SUMMARY OF ORAL PRESENTATIONS IN SESSION H9 OF THE AIA-HMI MEETING OF 
FEBRUARY 13--17, 2006 IN MONTEREY, CALIFORNIA

1. Mark Rast reviewed considerations that bear on the issue of how seismic 
waves are generated by turbulence, mostly in the few hundred km beneath 
the base of the photosphere. These involve the dynamics of collapse of 
cooling granules, the dynamics of gas flowing into the space evacuated 
by the granule as it accelerates downward, and the bow wave emitted by 
the cooled material as it descends. Mark described the dependence of the 
acoustic emission spectrum on parameters such as the characteristic 
depth at which the emission is excited with respect to the upper and 
lower turning points, using the analogy to organ pipe acoustics to 
illustrate how line profile asymmetries are diagnostic of the depth 
distribution of the acoustic excitation. Mark emphasized the need for 
both Doppler and intensity observations from the HMI.

2. Junwei Zhao reviewed studies he and Sasha Kosovichev have done on the 
effects photospheric magnetic fields have on time-distance diagnostics. 
His review was largely a summary of a paper they have recently published 
on this subject. He showed instances in which magnetic phase shifts in 
holographic signatures were significantly greater than those in 
time-distance diagnostics. He confirmed the basic vantage dependence of 
control correlation phase shifts found by Schunker et al., but showed 
instances in which this had no effect on models of subphotospheric sound 
speed anomalies. He showed instances in which masking a magnetic region 
had no apparent affect on the sum of the ingoing and outgoing travel 
time to the magnetic region from the quiet Sun many Mm away. Junwei 
emphasized the importance of continued study of magnetic effects 
notwithstanding his conviction that these would not introduce 
significant errors in time-distance diagnostics of flows and thermal 
anomalies in the subphotospheres of active regions.

3. Ashley Crouch reviewed theoretical models he has been working on with 
colleagues in Australia and Boulder of how magnetic fields interact with 
seismic waves. Their work has capitalized on aspects of the interation 
that explain absorption of p- and f-modes. The theory they have 
developed at length is heavily based on coupling between fast and slow 
magneto-acoustic gravity waves at depths at which the Alfven speed is 
comparable to the sound speed. Absorption of p- and f-modes can be 
characterized in terms of conversion of fast-mode energy to slow-mode 
energy by the coupling. Ash emphasized the importance of inclined fields 
in absorbing the p-modes, particularly the lower-degree p-modes, noting 
that an account for these in his MHD computations fits absorption and 
phase shift profiles derived from Hankel analysis remarkably well over a 
broad range of frequencies and horizontal wavenumbers. Ash elaborated on 
the relation of mode coupling to observable effects such as the 
dependence of the phase of the control correlations on vantage, 
discovered by Schunker et al. He expressed doubt that 
magnetically-induced phase shifts and absorption would be mitigated due 
to reflection of upcoming waves before the Alfven speed reached the 
sound speed as a general rule.

4. Sasha Kosovichev reviewed work on seismic emission from solar flares, of 
which he and V. Zharkova discovered the first instance in the flare of 
1996 July 9. Since then up to 15 other possible "sun quakes" have been 
reported by Alina Donea and Diana Besliu-Ionescu, of which Sasha 
analyzed five in detail. He confirmed the close spatial relation of the 
site of the seismic emission to impulsive hard X-ray emission detected 
by RHESSI. He reviewed the hypothesis that seismic transients from solar 
flares are the continuation of intense, downward-propagating shocks that 
result from ablation of the middle and upper chromospheric by energetic 
particles. Sasha emphasized the opportunity that HMI offers both for the 
study of flares and the use of seismic emission from flares in 
helioseismology.

5. Richard Wachter reviewed efforts to model magnetic effects on 
helioseismic signatures in sunspots. Zeeman splitting of the NiI 6768 
line (measured in high-resolution MDI Dopplergrams and to be similarly 
measured in HMI in the FiI 6173 line) results in effective broadening of 
the individual circularly-polarized line profiles when the magnetic 
field is inclined with respect to the line of sight. This results in a 
significantly less sensitive Doppler response in magnetic photospheres 
characteristic of sunspot umbrae than in the quiet Sun. The broadening 
of the observed line profiles is most pronounced in sunspot penumbrae if 
the sunspot is at disk center, because of the horizontal component of 
the field, resulting in a less sensitive Doppler response in penumbrae 
at disk center. Similarly, the sensitivity of MDI Dopplergrams at the 
center of a sunspot umbra (where the magnetic fields is nominally 
vertical) is thought to decrease significantly with increasing distance 
from disk center. Because of various technical ambiguities, this has yet 
to be confirmed by the observations. The relation between the Doppler 
sensitivity and the effective width of the individual circularly 
polarized profiles is nearly linear for MDI observations of sunspots. 
This result of radiative transfer computations has been confirmed by 
observations. This linear relation has been shown to break down for FeI 
6173, for which the individual circularly polarized components are 
completely split by magnetic fields characteristic of sunspot umbrae 
(keeping in mind that FeI 6173 has a greater Lande factor than NiI 6768).

Richard elaborated on the possibility of correcting the HMI sensitivity 
in sunspots by including an account for the greater effective line width 
in sunspots. The method he developed to correct MDI Doppler 
sensitivities might be adaptable to HMI in sunspot umbrae, given 
sufficient sampling of the line profiles to characterize fully split 
circularly polarized components. These considerations led to discussion 
later in session H9 of the practicality of sampling the line profile as 
thoroughly as possible at least in magnetic regions themselves. Jesper 
Schou explained technical practicalities that limit the spectral 
sampling to be implemented on the HMI. The HMI instrument will not have 
the on-board discrimination, for example, to treat magnetic regions 
preferentially.

6. Aaron Birch reviewed recent developments in seismic diagnostic tools 
designed for the quiet Sun. He gave a general review of the development 
of Born-approximation kernels for time-distance, holographic, and 
ring-diagram diagnostics, distinguishing between 2-dimensional (f-mode) 
and 3-dimensional (p-mode) applications. In the context of f-mode 
diagnostics, he reviewed work on the scattering of these by small 
magnetic elements, in which case the kernels show distinctive elliptical 
and hyperbolic fringes that characterize a medium in which waves 
propagate only horizontally in a horizontally uniform medium between 
scatterers. These fringes are clearly seen in travel-time perturbations 
measured by time-distance correlation techniques. He briefly mentioned 
the extension of the foregoing f-mode diagnostics to the applications in 
the scattering of f-modes to the p1-mode and the possibility of 
extrapolating to higher radial orders. The high-quality, concurrent 
Doppler, intensity and magnetic data that HMI will provide will greatly 
enhance both the quality and quantity of near-surface f- and p-mode 
scattering statistics.

Aaron also reviewed time-distance and holographic diagnostics of the 
supergranulation, illustrating how the horizontal scale of the 
supergranulation gives rise to an artifact 5--15~Mm beneath the 
photosphere that, somewhat diabolically, resembles signatures in 
previous models that have been attributed to a return of the surface 
outflow. Aaron emphasized the need for detailed modeling of wave 
progagation through supergranular-like flows as part of the HMI project.

7. Konstantin Parchevsky reviewed applications of numerical simulations of 
waves in the upper few Mm of the subphotosphere. He briefly reviewed 
work he has done simulating the propagation of waves from localized 
sources into magnetic flux tubes. In more recent simulations waves were 
produced stochastically outside of a circular, sourceless "magnetic 
region." Konstantin showed movies of the response to the magnetic region 
to the incoming waves both in the photosphere and in a vertical cut 
through the magnetic region. The photospheric wave amplitudes were 
considerably smaller in the sunspot, and characterized by a considerably 
greater characteristic horizontal scale. Konstantin suggested that the 
suppression of Doppler seismic signatures in sunspots is largely a 
result of the lack of seismic wave generation in sunspot 
subphotospheres, as opposed to absorption of incoming waves, which he 
understands is not significant in the hydrodynamics of his sunspot. The 
point was made that numerical simulations would be crucial to an 
understanding of magnetic and other surface effects in seismic 
diagnostics of active regions in the term of HMI. This has clear control 
applications to diagnostics of active region subphotospheres.