Title: 
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LINE ASYMMETRIES ABOVE AND BELOW THE ACOUSTIC CUT-OFF FREQUENCY AND THE EXCITATION OF SOLAR OSCILLATIONS

Lead Investigator:
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RAKESH NIGAM

E-Mail Address:
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rakesh@quake.stanford.edu

Team Members:
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Dr. P.H. Scherrer

Local Coordinator:
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Dr. A.G. Kosovichev

Lead Programmer:
=====


Technical Summary:
==================
 
        The power spectrum of solar oscillation modes shows varying amounts of
asymmetry. In helioseismology the eigenfrequencies were determined by
assuming that the power spectrum was symmetric and can be fitted by
a Lorentzian. This leads to systematic errors in the determination
of mode frequencies. The line asymmetry occurs
whenever the waves are excited by a localized source. Nigam and Kosovichev
(1996) have shown that the sense and the degree of the asymmetry depend
on the location and on the type of the excitation source.
Therefore, studying the line asymmetry will enable us to improve
the frequency measurements, find the depth, latitudinal distribution
and the physical nature of these localized sources that are responsible
for exciting the solar modes of different types and also investigate other
aspects of nonadiabaticity which get neglected when asymmetry is ignored.



Proposal Text:
=======================
	We plan to study the line profiles of the Doppler velocity, continuum
intensity and line depth oscillation power spectra of modes of
various angular degrees. First, we plan to investigate the m-averaged
spectra, and then to study the m-dependence of the line profile.
The study is carried out by comparing the observed power spectra with
the theoretical spectra computed for different types of the source
and different source  depths. The m-dependence will tell
us about the variations of the properties of oscillation sources
with latitude. We plan to pay a special attention to the frequency
shift between the velocity and intensity spectra. For this purpose,
high-cadence (30 sec) observations are particularly important
because the theoretical model predicts that the frequency shift is
most significant above the acoustic cut-off frequency.
Our model also predicts the variation of the location of the
power spectra peaks around and above the acoustic cut-off frequency
with the solar activity. Therefore, it is interesting to look for such
variations in the data. We would also like to investigate how
line asymmetry and the eigenfrequencies change with solar cycle
both below and above the acoustic cut-off frequency.


See Also:
=========
Abrams, D. and Kumar, P., APJ 1996
Jefferies, S.M., Osaki, Y., Shibahashi, H., Duvall, T.L., Jr.,
Harvey, J.W., Pomerantz, M.A., APJ 1994, 434, 795-800
Milford,P.N., Frank, Z., Gough, D.O.,  Kosovichev, A.G.
and Scherrer,P.H., 1993 in: Proc. GONG 1992 Conference,
Seismic investigation of the sun and stars,
ed. T.Brown, Astr. Soc. Pac., San Francisco, 97-100
Ronan, R.S., Cadora, K., LaBonte, B.J., Solar Phys. 150, 389-392