Previous Article
Next Article
Table of Contents

Time-Distance Helioseismology (1Dec93)
(from NOAO Highlights!, NOAO Newsletter No. 36, 1 December 1993)

The application of seismology to the study of the solar interior has
advanced almost solely by the prediction and measurement of the Sun's
frequencies of free oscillation. Direct measurement of the travel times
and distances of individual acoustic waves---the predominant approach in
terrestrial seismology---would appear to be more difficult in view of the
number, and stochastic nature, of solar seismic sources. T. Duvall
(NASA/Goddard Space Flight Ctr.), S. Jefferies (Bartol), J. Harvey
(NSO/T) and M. Pomerantz (Bartol) have shown that it is possible to
extract time-distance information from temporal cross correlations of
the intensity fluctuations on the solar surface. The basic concept is
simple: at the surface, an upward propagating wave from the interior is
reflected back downwards by the very steep density gradient at the
surface. As the wave travels into the interior it is refracted due to
the rapid increase of temperature with depth. The wave travel time is
the time taken between successive surface reflections. Consider two
locations at which waves traveling along subsurface curved ray paths
reach the solar surface. The resulting signal as observed at these two
locations is strongly correlated at a time difference corresponding to
the travel time along the path. By measuring this time difference as a
function of the distance of separation of the two surface locations, it
is possible to develop a plot of the travel time versus the distance
separation, a plot familiar from terrestrial seismology. In the initial
work, the new technique was used to show that acoustic waves with
frequencies greater than the acoustic cutoff frequency are not
significantly reflected by the solar atmosphere, with their
reflection coefficient being less than 2%. This approach opens the way
for seismic studies of local solar phenomena, such as subsurface
inhomogeneities near sunspots, and should help to refine global models
of the internal velocity stratification in the Sun.

Previous Article
Next Article
Table of Contents