Helioseismology
One way to study the Sun's interior is to study the propagation of waves in its body.
This is similar to
the way geologists learn about the interior of the Earth by studying seismic waves.
The resulting field of study is called helioseismology.
Velocity Fields
One can learn about waves in the interior by studying how the surface of the Sun vibrates.
This is done by dividing the surface of the Sun up into many small regions and
determining
the radial velocity of each tiny region from Doppler shifts.
The result is called the velocity field for the
Sun, and a diagram showing how the velocity field varies across the solar disk is called
a dopplergram. The top right image shows a typical dopplergram.
Solar Dopplergrams
As can be seen from the scale, the dopplergram is color-coded in radial velocity.
Darker colors
represent motion toward us (negative velocities)
and lighter colors represent motion away from us (positive velocities). We can see clearly in this
diagram the rotation of the Sun, with the left limb approaching us at about 2 km/s and the right
limb receding at about the same velocity. But superposed on this overall motion are
fluctuations that correspond to vertical motion of the Sun's surface in localized regions.
This dopplergram was obtained by the Michelson Doppler Imager (MDI) instrument
on the SOHO observatory,
which is orbiting the Sun 1.5 million kilometers sunward from the Earth.
This instrument is capable of measuring vertical displacement on the solar surface at a million
points simultaneously and can detect displacement velocities as small as 1 mm/s.
