Helioseismology II Posters
Presiding: M Woodard, NorthWest Research Associates, Inc.; M Rabello-Soares, Stanford University
SP13A-01 1330h
Five-Minute Power Maps From GONG and MDI.
The presence of magnetic active regions on the solar surface is well known to influence the detected power of the oscillation signal. We consider maps of the five-minute power in the velocity signal from Global Oscillations Network Group (GONG) observations covering much of the disk over multiple Carrington Rotations, and compare these in detail with magnetic and continuum intensity images, with estimates of the velocity power from ring diagram helioseismic analysis, and also with a small sample of contemporaneous MDI (Michelson Doppler Imager) data. The comparison of power maps with magnetograms is carried out at a pixel-by-pixel level, for averages over patches of 16× 16 degrees in heliographic latitude and longitude, and at some intermediate scales. This research was supported by the National Science Foundation and NASA.
SP13A-02 1330h
The Effects of Inclined Magnetic Field over Time-Distance Measurements
Recent observation by phase-sensitive acoustic holography has shown that the inclined magnetic field has significant effects over the local helioseismology measurements, and it was thus suggested that the acoustic wave phase shift is substantially caused by the photospheric magnetic field. We make the similar measurements by use of time-distance helioseismology technique over a sunspot when it rotated from the solar east limb to the west. We find that the wave travel time is often longer in the part of sunspot penumbra that is closer to the solar disk center, similar to what was found by acoustic holography analysis. We investigate the other possible reasons that may cause this effect, e.g., projection effect, Doppler velocity masking effect, and find these effects do not account for all the travel time variations. We perform time-distance inversions to investigate how these measurements affect interior structures inferred from inversions.
SP13A-03 1330h
The Effects of Magnetically-Induced Spectral Line Profile Changes on Helioseismic and Flare Observations
We have modeled the effect of changes in the shape of the spectral line used for the GONG and MDI observations, and we investigate the consequences for measurements of properties of oscillations and flares. We find that magnetic field measurements are not very sensitive to line shape changes, but velocity estimates do strongly depend on line variations. Using simulated observations of a flare we find that recently observed associated magnetic field changes are not due to line shape changes. On the other hand, a simulation of an oscillation indicates that at least part of the observed amplitude suppression in an active region is due to variations in the line shape. We also report preliminary results of the effect of vertical phase variations across the line profile on the helioseismic observations. This work is carried out through the National Solar Observatory Research Experiences for Undergraduate (REU) site program, which is co-funded by the Department of Defense in partnership with the National Science Foundation REU Program. This work utilizes data obtained by the Global Oscillation Network Group (GONG) project, 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 Astrofísica de Canarias, and Cerro Tololo Interamerican Observatory.
SP13A-04 1330h
How Well do the Oscillation Frequencies Track the Activity ?
Variations of solar oscillation frequencies with the solar cycle at both low and intermediate degrees are now well established. This is manifested by an increase in frequencies of the modes from minimum to maximum of the activity cycle. It is also known that the shifts are strongly correlatd with different activity indices on time scales of 72 and 108 days for MDI and GONG data respectively. However, earlier results from cycle 22 indicated that the frequencies change over shorter periods of weeks to months. It is therefore interesting to analyse mode frequencies and other mode parameters from time series of shorter duration. Here, we present results from an analysis where the frequencies are computed from 36 and 108 days long time series; the time series being shifted by three days over a period of about 5 months. The frequencies calculated from the 36 day long time series are much better correlated with the activity indices.
SP13A-05 1330h
Theoretical Predictions of Sunquake Waves
The propagation of ring waves across the surface of the sun in response to a flare initiated sunquake is modeled using Euler's equations of fluid dynamics. The solar convection zone is modeled as a plane parallel gas layer in hydrostatic equilibrium with an adiabatic temperature gradient. Small amplitude perturbations about this equilibrium state are described by the linearized Euler equations for an inviscid compressible fluid (the actual convective motions on the sun are neglected for the purpose of calculating the wave motions). The normal modes of oscillation of this solar model, which can be expressed in terms of generalized Laguerre polynomials, are used to construct the solution of an initial value problem for the linearized equations of motion. Assuming that the form of the initial velocity pulse is Gaussian, the solutions for the vertical velocity at the solar surface are computed as a function of time and compared to the observational data for the sunquake event of 9 July 1996. Model calculations of the position of the wave packet as a function of time predict arrival times that are a few minutes ahead of the observations (1 to 5 minutes) for the range of distances between 10 and 120 Mm from the point of impact or, equivalently, for the range of times between 15 and 50 minutes after the assumed time of impact of the flare ejecta (or shock wave) on the solar surface. It is concluded that the model is in good agreement with the observational data with an error of roughly 10% or 20%.
SP13A-06 1330h
Modeling Seismic Emission in the Quiet Sun
A major issue in the physics of seismic emission in the quiet Sun is the degree to which the emission from any particular location is episodic. Given our present understanding, this question is equivalent to that of how localized the sources of emission are at any particular moment. A variety of statistical tools are available to address this issue. For example, if seismic emission can be characterized in terms of relatively infrequent episodes sparsely distributed, then the distribution in amplitude of the source terms over space and time should be non-Gaussian. If the episodes of emission are densely disseminenated in space and time such that many phase-independent episodes would be expected in a space-time resolution element,then the distribution in amplitude approaches Gaussian statistics, and the distribution in power becomes exponential. Computational seismic holography focused at the solar surface from a subjacent vantage makes it possible to image acoustic sources and do statistics on the seismic source term. Earlier work by Donea, Lindsey and Braun, based on holographic imaging of acoustic sources, failed to detect a departure of source amplitudes from Gaussian statistics. This suggests that seismic sources are relatively dense on a spatial scale of 3~Mm and a temporal scale of 10~min. What this means in terms of the physics of acoustic excitation requires modeling. We will describe beginning efforts to model seismic emission in a standard model of the solar subphotosphere in terms of randomly distributed dipoles located close to the solar surface. A significant departute of the source amplitude distribution from Gaussian statistics is of fundamental importance to the utility of local helioseismic diagnostics to seismic emission in the quiet Sun.
SP13A-07 1330h
Systematic errors in Dopplergrams of active regions
Dopplergrams of magnetic regions are used in time-distance helioseismology and for the investigation of oscillations in sunspots. MDI Dopplergrams are produced onboard SOHO by measuring four filtergrams which are sensitive to intensity fluctuations in the wings of the Ni i absorption line at 6768 Å. The line of sight velocity is inferred from a lookup table which is based on the shape of the line in the quiet sun. In magnetic regions, however, the line shape changes drastically and molecular absorption lines appear close to the line's wavelength. This problem is equally relevant for the upcoming Helioseismic and Magnetic Imager (HMI) which measures velocities based on the same principles as MDI, using however a different absorption line (Fe i at 6173 Å). Based on high spectral resolution images of magnetic regions obtained by the ASP (Advanced Stokes Polarimeter) instrument we show that the MDI Doppler velocities are systematically underestimated in magnetic regions. We discuss possibilities to correct the velocity measurements in magnetic regions using intensity and magnetic field data.
SP13A-08 1330h
Automating the Quality Assurance Stage of GONG Data Processing
An Automated Image Rejection (AIR) program for image quality assurance has been developed for use in processing GONG data. It includes checks for detecting bad images due to weather, obstructions in the field of view (e.g. aircraft at Big Bear), and abnormal instrument operation. Data from a test period were reprocessed through our image calibration pipeline using the AIR program for image QA decisions. The dataset produced with AIR had a slightly higher duty cycle than the archived data from the test period. Comparison of the resulting peak mode frequency distributions, ring diagram derived flow patterns, and other helioseismology products shows close agreement with the products derived from the original dataset.
SP13A-09 1330h
Automating GONG's Angle Determination Pipeline
Recently, GONG started recording regular noon drift-scans throughout the Network (3 per week). This is in an effort to prevent spurious "wobbling" of GONG's merged images by providing regular "reality checks" on the true orientation of the site images. Wobbling can be very detrimental to local helioseismology analyses (A.K.A. the "Washing Machine Effect") Here we describe recent steps to automate the processing of the drift-scans once they arrive in Tucson.