Helioseismology (3) ...
The speed of sound (that is, the speed with which longitudinal compression waves propagate) in a
medium is a sensitive indication of the medium's internal properties. For example, the speed of sound through water
is influenced by its temperature and density.
Speed of Sound in the Sun
The image at right illustrates information about the speed of sound inside the Sun as inferred
from SOHO helioseismology.
The data are shown in the form of the fractional deviation of the square of the
sound speed s from that expected from the Standard Solar Model, with zero indicating perfect agreement.
The finite extent of the red area indicates the uncertainty in the measurement, and the data are plotted versus
the fraction of the solar radius Rsun.
We may note several things. First, the deviations are all very small (the vertical scale is greatly expanded
in this plot to emphasize deviations).
The largest deviations shown correspond to a fraction of a percent. Second, the largest deviations measured
are concentrated in two regions: a region just below the convection boundary at about 60-70 percent of the solar
radius and a region deep in the
interior in the energy-producing region lying inside about 25 percent of the solar radius.
It is believed that both deviations result from small differences in
helium concentration in these regions from that predicted by accepted models of the
Sun. Negative deviations
correspond
to higher helium concentration and positive deviations to reduced helium concentration relative
to that expected.
The left figure summarizes the variation of sound speed spatially inside the Sun as deduced from the SOHO
data, with red representing speeds
higher than expected and blue representing speeds slower than expected.
