  The polytropic process characterizes the thermodynamics of space plasma
particle populations. The polytropic index, ${\gamma}$, is particularly
important as it describes the thermodynamic behavior of the system by
quantifying the changes in temperature as the system is compressed or expanded.
Using Wind spacecraft plasma and magnetic field data during $01/1995 -
12/2018$, we investigate the thermodynamic evolution in 336 Interplanetary
Coronal Mass Ejection (ICME) events. For each event, we derive the index
${\gamma}$ in the sheath and magnetic ejecta structures, along with the pre-
and post- event regions. We then examine the distributions of all ${\gamma}$
indices in these four regions and derive the entropic gradient of each, which
is indicative of the ambient heating. We find that in the ICME sheath region,
where wave turbulence is expected to be highest, the thermodynamics takes
longest to recover into the original quasi-adiabatic process, while it recovers
faster in the quieter ejecta region. This pattern creates a thermodynamic
cycle, featuring a near adiabatic value ${\gamma}$ ~ ${\gamma}$${_a}$ (=5/3)
upstream of the ICMEs, ${\gamma}$${_a}$ - ${\gamma}$ ~ 0.26 in the sheaths,
${\gamma}$${_a}$ - ${\gamma}$ ~ 0.13 in the ICME ejecta, and recovers again to
${\gamma}$ ~ ${\gamma}$${_a}$ after the passage of the ICME. These results
expose the turbulent heating rates in the ICME plasma: the lower the polytropic
index from its adiabatic value and closer to its isothermal value, the larger
the entropic gradient, and thus, the rate of turbulent heating that heats the
ICME plasma.
