Helioseismology (2) ...
By studying how the solar velocity field changes with time, we may determine
how the Sun is vibrating.
The animation in the right panel illustrates how the velocity field varies with time
in a typical observation. The Sun vibrates with a complex set of frequencies, but the dominant
period is about five minutes.
These 5-minute oscillations are caused primarily by pressure waves that travel
through the region between the surface and the base of the convective zone.
Sound Waves in the Sun
The adjacent left image illustrates a calculation
of a standing acoustic (sound) wave in the body of the Sun,
with red and blue indicating displacements of opposite sign. The
animation below shows a greatly exaggerated possible vibrational mode.
(The amplitudes of the vibrations are much smaller than this.
They are highly exaggerated in this animation to make them easy to see.)
By decomposing the observed vibration of the Sun into a superposition of such modes,
it is possible to learn about the interior because the structure of the interior affects the wave
patterns that appear on the surface.
Constraints from Helioseismology
Presently, helioseismology is placing strong constraints on our
theories of the solar interior. The details are too technical for our introductory
presentation, but the basic idea is simple:
changes in the properties of the solar interior (for example, the amount of helium in a particular region)
will affect the way sound waves travel through the interior, and this will influence the way the surface
vibrates. Two important pieces of information obtained from helioseismology
are that the abundance of helium in the interior (but outside the core) is about the same as at the
surface, and that convection extends about 30 percent of the way to the center.
