Investigation title:  The p-mode sources as seen in oscillations with 
frequency greater than the acoustic-cutoff frequency.

Lead investigator: Tom Duvall, NASA/GSFC

Team Members:
Phil Scherrer
Stuart Jefferies
Jack Harvey
Pawan Kumar
Ted Tarbell
(+ Players to be named later)

SOI Coordinator: Tom Duvall

SSSC Programmer:

Abstract/Technical Summary:
The solar atmosphere is transperant to acoustic waves above a certain
frequency, termed the acoustic cutoff frequency.  The acoustic radiation
at higher frequencies
that we detect at the surface either comes directly from a source or
has followed a path from the source to a subsurface turning point and
then been refracted back to the surface.  It continues outwards
and is lost to the system after making its presence known to our 
detectors as it passes through the atmosphere.  The correlation between
the direct wave and the once-refracted wave leads to an approximately
sinusoidal pattern in the high-freqency nu-l power spectrum characteristic
of a two-beam interference pattern.  This sinusoidal pattern is what
we see as the extensions of the lower frequency p-mode ridges. The
peaks have been called pseudo-modes.  

The locations of the pseudo-modes are sensitive to the source position.
The source position has been derived for measurements of the pseudo-mode 
positions up to a frequency of 7 mHz.  This is greater than the peak
acoustic cutoff frequency of 5.3 mHz but not greatly so.  The pseudo-mode
positions at 7 mHz are still sensitive to the atmospheric structure
somewhat and the conclusions about the source position would be more
secure if the pseudo-mode positions were observed at higher frequencies.
It is the purpose of this investigation to obtain high-frequency 
observations, develop spectra, and measure via a modelling effort
the depth of the source of p-mode oscillations at higher frequencies
than have been done before.

Investigation plan:

Observations.
The basic observational plan is to obtain several sequences of 8 hours 
of images during the time before the structure program begins, 
so that all the time could be devoted to this campaign.
One important aspect of these observations is that high signal to noise
ratio is very important, much more so than with observations near the
peak of the power envelope near 3 mHz.  At frequencies above 3 mHz,
the power in the oscillation signal is falling off as the inverse 
fourth or fifth power of the frequency, depending on the observable.

We are thinking of high-resolution mode with 512x512 pixels centered
on the disk with 1 velocity and one intensity picture sent every
15 seconds.  The intensity should be the line center intensity as 
this will be formed the highest in the atmosphere and hence have the
highest signal level.  This should give the best throughput if we 
insist on the four-point doppler measurement with the calculation 
being done on board.  

Another approach might be to do only a two-point velocity and get
the intensity from these wing measurements.  So we are always in
the part of the line that is most sensitive to doppler shifts, at
the expense of linearity which may not be too important for this 
investigation.

This special campaign may require some custom programming of the
instrument.  This aspect will by handled by Tarbell.

Reduction.
The data would be put through a standard SOI analysis pipeline to
obtain 3d k-nu diagrams.  These would be averaged over angle to obtain
a single 2d k-nu diagram.  This part of the reduction would be handled
by Duvall and Scherrer.  Positions of the pseudo-mode features would
be determined from the spectrum by Jefferies and Harvey using 
algorithms used to analyze South Pole data.  

Analysis.
The estimation of the source depth from the frequencies of the
pseudo-modes will be done using established algorithms by Kumar.