Helioseismology IV
Presiding: J Schou, Stanford University; R Howe, National Solar Observatory
SP24A-01 15:30h
Comparison of Solar Subsurface Velocities Using Small-Scale Ring Analysis, Time-Distance and Holography
For the first time we compare subsurface horizontal velocity flows determined by all three common local helioseismic analysis methods on a very small scale (approximately 1-2 degrees). Holographic and time-distance techniques have been compared on small scales, however, ring analysis techniques have mainly been performed with 15 degree size tiles spaced 7.5 degrees apart. Here we use tiles approximately 2 degrees in size with a spacing of 1 degree and compare the results with those of time-distance and holography. All three techniques have been applied to full-disc Doppler velocity data with 4" resolution (0.2 deg at disc center) taken with the Michelson Doppler Imager instrument aboard SOHO during Carrington Rotation 1988 in March-April of 2002.
SP24A-02 15:45h
Solar Subsurface Flows of Active Region AR~0696
We use a ring-diagram analysis to determine the subsurface flows in the upper 16~Mm of the convection zone from high-resolution Global Oscillation Network Group (GONG) data obtained during the first two weeks of November 2004. The active region AR~0696 emerges near the eastern limb and moves across the disk during this time period. The region produced several terrestrially effective flares and halo CMEs during its transit across the disk. During its disk passage, AR~0696 is the only large active region in the northern hemisphere and almost the only flare producing region on the sun. This makes it a good candidate for investigating the relation between active regions, their flare activity, and associated subsurface flows. We will present the latest results. This work was supported by NASA grant NAG 5-11703.
SP24A-03 16:00h
Comparison of Mode Parameters Between Velocity and Intensity Acoustic Spectra via Ring Diagrams
We analyse the local acoustic spectra at different locations over the solar disk using both velocity and intensity images from MDI. These spectra were fitted to obtain different mode parameters: e.g., acoustic frequencies, mode amplitudes and life time using symmetric fits. We find differences between frequencies derived from velocity and intensity filtergrams, and it appears that the mode frequencies vary as a function of location on the disk. Since the apparent frequency shift between an oscillation observed in velocity and intensity can not be a property of the mode, the analysis is expected to provide important information about the driving and damping of local acoustic oscillations.
SP24A-04 16:15h
Do Supergranules Tend to Align in the North-South Direction?
We investigate the recent claim by Lisle, Rast, and Toomre (2004, ApJ) that supergranules tend to align preferentially in the north-south direction. We repeat their analysis with an extensive set of maps of the horizontal divergence of flow fields derived from time-distance helioseismology. We construct temporal averages of the divergence maps for different east-west tracking velocities (vx) and measure the rms values in the east-west (σx) and north-south (σy) directions from these maps. In agreement with Lisle et al. (2004) we find that, near the equator, the ratio σx/σy is maximum (and greater than one) for a tracking velocity 120 m/s above the Carrington velocity. In addition, we find that σx/σy displays a second local maximum at a tracking rate close to the Carrington velocity. We argue that the variations of the ratio σx/σy as a function of vx is a direct consequence and additional evidence of the wavelike power spectrum of supergranulation observed by Gizon, Duvall, and Schou (2003, Nature). Thus, a north-south alignment of supergranules is not the only explanation nor a necessary condition for a ratio σx/σy greater than one. Additionally, we also study the variations of the ratio σy/σx as a function of north-south velocity shifts (vy), and find that the ratio σy/σx also has two maxima of similar amplitude separated by approximately 120 m/s.
SP24A-05 16:30h
Flow Inversions for Phase-Sensitive Helioseismic Holography
Phase-sensitive holography has been used extensively to study solar subsurface mass flows. To date though, the ingression-egression correlation phases measured using phase-sensitive holography data have never been inverted. We present an inversion procedure, based on Born-approximation sensitivity functions and the MCD algorithm, for estimating subsurface flows from phase-sensitive holography measurements. We present some simple validation tests and example flow maps resulting from inversions of SOHO/MDI data.
SP24A-06 16:45h
Seismic Emission From Solar Flares
Local helioseismic diagnostics applied to helioseismic observations from the Michelson Doppler Imager (MDI) on the Solar Heliospheric Observatory (SOHO) have shown the clear signature of seismic emission from three flares during the advent of SOHO. All three of these flares showed the signatures of γ-ray emission indicating the involvement of accelerated protons. Two of the acoustically active flares were recent, October 28 and 29 of 2003, and were observed by RHESSI. In both of these instances, the sources of the acoustic emission acoustic source, determined by computational seismic holography, coincided closely with compact γ -ray signatures of protons. Elementary considerations ofenergy and momentum transfer appear to make chromospheric and photospheric heating by protons favorable for seismic emission from flares. If this is actually the case, proton diagnostics of flares from RHESSI would be useful for identifying acoustically active flares for the Helioseismic Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), and possibly for the SOHO/MDI. Given a clear understanding of the effects of flares on Doppler signatures in active regions, acoustic emission from flares can give us a powerful control utility for seismic diagnostics of active regions subphotospheres. This research has benefitted greatly from the keen insights of Valentina Zharkova, Gerald Share, Hugh Hudson, and Sam Krucker. It has been supported by grants from the Living with a Star and Supporting Research and Technology programs of the National Aeronautics and Space Administration and the Stellar Astronomy and Astrophysics branch of the National Science Foundation.