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 NCAR’s 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

The animation depicts two views of an H2 ionization field, one from below and one from above. Red (near the top) indicates areas of low ionization. Yellow, near the bottom, shows areas of high ionization. As bouncy drives the cool, dense H2 downward, heat causes the gas to ionize. Periodically a downward plume will reach supersonic velocities. These supersonic plumes are shown in green.