VETS Visualizations: Compressible Convection in an Ionizing Fluid

The Sun rings like a bell. By looking at the frequencies of this
ringing we can tell what the bell is made of. In the case of the Sun
we can tell the sound speed (temperature) of the gas, rotational
velocities, plus other properties, all as a function of depth. In
essence we can look inside the Sun by studying its vibrations. What is
it that rings the bell by exciting the oscillations? Probably
convective motions. Supersonic downdrafts may also play a
role. Researchers in NCARs High Altitude Observatory division are
attempting to better understand the processes that may influence the
acoustical properties of the Sun. Using complex numerical models, the
effects of ionization on the dynamics and transport properties of a
compressible, convective fluid are being studied. The fluid in these
experiments is pure hydrogen. Experiment conditions are intended to be
similar to those of the solar convection zone. Unlike conditions in
the Sun, however, only a fraction of the hydrogen is permitted to
ionize. The model becomes numerically unstable when 100% ionization
occurs. As the fraction of fluid permitted to ionize increases,
convective down flows become more vigorous, reaching supersonic
velocities. For an ionization fraction of .25, average peak velocities
reach Mach 1.2. For an ionization fraction of zero (an ideal gas),
peak vertical velocities only average around 0.5. For f>0.5, the
down flows become so vigorous that they become unresolvable using
present 256^3 numerical simulations. The existence of these supersonic
down flows may have implications for heat transport and solar acoustic
oscillations.