In classical turbulence theory, the energy contained in the large
scale ``eddies'' cascades to smaller scale eddies. This cascade of
energy to smaller scales occurs on all spatial scales down to a
dissipative scale, at which point the turbulent energy is converted
into thermal energy via some dissipation mechanism. The dissipation
rate of the turbulence is proportional to the energy contained in the
turbulence ([Minter & Spangler 1996]). A more complete discussion of turbulence is
in Tennekes & Lumley (1994) and Tatarski (1961). The heating
resulting from the dissipation of turbulence can be treated as a
thermal heating term. Minter & Spangler (1996) determined that the
turbulence in the diffuse ISM responsible for the scintillation of
pulsars and angular broadening of extragalactic radio sources was in
fact magnetohydrodynamical turbulence. This allowed for the direct
determination of the energy within the turbulence in the DIG since the
turbulent energy is contained in magnetic field and velocity
fluctuations, with density fluctuations only
being a tracer. From this information, Minter & Spangler (1997)
determined that the turbulent dissipation rate, and thus the turbulent
heating rate, is likely to be in the range 
for the DIG, assuming 
. This value is
approximately equal to the cooling rate of the DIG, indicating that
the heating of the DIG via the dissipation of turbulence could be an
important mechanism.
The magnitude of the turbulent energy can be estimated using
where 
is the
non-thermal velocity determined from spectral line observations and
equipartition of the turbulent energy between magnetic field
fluctuations and velocity fluctuations is assumed. In
H 
regions the non-thermal velocities are typically in the same
range as those observed in the DIG ([Reynolds 1985a]). The densities in
H regions are typically 3-4 orders of magnitude larger than in
the DIG resulting in a turbulent heating rate in H regions of
. The cooling rate, however, depends on the
density squared ([Reynolds 1990a]) so that the turbulent heating rate in
H regions is at least two orders of magnitude less than the
cooling rate. Therefore, heating via the dissipation of turbulence is
not a significant process in determining the observed line emissions
of H regions whereas it may be for the DIG.