共查询到20条相似文献，搜索用时 31 毫秒
1.
A. Eff‐Darwich C. Rgulo S.G. Korzennik F. Prez Hernndez T. Roca Corts 《Astronomische Nachrichten》2008,329(5):470-475
Data collected recently by the helioseismic experiments aboard the SOHO spacecraft have allowed the detection of low degree p‐modes with increasingly lower order n. In particular, the GOLF experiment is currently able to unambiguously identify low degree modes with frequencies as low as 1.3 mHz. The detection of p‐modes with very low frequency (i.e., low n), is difficult due to the low signal‐to‐noise ratio in this spectral region and its contamination by solar signals that are not of acoustic origin. To address this problem without using any theoretical a priory, we propose a methodology that relies only on the inversion of observed values to define a spectral window for the expected locations of these low frequency modes. The application of this method to 2920‐day‐long GOLF observations is presented and its results discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
2.
R. A. García S. Mathur J. Ballot A. Eff-Darwich S. J. Jiménez-Reyes S. G. Korzennik 《Solar physics》2008,251(1-2):119-133
The solar rotation profile is well constrained down to about 0.25R ⊙ 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 because of 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 ?≤3. In the work presented here, we used an 11.5-year-long time series to compute the rotational frequency splittings for modes ?≤3 using velocities measured with the GOLF instrument. We carried out a theoretical study of the influence of the low-degree modes in the region from 2 to 3.5 mHz on the inferred rotation profile as a function of their error bars. 相似文献
3.
During the declining phase of the longest solar minimum in a century, the arrival of the MESSENGER spacecraft at superior conjunction allowed the measurement of magnetohydrodynamic (MHD) waves in the solar corona with its 8 GHz radio frequency signal. MHD waves crossing the line of sight were measured via Faraday rotation fluctuations (FRFs) in the plane of polarization (PP) of MESSENGER’s signal. FRFs in previous observations of the solar corona (at greater offset distances) consisted of a turbulent spectrum that decreased in power with increasing frequency and distance from the Sun. Occasionally a spectral line, a distinct peak in the power spectral density spectrum around 4 to 8 mHz, was also observed in these early data sets at offset distances of about 5 to 10 solar radii. The MESSENGER FRF data set shows a spectral line at an offset distance between 1.55 to 1.85 solar radii with a frequency of 0.6±0.2 mHz. Other possible spectral lines may be at 1.2, 1.7, and 4.5 mHz; MHD waves with these same frequencies have been observed in X-ray data traveling along closed coronal loops at lower offset distances. An initial analysis of the MESSENGER spectral line(s) shows behavior similar to turbulent spectra: decreasing power with increasing frequency and distance from the Sun. Here we detail the steps taken to process the MESSENGER change in PP data set for the MHD wave investigation. 相似文献
4.
5.
We experiment with a method of measuring the frequency of solar p modes, intended to extend the passband for the variations of the frequency spectrum as high as possible. So far this passband is limited to a fraction of μ Hz for the classical analysis based on numerical fits of a theoretical line profile to a power spectrum averaged over periods lasting at least several weeks. This limit for the present analysis can be shifted to the mHz range, corresponding to some of the “5 min” oscillations, but in this range we use a lower resolution which allows us to separate odd and even p modes. We show an example of the results for long term variations and apply this analysis to search for a modulation of the p‐mode frequency spectrum by asymptotic series of solar g modes. A faint signal is found in the analysis of 10 years of GOLF data. This very preliminary result possibly indicates the detection of a small number of g modes of degree l = 1. A tentative determination of an observational value of the parameter P0 follows. P0 is the scaling factor of the asymptotic series of g modes and is a key data for solar core physics. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
6.
T. Toutain T. Appourchaux F. Baudin C. Fröhlich A. Gabriel P. Scherrer B. N. Andersen R. Bogart R. Bush W. Finsterle R. A. García G. Grec C. J. Henney J. T. Hoeksema A. Jiménez A. Kosovichev T. Roca Cortés S. Turck-Chièze R. Ulrich C. Wehrli 《Solar physics》1997,175(2):311-328
The three helioseismology instruments aboard SOHO observe solar p modes in velocity (GOLF and MDI) and in intensity (VIRGO
and MDI). Time series of two months duration are compared and confirm that the instruments indeed observe the same Sun to
a high degree of precision. Power spectra of 108 days are compared showing systematic differences between mode frequencies
measured in intensity and in velocity. Data coverage exceeds 97% for all the instruments during this interval. The weighted
mean differences (V-I) are −0.1 μHz for l=0, and −0.16 μHz for l=1. The source of this systematic difference may be due to
an asymmetry effect that is stronger for modes seen in intensity. Wavelet analysis is also used to compare the shape of the
forcing functions. In these data sets nearly all of the variations in mode amplitude are of solar origin. Some implications
for structure inversions are discussed. 相似文献
7.
GarcÍa R.A. Régulo C. Turck-Chièze S. Bertello L. Kosovichev A.G. Brun A.S. Couvidat S. Henney C.J. Lazrek M. Ulrich R.K. Varadi F. 《Solar physics》2001,200(1-2):361-379
Data recovered from the GOLF experiment on board the ESA/NASA SOHO spacecraft have been used to analyze the low-order low-degree solar velocity acoustic-mode spectrum below =1.5 mHz (i.e., 1n9,l2). Various techniques (periodogram, RLAvCS, homomorphic-deconvolution and RLSCSA) have been used and compared to avoid possible biases due to a given analysis method. In this work, the acoustic resonance modes sensitive to the solar central region are studied. Comparing results from the different analysis techniques, 10 modes below 1.5 mHz have been identified. 相似文献
8.
A. H. Gabriel G. Grec J. Charra J. -M. Robillot T. Roca Cortés S. Turck-Chièze R. Bocchia P. Boumier M. Cantin E. Cespédes B. Cougrand J. Crétolle L. Damé M. Decaudin P. Delache N. Denis R. Duc H. Dzitko E. Fossat J. -J. Fourmond R. A. García D. Gough C. Grivel J. M. Herreros H. Lagardère J. -P. Moalic P. L. Pallé N. Pétrou M. Sanchez R. Ulrich H. B. van der Raay 《Solar physics》1995,162(1-2):61-99
The GOLF experiment on the SOHO mission aims to study the internal structure of the sun by measuring the spectrum of global oscillations in the frequency range 10–7 to 10–2 Hz. Bothp andg mode oscillations will be investigated, with the emphasis on the low order long period waves which penetrate the solar core. The instrument employs an extension to space of the proven ground-based technique for measuring the mean line-of-sight velocity of the viewed solar surface. By avoiding the atmospheric disturbances experienced from the ground, and choosing a non-eclipsing orbit, GOLF aims to improve the instrumental sensitivity limit by an order of magnitude to 1 mm s–1 over 20 days for frequencies higher than 2.10–4 Hz. A sodium vapour resonance cell is used in a longitudinal magnetic field to sample the two wings of the solar absorption line. The addition of a small modulating field component enables the slope of the wings to be measured. This provides not only an internal calibration of the instrument sensitivity, but also offers a further possibility to recognise, and correct for, the solar background signal produced by the effects of solar magnetically active regions. The use of an additional rotating polariser enables measurement of the mean solar line-of-sight magnetic field, as a secondary objective. 相似文献
9.
A. H. Gabriel J. Charra G. Grec J.-M. Robillot T. Roca Cortés S. Turck-Chièze R. Ulrich S. Basu F. Baudin L. Bertello P. Boumier M. Charra J. Christensen-Dalsgaard M. Decaudin H. Dzitko T. Foglizzo E. Fossat R. A. García J. M. Herreros M. Lazrek P. L. Pallé N. Pétrou C. Renaud C. Régulo 《Solar physics》1997,175(2):207-226
GOLF in-flight commissioning and calibration was carried out during the first four months, most of which represented the cruise
phase of SOHO towards its final L1 orbit. The initial performance of GOLF is shown to be within the design specification,
for the entire instrument as well as for the separate sub-systems. Malfunctioning of the polarising mechanisms after 3 to
4 months operation has led to the adoption of an unplanned operating sequence in which these mechanisms are no longer used.
This mode, which measures only the blue wing of the solar sodium lines, detracts little from the detection and frequency measurements
of global oscillations, but does make more difficult the absolute velocity calibration, which is currently of the order of
20%. Data continuity in the new mode is extremely high and the instrument is producing exceptionally noise-free p-mode spectra.
The data set is particularly well suited to the study of effects due to the excitation mechanism of the modes, leading to
temporal variations in their amplitudes. The g modes have not yet been detected in this limited data set. In the present mode
of operation, there are no indications of any degradation which would limit the use of GOLF for up to 6 years or more. 相似文献
10.
We present observations of high frequency, intermediate degree, Ca-K line solar intensity oscillations. We compare the peak
frequencies determined from these 1991.4 observations with the peak frequencies from 1987.9 South Pole observations (Duvallet al., 1991) in that portion of the spatio-temporal diagram where the two datasets overlap (degrees between 30 and 150 and frequencies
between 4 and 6.6 mHz). We find that temporal changes are detectable in the high frequency spectrum and are particularly large
near 5.4 mHz. The m-averaged high frequency peaks decreased in frequency in 1991.4 compared to the peak frequencies measured
in 1987.9. The magnitude of the frequency shift is of the order of 10 μHz near 5.4 mHz, increases with degree, and decreases to near zero both above and below 5.4 mHz. It is unlikely that these temporal
changes in the high frequency spectrum are due to a change in the height of the subphotospheric acoustic source layer. A physical
mechanism for these frequency shifts has not yet been identified. 相似文献
11.
We present a comparative study of low-l solar p-mode parameters extracted by genetic-algorithm and ‘standard’ hill-climbing minimisation routines. To effect this we make
use of observations made in integrated sunlight by the Birmingham Solar-Oscillations Network (BiSON) and the GOLF instrument
on board the ESA/NASA SOHO satellite, in addition to artificial data. We find that over the central part of the p-mode range the two fitting routines return similar results. However, at low frequencies — where the S/N in the modes is low
and their resonant peaks narrow — we find that the genetic routine appears to offer more robust estimates of the underlying
parameters. 相似文献
12.
Solar flares presumably have an impact on the deepest layers of the solar atmosphere and yet the observational evidence for such an impact is scarce. Using ten years of measurements of the Na D1 and Na D2 Fraunhofer lines, measured by GOLF onboard SOHO, we show that this photospheric line is indeed affected by flares. The effect of individual flares is hidden by solar oscillations, but a statistical analysis based on conditional averaging reveals a clear signature. Although GOLF can only probe one single wavelength at a time, we show that both wings of the Na line can nevertheless be compared. The varying line asymmetry can be interpreted as an upward plasma motion from the lower solar atmosphere during the peak of the flare, followed by a downward motion. 相似文献
13.
Claus Fröhlich José Romero Hansjörg Roth Christoph Wehrli Bo N. Andersen Thierry Appourchaux Vicente Domingo Udo Telljohann Gabrielle Berthomieu Philippe Delache Janine Provost Thierry Toutain Dominique A. Crommelynck André Chevalier Alain Fichot Werner Däppen Douglas Gough Todd Hoeksema Antonio Jiménez Maria F. Gómez José M. Herreros Teodoro Roca Cortés Andrew R. Jones Judit M. Pap Richard C. Willson 《Solar physics》1995,162(1-2):101-128
The scientific objective of the VIRGO experiment (Variability of solar IRradiance and Gravity Oscillations) is to determine the characteristics of pressure and internal gravity oscillations by observing irradiance and radiance variations, to measure the solar total and spectral irradiance and to quantify their variability over periods of days to the duration of the mission. With these data helioseismological methods can be used to probe the solar interior. Certain characteristics of convection and its interaction with magnetic fields, related to, for example, activity, will be studied from the results of the irradiance monitoring and from the comparison of amplitudes and phases of the oscillations as manifest in brightness from VIRGO, in velocity from GOLF, and in both velocity and continuum intensity from SOI/MDI. The VIRGO experiment contains two different active-cavity radiometers for monitoring the solar constant, two three-channel sunphotometers (SPM) for the measurement of the spectral irradiance at 402, 500 and 862 nm, and a low-resolution imager (LOI) with 12 pixels, for the measurement of the radiance distribution over the solar disk at 500 um. In this paper the scientific objectives of VIRGO are presented, the instruments and the data acquisition and control system are described in detail, and their measured performance is given.died 13 October 1994 相似文献
14.
Maria A. Loukitcheva Sami K. Solanki Stephen White 《Astrophysics and Space Science》2008,313(1-3):197-200
The very nature of the solar chromosphere, its structuring and dynamics, remains far from being properly understood, in spite
of intensive research. Here we point out the potential of chromospheric observations at millimeter wavelengths to resolve
this long-standing problem. Computations carried out with a sophisticated dynamic model of the solar chromosphere due to Carlsson
and Stein demonstrate that millimeter emission is extremely sensitive to dynamic processes in the chromosphere and the appropriate
wavelengths to look for dynamic signatures are in the range 0.8–5.0 mm. The model also suggests that high resolution observations
at mm wavelengths, as will be provided by ALMA, will have the unique property of reacting to both the hot and the cool gas,
and thus will have the potential of distinguishing between rival models of the solar atmosphere. Thus, initial results obtained
from the observations of the quiet Sun at 3.5 mm with the BIMA array (resolution of 12″) reveal significant oscillations with
amplitudes of 50–150 K and frequencies of 1.5–8 mHz with a tendency toward short-period oscillations in internetwork and longer
periods in network regions. However higher spatial resolution, such as that provided by ALMA, is required for a clean separation
between the features within the solar atmosphere and for an adequate comparison with the output of the comprehensive dynamic
simulations. 相似文献
15.
Linear time-domain simulations of acoustic oscillations are unstable in the stellar convection zone. To overcome this problem it is customary to compute the oscillations of a stabilized background stellar model. The stabilization affects the result, however. Here we propose to use a perturbative approach (running the simulation twice) to approximately recover the acoustic wave field while preserving seismic reciprocity. To test the method we considered a 1D standard solar model. We found that the mode frequencies of the (unstable) standard solar model are well approximated by the perturbative approach within 1 μHz for low-degree modes with frequencies near 3 mHz. We also show that the perturbative approach is appropriate for correcting rotational-frequency kernels. Finally, we comment that the method can be generalized to wave propagation in 3D magnetized stellar interiors because the magnetic fields have stabilizing effects on convection. 相似文献
16.
A recent report that energetic particles measured in the solar wind may be influenced by solar gravity-mode (
-mode) oscillations motivated the search for
-mode signatures in the Ulysses solar wind plasma data. Ulysses solar wind plasma data from 1 March 1992 through the 12 April 1996 were examined in this study for signs of possible solar oscillations. The multi-taper method for spectral analysis was used to look for significant spectral peaks in the entire four-year data set, as well as in the smaller, more heliographically homogenous data set over the solar poles. Several frequencies satisfying certain significance requirements were found in the
-mode frequency range in both data sets that also agree with the previously published findings. However, these identifications are shown to be false detections, and hence the frequencies found cannot be identified as solar
modes. 相似文献
17.
M. Lazrek F. Baudin L. Bertello P. Boumier J. Charra D. Fierry-Fraillon E. Fossat A. H. Gabriel R. A. García B. Gelly C. Gouiffes G. Grec P. L. Pallé F. Pérez Hernández C. Régulo C. Renaud J.-M. Robillot T. Roca Cortés S. Turck-Chièze R. K. Ulrich 《Solar physics》1997,175(2):227-246
The GOLF experiment on the SOHO mission aims to study the internal structure of the Sun by measuring the spectrum of global
oscillations in the frequency range 10-7 to 10-2 Hz. Here we present the results of the analysis of the first 8 months of data. Special emphasis is put into the frequency
determination of the p modes, as well as the splitting in the multiplets due to rotation. For both, we show that the improvement
in S/N level with respect to the ground-based networks and other experiments is essential in achieving a very low-degree frequency
table with small errors ∼ 2 parts in 10-5). On the other hand, the splitting found seems to favour a solar core which does not rotate slower than its surface. The
line widths do agree with theoretical expectations and other observations. 相似文献
18.
Guzik Joyce A. Neuforge-Verheecke Corinne Young A. Cody Epstein Richard I. Poulin Francis M. Schissel Jason R. 《Solar physics》2001,200(1-2):305-321
We summarize the physical input and assumptions commonly adopted in modern standard solar models that also produce good agreement with solar oscillation frequencies. We discuss two motivations for considering non-standard models: the solar neutrino problem and surface lithium abundance problem. We begin to explore the potential for mixed core models to solve the neutrino problem, and compare the structure, neutrino flux, and oscillation frequency predictions for several models in which the inner 25% of the radius is homogenized, taking into account the effects of non-local equilibrium abundances of 3He. The results for the neutrino flux and helioseismic predictions are far from satisfactory, but such models have the potential to reduce the predicted 7Be/8B neutrino flux ratio, and further studies are warranted. Finally, we discuss how much the neutrino problem can be alleviated in the framework of the standard solar model by using reaction rates, abundances and neutrino capture cross-sections at the limits of their uncertainties, while still satisfying the constraints of helioseismology. 相似文献
19.
Gravity modes are the best probes to infer the properties of the solar radiative zone, which represents 98% of the Sun’s total mass. It is usually assumed that high-frequency g modes give information about the structure of the solar interior whereas low-frequency g modes are more sensitive to the solar dynamics (the internal rotation). In this work, we develop a new methodology, based on the analysis of the almost constant separation of the dipole gravity modes, to introduce new constraints on the solar models. To validate this analysis procedure, several solar models – including different physical processes and either old or new chemical abundances (from, respectively, Grevesse and Noels (Origin and Evolution of the Elements 199, Cambridge University Press, Cambridge, 15, 1993) and Asplund, Grevesse, and Sauval (Cosmic Abundances as Records of Stellar Evolution and Nucleosynthesis CS-336, Astron. Soc. Pac., San Francisco, 25?–?38, 2005)) – have been compared to another model used as a reference. The analysis clearly shows that this methodology has enough sensitivity to distinguish among some of the models, in particular, among those with different compositions. The comparison of the models with the g-mode asymptotic signature detected in GOLF data favors the ones with old abundances. Therefore, the physics of the core – obtained through the analysis of the g-mode properties – is in agreement with the results obtained in the previous studies based on the acoustic modes, which are mostly sensitive to more external layers of the Sun. 相似文献
20.
It is presently widely accepted that the solar low p modes show asymmetric profiles when their power spectrum is analysed and that the fact of fitting symmetric profiles yields systematic effects in the obtained frequencies which could affect the results of inversions. In this paper the low p-mode profiles are analysed using wavelets to denoise the power spectra of the modes. This denoising method is applied both to artificial data generated by Kosovichev (the Hare and Hound exercise) and to real data obtained with the GOLF instrument. The asymmetries as well as the frequencies obtained are studied in both cases. The results show that although the obtained p-mode profiles present a slightly negative asymmetry, the use of symmetric profiles to fit the power spectra does not introduce any systematic effect in the obtained frequencies. 相似文献