889 research outputs found
The onset of solar cycle 24: What global acoustic modes are telling us
We study the response of the low-degree, solar p-mode frequencies to the
unusually extended minimum of solar surface activity since 2007. A total of
4768 days of observations collected by the space-based, Sun-as-a-star
helioseismic GOLF instrument are analyzed. A multi-step iterative
maximum-likelihood fitting method is applied to subseries of 365 days and 91.25
days to extract the p-mode parameters. Temporal variations of the l=0, 1, and 2
p-mode frequencies are then obtained from April 1996 to May 2009. While the
p-mode frequency shifts are closely correlated with solar surface activity
proxies during the past solar cycles, the frequency shifts of the l=0 and l=2
modes show an increase from the second half of 2007, when no significant
surface activity is observable. On the other hand, the l=1 modes follow the
general decreasing trend of the solar surface activity. The different
behaviours between the l=0 and l=2 modes and the l=1 modes can be interpreted
as different geometrical responses to the spatial distribution of the solar
magnetic field beneath the surface of the Sun. The analysis of the low-degree,
solar p-mode frequency shifts indicates that the solar activity cycle 24
started late 2007, despite the absence of activity on the solar surface.Comment: To be accepted by A&A (with minor revisions), 4 pages, 3 figures, 1
tabl
Are short-term variations in solar oscillation frequencies the signature of a second solar dynamo?
In addition to the well-known 11-year solar cycle, the Sun's magnetic
activity also shows significant variation on shorter time scales, e.g. between
one and two years. We observe a quasi-biennial (2-year) signal in the solar
p-mode oscillation frequencies, which are sensitive probes of the solar
interior. The signal is visible in Sun-as-a-star data observed by different
instruments and here we describe the results obtained using BiSON, GOLF, and
VIRGO data. Our results imply that the 2-year signal is susceptible to the
influence of the main 11-year solar cycle. However, the source of the signal
appears to be separate from that of the 11-year cycle. We speculate as to
whether it might be the signature of a second dynamo, located in the region of
near-surface rotational shear.Comment: 6 pages, 2 figures, proceedings for SOHO-24/GONG 2010 conference, to
be published in JPC
Analysis of the solar cycle and core rotation using 15 years of Mark-I observations:1984-1999. I. The solar cycle
High quality observations of the low-degree acoustic modes (p-modes) exist
for almost two complete solar cycles using the solar spectrophotometer Mark-I,
located at the Observatorio del Teide (Tenerife, Spain) and operating now as
part of the Birmingham Solar Oscillations Network (BiSON). We have performed a
Fourier analysis of 30 calibrated time-series of one year duration covering a
total period of 15 years between 1984 and 1999. Applying different techniques
to the resulting power spectra, we study the signature of the solar activity
changes on the low-degree p-modes. We show that the variation of the central
frequencies and the total velocity power (TVP) changes. A new method of
simultaneous fit is developed and a special effort has been made to study the
frequency-dependence of the frequency shift. The results confirm a variation of
the central frequencies of acoustic modes of about 450 nHz, peak-to-peak, on
average for low degree modes between 2.5 and 3.7 mHz. The TVP is
anti-correlated with the common activity indices with a decrease of about 20%
between the minimum and the maximum of solar cycle 22. The results are compared
with those obtained for intermediate degrees, using the LOWL data. The
frequency shift is found to increase with the degree with a weak l-dependence
similar to that of the inverse mode mass. This verifies earlier suggestions
that near surface effects are predominant.Comment: Accepted by A&A October 3 200
The complementary roles of interferometry and asteroseismology in determining the mass of solar-type stars
How important is an independent diameter measurement for the determination of
stellar parameters of solar-type stars? When coupled with seismic observables,
how well can we determine the stellar mass? If we can determine the radius of
the star to between 1% and 4%, how does this affect the theoretical
uncertainties? Interferometry can provide an independent radius determination
and it has been suggested that we should expect at least a 4% precision on such
a measurement for nearby solar-type stars. This study aims to provide both
qualitative and quantitive answers to these questions for a star such as our
Sun, where seismic information will be available. We show that the importance
of an independent radius measurement depends on the combination of observables
available and the size of the measurement errors. It is important for
determining all stellar parameters and in particular the mass, where a good
radius measurement can even allow us to determine the mass with a precision
better than 2%. Our results also show that measuring the small frequency
separation significantly improves the determination of the evolutionary stage
and the mixing-length parameter.Comment: 7 pages text; 6 pages ref+figures+tables; accepted by ApJ; uses
emulateapj
Soil and leaf mineral element contents in mediterranean vineyards: bioaccumulation and potential soil pollution
The study reported here concerns the geochemical distributions of macro- and trace elements (including potentially toxic elements, PTEs) in the vineyard soils of Alcubillas, which is one of the oldest, albeit not world-renowned, wine-growing areas in La Mancha (Central Spain). Soil and leaf samples were analyzed by X-ray fluorescence spectrometry to ascertain the levels of various elements in the soil and the plant. The potential toxicity of the elements was assessed with regard to the development of the vineyard. Despite the fact that fertilizers and pesticides are employed in the vineyards in this area, the results showed that the levels of trace elements in the soil samples did not exceed the reference values according the pedogeochemical values for the region and Spain. This finding suggests that the study area is not polluted, and therefore, there are hardly any traces of anthropogenic contamination. The Biological Absorption Coefficient (BAC) was calculated to assess the assimilation of various elements from the soil to the leaves, and differences were found in the element absorption capacity of the vines. Some elements were not taken up by Vitis vinifera despite elements like Zr and Rb being present in relatively high concentrations in the soil. The production in these soils does not represent a threat to human health or the ecosystem, because the farmers in this area are extremely careful to preserve the environment and they only farm to achieve moderate yields of grapes per hectar
Influence of Low-Degree High-Order p-Mode Splittings on the Solar Rotation Profile
The solar rotation profile is well constrained down to about 0.25 R thanks to
the study of acoustic modes. Since the radius of the inner turning point of a
resonant acoustic mode is inversely proportional to the ratio of its frequency
to its degree, only the low-degree p modes reach the core. The higher the order
of these modes, the deeper they penetrate into the Sun and thus they carry more
diagnostic information on the inner regions. Unfortunately, the estimates of
frequency splittings at high frequency from Sun-as-a-star measurements have
higher observational errors due to mode blending, resulting in weaker
constraints on the rotation profile in the inner core. Therefore inversions for
the solar internal rotation use only modes below 2.4 mHz for l < 4. In the work
presented here, we used an 11.5 year-long time series to compute the rotational
frequency splittings for modes l < 4 using velocities measured with the GOLF
instrument. We carried out a theoretical study of the influence of the
low-degree modes in the region 2 to 3.5 mHz on the inferred rotation profile as
a function of their error bars.Comment: Accepted for publication in Solar Physics. 17 Pages, 9 figure
Solar Activity Phases and Intermediate-degree Mode Frequencies
We analyze intermediate degree p-mode eigenfrequencies measured by GONG and
MDI/SOHO over a solar cycle to study the source of their variability. We carry
out a correlation analysis between the change in frequencies and several
measures of the Sun's magnetic activity that are sensitive to changes at
different levels in the solar atmosphere. The observations span a period of
about 12 years starting from mid-1996 (the minimum of cycle 23) to early-2008
(near minimum of cycle 24), corresponding to a nearly complete solar activity
cycle. We demonstrate that the frequencies do vary in phase with the solar
activity indices, however the degree of correlation differs from phase to phase
of the cycle. During the rising and declining phases, the mode frequency shifts
are strongly correlated with the activity proxies whereas during the
high-activity period, the shifts have significantly lower correlation with all
activity proxies, except for the 10.7-cm radio flux. In particular, the proxies
that are only influenced by the variation of the strong component of the
magnetic field in the photosphere have a much lower correlation at the
high-activity period. On the other hand, the shifts are better correlated with
the proxies sensitive to changes in the weak component of the magnetic field.
Our correlation analysis suggests that more than 90% of the variation in the
oscillation frequencies in all activity phases can be explained by changes in
both components of the magnetic field. Further, the slopes obtained from the
linear regression analysis also differ from phase to phase and show a strong
correlation with the correlation coefficients between frequency shifts and
solar activity.Comment: Accepted for publication in Astrophysical Journal (April 20, 2009
issue
Constraining the properties of delta Scuti stars using spectroscopic eclipsing binary systems
Many stars exhibit stellar pulsations, favoring them for asteroseismic
analyses. Interpreting the oscillations requires some knowledge of the
oscillation mode geometry (spherical degree, radial and azimuthal orders). The
delta Scuti stars (1.5 - 2.5 M_sol) often show just one or few pulsation
frequencies. Although this may promise a successful seismological analysis, we
may not know enough about either the mode or the star to use the oscillation
frequency to improve the determination of the stellar model, or probe the
star's structure. For the observed frequencies to be used successfully as
seismic probes of these objects, we need to concentrate on stars for which we
can reduce the number of free parameters in the problem, such as binary systems
or open clusters. We investigate how much our understanding of a delta Scuti
star is improved when it is in a detached eclipsing binary system instead of
being a single field star. We use singular value decomposition to explore the
precision we expect in stellar parameters (mass, age and chemical composition)
for both cases. We examine how the parameter uncertainties propagate to the
luminosity - effective temperature diagram and determine when the effort of
obtaining a new measurement is justified. We show that for the single star, a
correct identification of the oscillation mode is necessary to produce strong
constraints on the stellar model properties, while for the binary system the
observations without the pulsation mode provide the same or better constraints
on the stellar parameters. In the latter case, ...Comment: emulateapj 16 pages, accepted Ap
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