  Compared to the Sun, the atmospheric structure and convective flow in Procyon
A exhibit the following characteristics: (1) the highly superadiabatic
transition layer (SAL) is located at much shallower optical depth; it is in a
dynamically active region, and its outer region is located part of the time in
the optically thin atmosphere; (2) the outer region of the SAL moves from an
optically thin region to thick region and back again over a time of 20-30
minutes. This motion, which is driven by the granulation, takes place in a time
approximately half the turnover time of the largest granules; The main reason
for the radically different radiative-convective behaviour in Procyon A
compared to the Sun is the role played by turbulent eddies in determining the
overall flow/thermal structure. The turbulent pressure and turbulent kinetic
energy can exceed 50 % of the local gas pressure (compared to about 10-20 % in
the Sun). The Procyon A simulation thus reveals two distinct timescales - the
autocorrelation time of the vertical velocity and the characteristic timescale
of the SAL which is tied to granulation. Just below the surface the
autocorrelation decay time is about 5 minutes in Procyon A, and the SAL motion
timescale is 20-30 mins. When the SAL penetrates the optically thin region
there are efficient radiative losses and the peak of the SAL is low. We
speculate that these losses damp out the relative amplitudes in luminosity
(temperature fluctuations) compared to velocity (Doppler). Although this will
not affect the frequencies of the peaks in the power spectrum, it will probably
lower the average amplitude of the peaks relative to the noise background.
