The numerical program CLOUDY (version 90.02) is used to model the diffuse interstellar gas (DIG) near the Galactic midplane (see Ferland 1996 for a complete description of CLOUDY). Briefly, CLOUDY calculates the physical conditions of a dilute gas when heated and ionized by an incident radiation field. This is done by ``simultaneously solving the equations of statistical and thermal equilibrium, equations which balance ionization-neutralization processes, and heating-cooling processes, respectively'' (Ferland 1996). The resulting emission-line spectra for many transitions are determined and can be compared to observations.
The interstellar medium is composed of several different phases. We use CLOUDY to model only the diffuse ionized component which we assume is physically distinct, i.e., the DIG is composed of nearly fully ionized hydrogen. Observations of the DIG are used to constrain the physical parameters defined in CLOUDY. Below is a list of all the user defined parameters.
The incident radiation field: The incident radiation field
is defined by an intensity and a shape. The observed surface flux of
hydrogen ionizing photons in the DIG 
is used to define the intensity. The grid of stellar
atmosphere models determined by Kurucz (1991) are used to specify the
shape. N.B., the effective temperature defined here is only a
measure of the shape of the radiation field and does not define in any
way the intensity level. A range of effective temperatures, 
, are explored (see below and §4).
The geometry: CLOUDY is intrinsically a one-dimensional code. We define the geometry to be plane-parallel and the incident radiation field is set at the Galactic midplane. (That is, the geometry is a slab which is ionized from below.)
The chemical composition: We use the average interstellar medium abundances from Cowie & Songaila (1986). This is representative of the warm and cold phases of the ISM.
The density law: An exponential density law is defined
with a local total hydrogen density at the midplane of
and a vertical scale height of 
(Reynolds
1993). (In all cases the total hydrogen density is approximately
equal to the electron density because the hydrogen is fully ionized.)
The local density is the density inside the clumps which
occupy a volume defined by the filling factor (see below). In this
paper we only explore regions near the Galactic midplane and therefore
the electron density is essentially constant.
The filling factor: The filling factor for the DIG has been
determined by using H
emission measures and dispersion measures
from pulsars in globular clusters to be 
(Reynolds 1991).
We adopt a filling factor of 0.25.
The additional heating: We adopt a thermal heating rate
due to the dissipation of turbulence of 
and
for densities of 
and 
,
respectively (Minter & Spangler 1997). This heating is included by
adding the turbulent heating rate to the equations which balance the
thermal processes.
The program CLOUDY is used to generate a series of four different
models, each of which explores stellar effective temperatures from
. Various physical properties
,
, etc. 
and emission line intensities 
,
,
etc. are calculated by CLOUDY as a function of position from the
Galactic midplane. All results shown here are determined by averaging
these properties over the first 100¸ of the simulation. The four
models are derived using two different densities, 
and 
, and allowing the turbulent heating
to be turned on and off.