Far Ultraviolet ( FUV , 6 eV < h \nu < 13.6 eV ) radiation has been recognized as the main source of heating of the neutral interstellar gas , and , as a consequence , it determines whether the thermal balance of the neutral gas results in cold ( T \sim 50 - 100 K ) clouds ( CNM ) , warm ( T \sim 10 ^ { 4 } K ) clouds ( WNM ) , or a combination of the two .
High FUV fields convert the neutral gas to WNM , while low fields result in CNM .
The knowledge of how these fractions depend on the FUV sources ( i.e .
the star formation rate , the IMF , and the size distribution of associations ) is a basic step in building any detailed model of the large scale behavior of the ISM and the mutual relation between the ISM and the star formation rate in a galaxy .

The sources of FUV radiation are the short-lived massive stars that generally originate in associations that form in Giant Molecular Clouds present in the galactic disk .
Using McKee & Williams ’ ( 1997 ) distribution of birthrates for OB associations in the Galaxy , we determine the expected behavior of the time-dependent FUV field for random positions in the local ISM .
The FUV field is calculated in two bands ( 912 - 1100 Å and 912 - 2070 Å ) and at the wavelength 1400 Å .
In terms of U _ { -17 } \equiv U / ( 10 ^ { -17 } erg cm ^ { -3 } Å ^ { -1 } ) , where U is the energy density of the radiation field in some band , we find ( mean , median ) values at the solar circle of U _ { -17 } = ( 15.7 , 7.4 ) and ( 14.2 , 7.2 ) for the [ 912-1100 Å ] and [ 912-2070 Å ] bands , respectively .
At 1400 ~ { } \AA we find ( mean , median ) values of U _ { -17 } = ( 14.4 , 7.5 ) .
Our median value for the [ 912-2070 Å ] band is G _ { 0 } = 1.6 times Habing ’ s ( 1968 ) value for the radiation field at the solar circle in this band , and quite close to Draine ’ s ( 1976 ) value , G _ { 0 } = 1.7 .
Both the latter values are based on observations of sources of FUV radiation in the solar neighborhood , so all three values are close to observed values .
Due to attenuation by dust , only associations within about 500 pc contribute significantly to the energy density at a given point .
Large angle scattering produces a diffuse field that is about 10 % of the field produced by the sum of direct and small angle ( < 5 ^ { o } ) scattering from discrete sources ( the associations ) , as observed .
At a point exposed to the median radiation field , the brightest association typically produces about 20 % of the total energy density .
At a point exposed to an above average radiation field , the brightest association produces most of the energy density .
Therefore , the FUV field is asymmetric at a given point , and the asymmetry grows for higher fields .

The FUV field fluctuates with a variety of amplitudes , the larger ones being less frequent .
The mean field is about twice the median field because of these fluctuations , or spikes , in the radiation field .
These spikes , which last \sim 30 Myr , are caused by the infrequent birth of nearby associations .
For spikes that are significantly higher than the mean field , the time interval between spikes is \sim 2 U _ { -15 } ^ { 3 / 2 } Gyr .
We also model shorter duration spikes caused by runaway OB stars .
The presence of a fluctuating heating rate created by the fluctuating FUV field converts CNM to WNM and vice versa .