C+
C NAME:
C	ThomsonBase
C PURPOSE:
C	(Used internally only)
C	Determines the intensity scattered from single coronal electron
C	by Thomson scattering
C CALLING SEQUENCE:
	function ThomsonBase(ElSun,SinChi,U, P,rIt,rItr)
C INPUTS:
C	ElSun	real	Electron-Sun distance (solar radii) = 1./sine(Omega)
C	SinChi	real	angle Sun-Electron-Observer (degrees)
C	U	real	limb darkening coefficient
C OUTPUTS:
C	ThomsonBase
C		real	scattered intensity
C	P	real	polarization; P=rItr/ThomsonBase (-1<=P<=1)
C	rItr	real	tangential minus radial intensity; same units as ThomsonBase.
C	rIt	real	radial intensity; same units as ThomsonBase
C CALLS:
C	ThomsonSoup
C PROCEDURE:
C >	See Chapt. 6, Sect. B of Billings' "A guide to the solar corona" (p.150)
C	Sigma	= Thomson cross section=7.96x10^-26 cm^2/sterad
C	Omega	= angular size of Sun from electron
C	Chi	= angle Sun-Electron-Observer
C	I0	= intensity at disk center
C	RSun	= solar radius = 7x10^10 cm
C
C	Billings specifies the scattered intensity in the form
C		I=0.5*Pi*Sigma*I0*Func(Omega,Chi) (erg/s/sterad)
C	This subroutine only deals with Func(Omega,Chi):
C		From Eq. 18: rIt  = (1-u)*CC+u*DD
C		From Eq. 19: rItr = sin^2(Chi)*[(1-u)AA+u*DD]
C >	The last two arguments (rIt, rItr) are returned for use by
C	functions ThomsonLOS*.
C MODIFICATION HISTORY:
C	JUL-1996, Paul Hick (UCSD/CASS; pphick@ucsd.edu)
C-
	    real	ElSun
	    real	SinChi
	    real	U
	    real	P
	    real	rIt
	    real	rItr

	call ThomsonSoup(ElSun,U,rIt,rItr)

	rItr	= rItr*SinChi*SinChi 		! Eq. (19), It-Ir

	ThomsonBase = rIt+rIt-rItr		! It+Ir
	P = rItr/ThomsonBase			! (It-Ir)/(It+Ir)

	return
	end