Helioseismology III
Presiding: J Schou, Stanford University; R Howe, National Solar Observatory
SP23C-01 INVITED 13:30h
Parker Lecture: Local Helioseismology
This brief introduction to the science of local helioseismolgy - the study of travelling sound waves in addition to the standing waves familiar from global helioseismology - will demonstrate how we can now probe the temporally varying spatial inhomogeneities within the solar interior, and illustrate this with results on meridional flows and torsional oscillations, sunspot and active region structure, supergranulation, images of farside of the Sun, as well as signatures of flares.
http://www2.nso.edu/staff/jleibacher/parker.html
SP23C-02 14:00h
Direct Measurement of Wave Kernels in Time-Distance Helioseismology
Solar f-mode waves are surface-gravity waves which propagate horizontally in a thin layer near the photosphere with a dispersion relation approximately that of deep water waves. At the power maximum near 3 mHz, the wavelength of 5 Mm is large enough for various wave scattering properties to be observable. Gizon and Birch (2002,ApJ,571,966) have calculated kernels, in the Born approximation, for the sensitivity of wave travel times to local changes in damping rate and source strength. In this work, using isolated small magnetic features as approximate point source scatterers, such a kernel has been measured. The observed kernel contains similar features to a theoretical damping kernel but not for a source kernel. A full understanding of the effect of small magnetic features on the waves will require more detailed modeling.
SP23C-03 14:15h
Towards Assessing, Understanding, and Correcting the Influence of Surface Magnetism in Local Helioseismology
Efforts to probe subsurface wave-speed variations and mass flows near and under solar active regions are complicated and potentially compromised by strong phase and amplitude perturbations introduced in the photosphere by magnetic fields. Recently it has been shown that the phase distortions correlate with surface magnetic field strength and may be corrected to image wave-speed variations underlying active regions. A strong phase asymmetry between waves arriving into and departing from a magnetic region is also shown to produce spurious signatures of horizontal outflows below active regions. Further evidence of the photospheric origin of these phase distortions, as well as a key to their physical origin, is also presented. Using MDI observations, from the SOHO spacecraft, we perform ingression control correlations in the inclined magnetic fields of sunspot penumbra and demonstrate that incoming acoustic waves produce photospheric motion that describes an ellipse tilted towards the inclination of the magnetic field. A consequence is that the phase of the correlation varies with the viewing angle with respect to the field direction. Observations of the vector components of the photospheric fields could be used to correct these phase perturbations analogous to the procedures already developed using line-of-sight magnetograms. A physical understanding of surface distortions will come through MHD modeling, including simulations of the interaction of acoustic and magneto-acoustic-gravity waves with prescribed magnetic and sound-speed perturbations and flows (artificial data). The development of appropriate tools for assessing and correcting the effects of the surface magnetism will be vital for the interpretation of helioseismic data from the upcoming HMI mission on SDO. DCB and CL gratefully acknowledge support from the NSF (SAA/AST) and NASA (LWS, SR&T).
http://www.cora.nwra.com/~dbraun
SP23C-04 14:30h
Modelling the Interaction of p-modes With Sunspots
Sunspots absorb energy from and shift the phase of f- and p-modes incident upon them. One promising absorption mechanism is partial conversion to slow magnetoacoustic-gravity waves (and Alfvén waves), which guide energy along the magnetic field away from the acoustic cavity. Recent mode conversion calculations by Cally, Crouch, and Braun have shown that simple sunspot models with non-vertical magnetic fields can produce ample absorption to explain the observations, along with phase shift predictions that agree remarkably well with the Hankel analysis data. In this investigation, we further test the mode conversion hypothesis. We use a realistic solar model that accounts for both the magnetic and thermal influences associated with a sunspot. Our model has several adjustable parameters - the field strength and inclination can vary (crudely) across the spot. We employ a genetic algorithm to adjust these parameters to optimize the agreement between the model predictions and the observations. At this stage, our model is too simple to perform quantitative forward modelling. However, the genetic algorithm allows us to rigorously test the model. We will discuss the results of this testing in detail. Broadly speaking, our findings are consistent with those of Cally, Crouch, and Braun: the predicted phase shifts are in excellent agreement with the Hankel analysis data, and the corresponding absorption coefficients are generally ample to explain the observations. While there remain several uncertainties, our results further verify that mode conversion is a significant process in sunspot acoustics.
SP23C-05 14:45h
Ring-Diagram Analysis of Data From the Mt Wilson Magneto-Optic Filter
Full-disc high-resolution (4") Doppler data are available at a one-minute cadence on a near-daily basis from the Magneto-Optic Filter on the Mt Wilson 60-ft Tower Telescope from 1988 onwards. Although coming from an instrument that is less stable than the space-based MDI and has a lower duty cycle than the 6-site GONG+ network, the Mt Wilson series is virtually the sole source of data extending back before 1996 to which the techniques of local-area heleioseismology might be usefully applied. Ring-diagram analysis, which in many respects is just an extension of the techniques of high-degree global-mode helioseismology, is particularly promising for these data. Here we report on initial analyses of selected time samples from the Mt Wilson data series, comparing them where possible with results obtained from other data sources during periods of overlap.
http://rick.stanford.edu/pubs/spd05/mwo/