Analysis of spacecraft shock-crossing data on the magnetosheath thickn
ess to determine the polytropic index gamma of the solar wind is shown
to overestimate the value for gamma when the gasdynamic (GD) equation
s are used for the shock jump. This is done by comparing GD estimates
for gamma extrapolated from observed data points to values obtained wh
en more appropriate MHD equations are used. As a result of this bias a
shock-crossing data analysis carried out recently using the GD equati
ons appears to have obtained a value for gamma that is over 0.5 too la
rge. This is in part associated with the breakdown of GD with respect
to nonzero angle between the bow-shock normal and the magnetic field f
or finite Alfven Mach numbers. The magnetosonic phenomenological model
(sonic Mach number replaced with the magnetosonic Mach number in the
GD equations), which was also recently used to determine gamma, is fou
nd to reduce this bias by 70-90%. This analysis also strongly suggests
that the phenomenological model inadequately treats variations in the
angle between the bow-shock normal and the magnetic field, which may
also produce systematic errors in the experimental determination of ga
mma using this model.