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Measurement of High-Degree Solar Oscillation Frequencies (1Dec94)
(from NOAO HIGHLIGHTS!, NOAO Newsletter No. 40, 1 December 1994)

The chief method of testing models of solar interior structure and the
physical processes occurring within the Sun involves studying the acoustic
oscillations within the Sun that are observed as coherent radial
displacements across the entire solar surface. Kurt Bachmann (NSO), Tom
Duvall (NASA/Goddard), Jack Harvey (NSO), and Frank Hill (NSO) have
made a new measurement of high-degree solar oscillation frequencies using
the High-L Helioseismometer (HLH) recently installed on top of the NSO
Kitt Peak Vacuum Telescope. These frequencies represent an improvement
in accuracy and spatial resolution over previous measurements; they are
allowing solar astronomers to discriminate among theoretical treatments of
the convection zone, the outer region of the Sun that is difficult to model.

The improved frequencies reflect advances in both instrumentation and
computing capabilities. The HLH detector is a 1000 X 1024 pixel CCD that
uses a fast guiding system to maintain good spatial resolution during long
exposures. The oscillations measured by the HLH detector are of smaller
spatial extent represented by spherical harmonics of higher degree than the
oscillations that will be observed by the GONG (Global Oscillation Network
Group) instruments.

Bachmann, Duvall, Harvey, and Hill have made detailed comparisons of
their observed frequencies with predictions from three different models, as
shown in the figure. The clear disagreement of two of the models suggests
that the treatments of convection, low-temperature opacities, and the
equation of state in the third model are superior. The HLH research program
serves as ground-based support for the Solar Oscillations Imager mission
experiment on the SOHO satellite to be launched in 1995. The complete list
of over 5000 observed frequencies from this study is available.

                            [Figure not included]

        HLH frequencies (small dots) agree better with the Los Alamos
        model (filled squares, Guzik and Cox 1992) than with the Denmark
        (large dots, Christensen-Dalsgaard, Proffitt, and Thompson 1993) or
        Yale (plus signs, Guenther, Demarque, Kim, and Pinsonneault 1992)
        models.

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