. Contrary to earlier observations the frequencies of f-modes are
found to be close to the theoretically computed values for a standard solar
model. The f-mode being essentially a surface mode is a valuable diagnostic
probe of the properties of the solar surface, and also provides an independent
measure of solar radius. The estimated solar radius is found to be about 0.03%
less than what is traditionally used in construction of standard solar models.
If this decrease in solar radius is confirmed then the current solar models as
well as inversion results will need to be revised. The rotational splittings of
the f-modes yield an independent measure of the rotation rate near the solar
surface, which is compared with other measurements.Comment: 5 pages, A&A-TeX, 5 figure
High frequency and high wavenumber solar oscillations
We determine the frequencies of solar oscillations covering a wide range of
degree (100< l <4000) and frequency (1.5 <\nu<10 mHz) using the ring diagram
technique applied to power spectra obtained from MDI (Michelson Doppler Imager)
data. The f-mode ridge extends up to degree of approximately 3000, where the
line width becomes very large, implying a damping time which is comparable to
the time period. The frequencies of high degree f-modes are significantly
different from those given by the simple dispersion relation \omega^2=gk. The
f-mode peaks in power spectra are distinctly asymmetric and use of asymmetric
profile increases the fitted frequency bringing them closer to the frequencies
computed for a solar model.Comment: Revised version. 1.2 mHz features identified as artifacts of data
analysis. Accepted for publication in Ap
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
Revisiting the solar tachocline: Average properties and temporal variations
The tachocline is believed to be the region where the solar dynamo operates.
With over a solar cycle's worth of data available from the MDI and GONG
instruments, we are in a position to investigate not merely the average
structure of the solar tachocline, but also its time variations. We determine
the properties of the tachocline as a function of time by fitting a
two-dimensional model that takes latitudinal variations of the tachocline
properties into account. We confirm that if we consider central position of the
tachocline, it is prolate. Our results show that the tachocline is thicker at
higher latitudes than the equator, making the overall shape of the tachocline
more complex. Of the tachocline properties examined, the transition of the
rotation rate across the tachocline, and to some extent the position of the
tachocline, show some temporal variations
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
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 variations of large scale flows in the Sun
Using data from the Michelson Doppler Imager (MDI) instrument on board the
Solar and Heliospheric Observatory (SOHO), we study the large-scale velocity
fields in the outer part of the solar convection zone using the ring diagram
technique. We use observations from four different times to study possible
temporal variations in flow velocity. We find definite changes in both the
zonal and meridional components of the flows. The amplitude of the zonal flow
appears to increase with solar activity and the flow pattern also shifts
towards lower latitude with time.Comment: To appear in Solar Physic
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