Observations Of Solar Oscillations Research Articles

Progress in Global Helioseismology: A New Light on the Solar Modeling Problem and Its Implications for Solar-Like Stars

Since the first observations of solar oscillations in 1962, helioseismology has probably been one of the most successful fields of astrophysics. Data of unprecedented quality were obtained through the implementation of networks of ground-based observatories such as the GONG project or the BiSON network, coupled with space-based telescopes such as SOHO, Solar Orbiter and SDO missions. Besides the improvement of observational data, solar seismologists developed sophisticated techniques to infer the internal structure of the Sun from its eigenfrequencies. These methods, then already extensively used in the field of Geophysics, are called inversion techniques. They allowed to precisely determine the position of the solar convective envelope, the helium abundance in this region and the internal radial profiles of given thermodynamic quantities. Back in 1990s these comparisons showed a very high agreement between solar models and the Sun. However, the downward revision of the CNO surface abundances in the Sun in 2005, confirmed in 2009, induced a drastic reduction of this agreement leading to the so-called solar modelling problem. More than ten years later, in the era of the space-based photometry missions which have established asteroseismology of solar-like stars as a standard approach to obtain their masses, radii and ages, the solar modelling problem still awaits a solution. In this paper, we will present the results of new helioseismic inversions, discuss the current uncertainties of solar models as well as some possible solutions to the solar modelling problem. We will show how helioseismology can help us grasp what is amiss in our solar models. We will also show that, far from being an argument about details of solar models, the solar problem has significant implications for seismology of solar-like stars, on the main sequence and beyond, impacting asteroseismology as a whole as well as the fields requiring precise and accurate knowledge of stellar masses, radii and ages, such as Galactic archaeology and exoplanetology.

Signal and noise in helioseismic holography

Context. Helioseismic holography is an imaging technique used to study heterogeneities and flows in the solar interior from observations of solar oscillations at the surface. Holographic images contain noise due to the stochastic nature of solar oscillations. Aims. We aim to provide a theoretical framework for modeling signal and noise in Porter–Bojarski helioseismic holography. Methods. The wave equation may be recast into a Helmholtz-like equation, so as to connect with the acoustics literature and define the holography Green’s function in a meaningful way. Sources of wave excitation are assumed to be stationary, horizontally homogeneous, and spatially uncorrelated. Using the first Born approximation we calculated holographic images in the presence of perturbations in sound-speed, density, flows, and source covariance, as well as the noise level as a function of position. This work is a direct extension of the methods used in time-distance helioseismology to model signal and noise. Results. To illustrate the theory, we compute the holographic image intensity numerically for a buried sound-speed perturbation at different depths in the solar interior. The reference Green’s function is obtained for a spherically-symmetric solar model using a finite-element solver in the frequency domain. Below the pupil area on the surface, we find that the spatial resolution of the holographic image intensity is very close to half the local wavelength. For a sound-speed perturbation of size comparable to the local spatial resolution, the signal-to-noise ratio is approximately constant with depth. Averaging the image intensity over a number N of frequencies above 3 mHz increases the signal-to-noise ratio by a factor nearly equal to the square root of N. This may not be the case at lower frequencies, where large variations in the holographic signal are due to the contributions from the long-lived modes of oscillation.

Observations of ULF waves in the solar corona and in the solar wind at the Earth's orbit

Signs were looked for that would indicate a possible connection between plasma velocity oscillations observed in the region of solar coronal holes and magnetic field oscillations as recorded in the interplanetary medium. The problem appears to be quite important since the presence of large-scale ULF waves in the solar wind can increase geoeffectiveness of high speed streams in the interplanetary plasma. Observations of solar oscillations in the FeI 6569Å spectral line in a coronal hole were taken as a basis. The measurements were carried out at the Horizontal Automated Solar Telescope of the Sayan Solar Observatory. High speed solar wind stream ejected from the coronal hole reached the Earth's orbit after approximately 60h. The spectra of solar oscillations were compared with those of ultra low frequency (ULF) oscillations of the interplanetary magnetic field (IMF) at libration point L1. The oscillations were recorded with the ACE magnetometer when the leading edge of the high speed stream, bringing increased ULF wave activity, reached the Earth. The spectra of solar oscillations had a sharp peak at about 3.4–3.6mHz. The spectrum of the solar wind ULF oscillations is much more complex, being formed by different sources. Nevertheless, ULF oscillations of the IMF often had peaks that were close in frequency to those of the solar oscillations. Analysis of the ULF wave spectra observed in the 92 high speed streams confirmed the presence of 3- and 5-min oscillations in the total wave spectrum. It is emphasized that the results cannot be regarded as proving a direct connection between solar oscillations and ULF waves at the Earth's orbit even though they do support such a possibility. Additional research is needed involving IMF wave trajectory calculations.

Time–Distance Helioseismology Data-Analysis Pipeline for Helioseismic and Magnetic Imager Onboard Solar Dynamics Observatory (SDO/HMI) and Its Initial Results

The Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO/HMI) provides continuous full-disk observations of solar oscillations. We develop a data-analysis pipeline based on the time-distance helioseismology method to measure acoustic travel times using HMI Doppler-shift observations, and infer solar interior properties by inverting these measurements. The pipeline is used for routine production of near-real-time full-disk maps of subsurface wave-speed perturbations and horizontal flow velocities for depths ranging from 0 to 20 Mm, every eight hours. In addition, Carrington synoptic maps for the subsurface properties are made from these full-disk maps. The pipeline can also be used for selected target areas and time periods. We explain details of the pipeline organization and procedures, including processing of the HMI Doppler observations, measurements of the travel times, inversions, and constructions of the full-disk and synoptic maps. Some initial results from the pipeline, including full-disk flow maps, sunspot subsurface flow fields, and the interior rotation and meridional flow speeds, are presented.

HMI time-distance pipeline: An overview and data products

The Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory provides uninterrupted high-resolution observations of solar oscillations over the entire disk. This gives a unique opportunity for mapping subsurface flows and wave-speed structures and investigating their role in the Sun's dynamics and magnetic activity on various scales by methods of local helioseismology. A data analysis pipeline for the time-distance helioseismology analysis has been developed and implemented at the SDO Joint Science Operation Center (JSOC) at Stanford. It provides near-real time processing of the helioseismology data. We provide an overview of this pipeline, including the data flow procedures, measurement and inversion codes, and our data products.

Local Helioseismology: Three-Dimensional Imaging of the Solar Interior

The Sun supports a rich spectrum of internal waves that are continuously excited by turbulent convection. The Global Oscillation Network Group (GONG) network and the SOHO/MDI (Solar and Heliospheric Observatory/Michelson Doppler Imager) space instrument provide an exceptional database of spatially resolved observations of solar oscillations, covering more than an entire sunspot cycle (11 years). Local helioseismology is a set of tools for probing the solar interior in three dimensions using measurements of wave travel times and local mode frequencies. Local helioseismology has discovered (a) near-surface vector flows associated with convection, (b) 250 m s−1subsurface horizontal outflows around sunspots, (c) ∼50 m s−1extended horizontal flows around active regions (converging near the surface and diverging below), (d) the effect of the Coriolis force on convective flows and active region flows, (e) the subsurface signature of the 15 m s−1poleward meridional flow, (f) a ±5 m s−1time-varying depth-dependent component of the meridional circulation around the mean latitude of activity, and (g) magnetic activity on the farside of the Sun.

Properties of high-degree oscillation modes of the Sun observed with Hinode/SOT

Aims. With the Solar Optical Telescope on Hinode, we investigate the basic properties of high-degree solar oscillations observed at two levels in the solar atmosphere, in the G-band (formed in the photosphere) and in the Ca II H line (chromospheric emission). Methods. We analyzed the data by calculating the individual power spectra as well as the cross-spectral properties, i.e., coherence and phase shift. The observational properties are compared with a simple theoretical model, which includes the effects of correlated noise. Results. The results reveal significant frequency shifts between the Ca II H and G-band spectra, in particular above the acoustic cut-off frequency for pseudo-modes. The cross-spectrum phase shows peaks associated with the acoustic oscillation (p-mode) lines, and begins to increase with frequency around the acoustic cut-off. However, we find no phase shift for the (surface gravity wave) f-mode. The observed properties for the p-modes are qualitatively reproduced in a simple model with a correlated background if the correlated noise level in the Ca II H data is higher than in the G-band data. These results suggest that multi-wavelength observations of solar oscillations, in combination with the traditional intensity-velocity observations, may help to determine the level of the correlated background noise and to determine the type of wave excitation sources on the Sun.

The structure of the solar convective overshooting zone

Using a non-local theory of convection, we calculated the structure of the solar convection zone, paying special attention to the detailed structure of the lower overshooting zone. Our results show that an extended transition zone exists near the bottom of the convection zone, where the temperature gradient turns smoothly from adiabatic in the convection zone to radiative in solar interior. A super-radiative temperature region is found in the overshooting zone under the solar convection zone, where and . The extension of the super-radiative region (defined by l is about 0.63-HP (0.053-R⊙). A careful comparison of the distribution of adiabatic sound speed and density with the local one is carried out. It is found, strikingly, that the distribution of adiabatic sound speed and density of our model is roughly consistent with the results of reversion from solar oscillation observations.

A strategy for fitting partially blended ridges in GONG solar p-mode spectra

Observations of solar oscillations by the Global Oscillation Network Group (GONG) using a network of telescopes deployed around the Earth eliminate tempo- ral aliases from the oscillation power spectra to a very substantial extent, making the measurement of oscillation mode parameters much cleaner than with single-site ob- servations. However, the problem of spatial aliasing, which arises from the fact that we see only one side of the Sun, still causes some problems. We discuss some of the issues aecting the estimation of medium-l solar acoustic mode parameters from the data obtained by the GONG net- work, and in particular the contamination by spatial leak- age. We determine the areas in thel plane in which such leakage is a problem, and nally we describe a method for improving the mode frequency estimation in such regions, thus extending the range of modes for which useful fre- quency estimates can be obtained.

Data for Probing the Sun

The observations of solar oscillations provide an unrivalled, precise way of probing the solar interior. In this paper, I consider the observations and their interpretation in terms of the physics of the Sun. The oscillations that we are concerned with here are the so-called p modes, i.e. oscillations for which pressure is the restoring force. The modes for which gravity is the restoring force have yet to be unambiguously detected on the Sun. The observations are made either as Doppler velocity or as intensity and are, in general, very small effects. To get an impression of the precision required, consider that in integrated velocity the total signal is ~ 1 m s−1 with the strongest individual modes being about 15-20 cm s−1. The weakest, detected modes are of order a few mm s−1. When this signal is measured as a Doppler shift, v/c is a few parts in 1011. The observations are made by a variety of instruments on Earth or in Space which can be simply divided into those which observe the Sun as a star and those which image the solar surface into many pixels Although there are many different observers using many different techniques, in all cases one is analysing light emitted from a region relatively high in the atmosphere of the Sun. When one considers how these measurements can be interpreted in terms of the solar oscillations, two issues arise: 1.Roughly where in the solar atmosphere are the lines formed?2.How different are the heights of formation for different lines?

Seismological Comparisons of Solar Models with Element Diffusion Using the MHD, Opal, and Sireff Equations of State

We compare the thermodynamic and helioseismic properties of solar models evolved using three different equation of state (EOS) treatments: the Mihalas, D{umlt a}ppen & Hummer EOS tables (MHD); the latest Rogers, Swenson, & Iglesias EOS tables (OPAL), and a new analytical EOS (SIREFF) developed by Swenson {ital et al.} All of the models include diffusive settling of helium and heavier elements. The models use updated OPAL opacity tables based on the 1993 Grevesse & Noels solar element mixture, incorporating 21 elements instead of the 14 elements used for earlier tables. The properties of solar models that are evolved with the SIREFF EOS agree closely with those of models evolved using the OPAL or MHD tables. However, unlike the MHD or OPAL EOS tables, the SIREFF in-line EOS can readily account for variations in overall Z abundance and the element mixture resulting from nuclear processing and diffusive element settling. Accounting for Z abundance variations in the EOS has a small, but non-negligible, effect on model properties (e.g., pressure or squared sound speed), as much as 0.2{percent} at the solar center and in the convection zone. The OPAL and SIREFF equations of state include electron exchange, which produces models requiring a slightlymore » higher initial helium abundance, and increases the convection zone depth compared to models using the MHD EOS. However, the updated OPAL opacities are as much as 5{percent} lower near the convection zone base, resulting in a small decrease in convection zone depth. The calculated low-degree nonadiabatic frequencies for all of the models agree with the observed frequencies to within a few microhertz (0.1{percent}). The SIREFF analytical calibrations are intended to work over a wide range of interior conditions found in stellar models of mass greater than 0.25M{sub {circle_dot}} and evolutionary states from pre-main-sequence through the asymptotic giant branch (AGB). It is significant that the SIREFF EOS produces solar models that both measure up to the stringent requirements imposed by solar oscillation observations and inferences, and are more versatile than EOS tables. {copyright} {ital 1997} {ital The American Astronomical Society}« less

The state of the art in helioseismic ground-based experiments

The new results obtained from the observation of solar oscillations over the past decade, have a direct impact on our knowledge of the Sun's interior. As a consequence, a great interest in helioseismology has arisen and is reflected in the development of new observational projects as well as new analyse and inversion techniques. In this review we will describe the present ground-based observational programmes, which, unlike the space ones, are mostly designed to produce high quality data over very long time spans (up to solar cycle time scales). The characteristics of the various observational programmes, single-site and network, will be described together with their performances, the main results obtained up to now, and some other logistical aspects.

A pulsar inside the Sun?

The Crimean observation of solar oscillations in 1974–1982 showed that the basic period of pulsation of the Sun hidden in its deep interior was equal to P0=160.0101±0.0001min. More recently, the period was changed to the new value P1=159.9662±0.0006min, which almost coincided with the annual sidelobe of the former period P0. The amplitude of the P1 oscillation has increased considerably over 1994–1995. We substantiate the hypothesis that a) the change in the period was caused by the interaction of the P0 oscillation with the rapid rotation of the solar core and that b) the latter has the form of a compact, highly magnetized object like a neutron star rotating with sidereal period P1.

Alaska as a site for high duty-cycle solar observations

The day/night cycle at a single observatory prevents definitive observations of many aspects of solar activity, convection, and oscillations with timescales near 1 day. Solutions to this problem include multi-site networks, spacecraft observatories, and observations from high-latitude sites during their summer season. We report here on our experience in using Alaska as a high-latitude site for observations of solar oscillations.

Golf: A resonance spectrometer for the observation of solar oscillations

GOLF (Global Oscillations at Low Frequencies) is an instrument to study the line-of-sight velocity of the solar photosphere, to be flown on the SOHO satellite in 1995. It uses a sodium vapour cell in resonance scattering mode, in order to measure the absolute Doppler shift of the solar sodium absorption lines. We detail laboratory tests to determine the performances of the cell built for the experiment. The results are in good agreement with numerical simulations of the resonance processes. As a final result, we can conclude that the level of performances required for the flight instrument will be obtained.

Observations of Solar Oscillations in He I 10830 Å

In continuation of a series of studies devoted to the dynamics of the solar photosphere and chromosphere we have attempted to further extend the range of heights in the atmosphere towards the transition region by including observations of the He I 10830 line. We have recorded simultaneous time series of He I 10830 and Mg I 8807 spectra in the quiet solar atmosphere using the echelle spectrograph at the German Vacuum Tower Telescope in Izaña, Tenerife. The velocity signal derived from the Doppler shifts of He 10830 clearly reveals oscillatory motions. The intensity of He 10830, on the other hand, is hardly affected by the oscillations. In the cell interior the 3-min oscillations prevail. Longer periods are found ain the cell boundaries of the chromospheric network where He absorption is enhanced. The V–V phase difference spectrum between the oscillations of He 10830 and those of Mg 8807 confirms previous observations of a non-propagating component that dominates the acoustic wave spectrum in the chromosphere.

Golf: A resonance spectrometer for the observation of solar oscillations I) Numerical model of the sodium cell response

GOLF (Global Oscillations at Low Frequencies) is an instrument to study the line-of-sight velocity of the solar photosphere, to be flown on the SOHO satellite in 1995. It uses a sodium vapour cell in resonance scattering mode, in order to measure the absolute Doppler shift of the solar sodium absorption lines. We have developed a model of the resonance cell performance. We describe here the main characteristics of the model, and report the most important results concerning the performance of the cell and its dependance on temperature.

Preliminary Results from the SOVA Experiment on Board the European Retrievable Carrier (EURECA)

The experiment SOVA (SOlar VAriability) on board EURECA (EUropean REtrievable CArrier) contains two types of radiometers sunphotometers for measuring the total and spectral irradiance at five wavelengths between 335 nm and 862 nm. Besides the direct observation of variations of the solar irradiance, these measurements will also be used for the observation of solar oscillations. Preliminary results are presented from the observations of total and spectral irradiance during the first five months of the mission (August to December 1992).

Some aspects of the theory of solar oscillations

Abstract The study of nonradial oscillations of stars, based on the seminal work by Cowling half a century ago, is increasingly providing information about the interiors of stars. Observations of solar oscillations have led to detailed inferences about the structure and rotation of the solar interior. Nonradial oscillations have also been studied in white dwarfs and in rapidly oscillating Ap stars, and indications of solar-like oscillations have been detected in a few stars. Here I give an overview of some properties of nonradial oscillations, and discuss the diagnostic potential of low-degree modes for probing the core of the Sun and other stars. The observed solar frequencies appear to exclude several of the proposed solutions to the solar neutrino problem that involve modifications to solar models.

The p-mode oscillation spectra of an evolving 1-solar-mass sun-like star

view Abstract Citations (30) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The p-Mode Oscillation Spectra of an Evolving 1 Msun Sun-like Star Guenther, D. B. Abstract Calculations of the p-mode frequencies and first-order and second-order spacings of a 1-solar-mass star with solar composition are presented for six models ranging from the zero-age main sequence to the base of the giant branch. When compared to observations of solar oscillations, the calculated frequency spacings reveal the sun's deep interior to have the structure of a younger than 4.89 Gyr old stellar model and the sun's middle regions to have the structure of an older than 4.89 Gyr old stellar model. This structure is consistent with nonstandard modeling schemes proposed to reduce the predicted solar neutrino flux. Publication: The Astrophysical Journal Pub Date: July 1991 DOI: 10.1086/170195 Bibcode: 1991ApJ...375..352G Keywords: P Waves; Stellar Evolution; Stellar Interiors; Stellar Models; Stellar Oscillations; Stellar Spectra; Giant Stars; Main Sequence Stars; Solar Interior; Solar Neutrinos; Stellar Composition; Sun; Astrophysics; STARS: EVOLUTION; STARS: PULSATION; SUN: INTERIOR; SUN: OSCILLATIONS full text sources ADS |