Helioseismology I Posters
Presiding: M Woodard, NorthWest Research Associates, Inc.; M Rabello-Soares, Stanford University
SP11B-01 0830h
Time-Distance Helioseismology: Impact of Phase-Speed Filters on Travel-Time Measurements
In time-distance helioseismology most procedures to compute travel-time maps use Gaussian phase-speed filters to select different parts of the wave propagation diagram. The mean phase speed of the filter is usually derived using ray theory and a solar model, but there is no obvious rule for choosing the width of the filter. We study how the filter widths impact travel-time measurements and show what widths give the best signal-to-noise ratio for measurements of sound-speed perturbations.
SP11B-02 0830h
Effect of phase speed filters on time-distance correlations of acoustic waves on the Sun.
Use of phase-speed filters in time-distance helioseismic measurements is crucial to obtain spatially resolved information about localised sub-surface structures. These filters have to be chosen such that the travel times of the waves that are filtered in are themselves not affected by the filtering process. Here we derive analytically the cross-correlation signal that results from phase-speed filtered signals, assuming plane wave conditions. The resulting wavelet explicitly depends on the parameters of the filters, such as the phase-speed and its dispersion, in contrast to the currently used Gabor wavelet, and hence accounts for any filter induced changes in travel times. Alternatively, this new wavelet allows the determination of optimum parameters for the filters.
SP11B-03 0830h
A Potential New Method for Measuring the Solar Diameter Changes with Time Distance Helioseismology
We present a new possibility of measuring the solar diameter change using multiple bounce time-distance analysis. We show that the 2D cross-correlation function can be used to investigate travel time and path length changes to separate the effects of sound speed and travel distance perturbations. We can interpret the travel distance perturbation as the results of solar radius change. This method can be used to search for solar radius changes associated with solar activity or for comparison between o bserved solar radius and that used in solar models. We also present an estimate of the accuracy of the method and some preliminary results obtained with GONG low-resol ution images for the period 1996-1997 which corresponds to the minimum of solar activity. This work utilizes data obtained by the Global Oscillation Network Group (GONG) Program, managed by the National Solar Observatory, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. The data were acquired by instruments operated by the Big Bear Solar Observatory, High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de Astrofisico de Canarias, and Cerro Tololo Interamerican Observatory. Serebryanskiy A.V. supported by grant of NSF-NATO Postdoctoral Fellowship in Science and Engineering for Scientists from NATO Partner Countries.
SP11B-04 0830h
GONG Travel Time Measurements of Meridional Flows for a Decade
We present measurements of sub-surface meridional flows derived from the difference in travel time between northward and southward propagating sound waves, and their variations during the period from May 1995 to February 2004. We show that for measurements at depths of 20 Mm or deeper, the original low spatial resolution GONG data can be used to extend our study of temporal variations of meridional flows back to the previous solar minimum. In addition to the usual approach using velocity images, we demonstrate that spherical harmonic (SH) coefficient time series for m = 0 provide an effective means for filtering for waves that are propagating directly northward or southward. Another advantage of using SH coefficient time series more generally is that phase velocity filtering can be done for any l range, without an implicit of plane-wave assumption.
SP11B-05 0830h
Sensitivity of time-distance helioseismic measurements to modulation of oscillation amplitudes
Spatial modulation of oscillation amplitudes in solar active regions arise from several causes, most notably due to the absorption of acoustic waves by sunspots and due to incorrect measurements induced by the changes in the spectral line profiles used in the observations. We show that these modulations may introduce significant changes in the travel times of acoustic waves in addition to those arising directly from the real physical causes. These changes are caused by the convolution of the wave-number spectrum of the modulating function with the power spectra of the oscillations. We develop an empirical 'deconvolution' strategy based on the modulation of oscillation power in each pixel of Dopplergrams and test its effectiveness. These corrections are important in inferring correctly the sub-surface structure and dynamics of localized strong perturbations such as sunspots.
SP11B-06 0830h
Local Helioseismic Mode Frequency Shifts With Magnetic Activity, From GONG and MDI
We use the ring-diagram technique of local helioseismology to study the frequency shifts of high-degree solar acoustic modes from over 600 days of data from the Global Oscillations Network Group (GONG), covering the period 2001-2004. The data are compared with contemporaneous data from the Michelson Doppler Imager (MDI) dynamics program, where available. We examine both synoptic charts and the day-to-day variations in selected active regions. The results, once instrumental effects have been removed, show strong dependence of the mode frequency on the local magnetic flux, with the frequencies generally increasing with magnetic index. We relate these findings to results from global modes. This work was supported by the National Science Foundation and NASA
SP11B-07 0830h
Zonally Averaged Meridional Flows Determined by Ring Analysis of Untracked Data
Measurements of near-surface meridional flow profiles can be obtained from ring-diagram analysis. Ring-diagram analysis ordinarily relies on tracking of the targeted regions of study in order to distinguish the physical targets spatially and to remove the large contribution of solar rotational velocity to the frequency splittings. This, together with the effects of geometrical foreshortening, limits the time period for any one analysis to a few days. It also has the potential for introducing systematic biases as the aspect of each tracked region changes during its disc passage. In determining the zonally averaged meridional flow field, however, we can overcome these problems by analyzing untracked latitude zones for periods up to or even exceeding a full solar rotation. We present results obtained from several full rotations of MDI Doppler data during selected Dynamics Campaigns of nearly continuous coverage. We also compare these results with those obtained by averaging the results of traditional tracked analyses.
http://rick.stanford.edu/pubs/spd05/notrack/
SP11B-08 0830h
New Steps Towards the Unbiased Characterization of High-Degree Mode Frequencies
Bias in the characterization of high-degree mode frequency results from the blending of individual modes into ridges as mode lifetimes get shorter and spatial leaks get closer in frequency at high degrees. To recover the actual underlying mode frequency from fitting the ridge, an accurate model of the amplitude of the peaks that contribute to the ridge power distribution is crucial. Such a model requires that the instrumental characteristics be very well understood and very precisely measured. We here present new results from our continuing effort to estimate unbiased high-degree mode frequencies using full-disk data from the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). The methodology is based on the extensive analysis presented in Korzennik et al (2004) - and will in turn be beneficial to MDI, GONG and eventually HMI. The key improvement on our previous work is the re-decomposition of the images onto spherical harmonic component. This new spatial decomposition incorporates specific MDI instrumental characteristics, like a more accurate plate scale, our best model of the image distortion, the image orientation, etc... These instrumental effects were introduced one at a time and their effect carefully compared to the predictions of our model. As expected, by including these corrections in the spatial decomposition, the residual corrections that must be applied to the ridge frequency are reduced, leading to a less biased frequency estimation.
SP11B-09 0830h
Searching for GONG p-Mode Frequency Variations Generated by Perturbations Near the Base of the Convection Zone
We have analysed p-mode frequency variations from GONG data in the period of 1996-2003 to search for solar cycle variations generated by perturbations near the base of the solar convection zone. The scaled relative frequency change variation with horizontal phase velocity from GONG data is consistent with a previous study based on MDI data. The scaled relative frequncy change shows solar cycle variations for the horizontal phase velocity greater than a critical value, which corresponds t o a depth near the base of the convection zone. The magnitude of the change correlates with the surface magnetic activity. In comparison with models, the observed frequency change could be generated by a perturbation of -Δ Γ1/Γ1(=-2Δ c/c) ≈ 2-6 × 10-5 at r ≈ 0.65 - 0.67 R⊙, if the FWHM of the Gaussian perturbation is 0.05 R⊙. This work utilizes data obtained by the Global Oscillation Network Group (GONG) Program, managed by the National Solar Observatory, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. The data were acquired by instruments operated by the Big Bear Solar Observatory, High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de Astrofisico de Canarias, and Cerro Tololo Interamerican Observatory. Serebryanskiy A.V. supported by grant of NSF-NATO Postdoctoral Fellowship in Science and Engineering for Scientists from NATO Partner Countries.
SP11B-10 0830h
The structure of the near-surface layers of the Sun: asphericity and time variation
We present results on the structure of the near-surface layers of the Sun. These results were obtained by inverting frequencies of high-degree solar modes obtained by using "ring diagrams". We have results for seven epochs from June 1996 to March 2002. The frequencies of each epoch were obtained from ring diagrams constructed from MDI Dopplergrams spanning one complete Carrington rotation. We find that there is a substantial latitudinal variation of both sound speed and the adiabatic index Γ1 in the outer 2% of the Sun. In addition, it appears that both the sound-speed and Γ1 profiles change with change in the level of solar activity.
SP11B-11 0830h
Simulations Of Acoustic-Flow Interaction In Spherical Geometry: Steps Toward Validating Helioseismology
We simulate acoustic wave interaction with flows in spherical geometry with the specific intent of using them as artificial data for validation of helioseismology. The numerical procedure is pseudo-spectral; we employ a spherical harmonic representation of the spherical surface, compact finite differences in the radial direction and a fourth order Runge-Kutta time stepping scheme. We also excite surface gravity modes, modeling all waves as linear perturbations to the background state so as to gain further insight into wave-flow interaction. Towards validation, we apply techniques of helioseismology to the artificial data to determine the efficacy of the helioseismic inversion procedure. In other words, we are attempting the forward problem.
SP11B-12 0830h
Cross-Spectral Signatures in Global Helioseismology Data of Large-Scale Flow in the Sun
Large-scale flows in the Sun's interior have been studied using a variety of helioseismic techniques, including spectral analysis of spherical harmonic time series of photospheric velocity oscillations. Detailed maps of differential rotation have been obtained from measurements of the frequencies of resonance peaks in the power spectra. Flows can also affect power spectra in subtler ways, e.g., by their influence on the widths of resonance peaks. In addition to their spectral signature, flows and other aspherical perturbations also produce cross-spectral signatures, via the mode-coupling effect of a flow. Cross power spectra of time series of coefficients in the spherical-harmonic decomposition of SOHO/MDI medium-ℓ velocity images have been computed and are being compared with theoretical predictions. The results of a preliminary comparison of observed and theoretically predicted cross spectra for differential rotation and meridional circulation will be presented. A program to systematically map large-scale solar internal flow using cross-spectral data will be described. The author acknowledges useful discussions with colleagues, especially Doug Braun, Yuhong Fan, Aaron Birch, and Jesper Schou. He is also grateful to Jesper Schou for help in acquiring MDI data products and to NASA for support under contract NAS5-3114. The Solar Oscillations Investigation- Michelson Doppler Imager experiment on SOHO is supported by NASA contract NAG5-3077 at Stanford University. SOHO is a project of international cooperation between ESA and NASA.
SP11B-13 0830h
Characteristic Patterns of the Mode Frequencies With Solar Activity Indices
The variability of solar p-mode oscillation frequencies during the course of a solar cycle is the result of perturbations due to changing average magnetic flux near the solar surface and is an important clue toward understanding the mechanisms of solar activity. We analyze temporal relationships among p-mode frequencies measured by GONG and MDI having ℓ ≤ 200 and 1.5 mHz ≤ ν ≤ 3.5 mHz to establish the presence of characteristic loop patterns with the 10.7-cm flux (F10), the Kitt Peak Magnetic Index (KPMI), International Sunspot Number (RZ), and the radiative indices: He I equivalent width and Mg II core-to-wing ratio. The data cover the ascending and descending phases of the current solar activity cycle 23. Our preliminary results indicate that p-mode frequencies vary closely with F10z and KPMI but lead the radiative indices and lag RZ by approximately one month. This implies that the more generalized magnetic flux responsible for F10 and KPMI, which is known to follow the emergence of the powerful localized flux of sunspots, is more closely linked to the variation of intermediate-degree p-mode frequencies.
SP11B-14 0830h
The GONG Farside Project
The GONG program is currently providing near-real-time helioseismic images of the farside of the Sun. The continuous stream of low resolution images, obtained from the 6 earth based GONG stations, are merged into a single data series that are the input to the farside pipeline. In order to validate the farside images, it is crucial to compare the results obtained from different instruments. We show comparisons between the farside images provided by the MDI instrument and the GONG ones. New aditions to the pipeline will allow us to create full-hemisphere farside images, examples of the latest are shown in this poster. Our efforts are now concentrated in calibrating the farside signal so it became a reliable solar activity forecasting tool. We are also testing single-skip acoustic power holography at 5-7 mHz as a prospective means of reinforcing the signatures of active regions crossing the the east and west limb and monitoring acoustic emission in the neighborhoods of Sun's the poles. This work utilizes data obtained by the Global Oscillation Network Group (GONG) Program, managed by the National Solar Observatory, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. The data were acquired by instruments operated by the Big Bear Solar Observatory, High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de Astrofisico de Canarias, and Cerro Tololo Interamerican Observatory, as well as the Michaelson Doppler Imager on SoHO, a mission of international cooperation between ESA and NASA. This work has been supported by the NASA Living with a Star - Targeted Research and Technology program.
http://gong.nso.edu/data/farside