C+
C NAME:
C	ThomsonPDistance
C PURPOSE:
C	Find the distance to the plane of the sky where a single electron
C	would have a given polarization.
C CALLING SEQUENCE:
	function ThomsonPDistance(ElSun,U,P)
C INPUTS:
C	ElSun	real	distance Sun-Electron (in solar radii)
C	U	real	limb darkening coefficient
C	P	real	polarization	(-1<=P<=1)
C OUTPUTS:
C	ThomsonPDistance
C		real	distance to the plane of the sky (solar radii) (>=0)
C			(ElSun*cos(Chi))
C CALLS:
C	ThomsonSoup, BadR4
C PROCEDURE:
C >	The distance to the plane of the sky is given as a positive number.
C	There are two locations on either side of the plane of the sky
C	which match the polarization.
C >	An electron can only produce a positive polarization. If a negative
C	polarization is specified, then D=BadR4() is returned
C >	An electron can only produce a polarization below a certain maximum
C	(less than one). If a polarization above the maximum value is specified
C	the D=-1. is returned.
C MODIFICATION HISTORY:
C	JUL-1996, Paul Hick (UCSD/CASS; pphick@ucsd.edu)
C-
	    real	ElSun
	    real	U
	    real	P

	call ThomsonSoup(ElSun,U,rIt,rItr)

	Q = P*(2*rIt/rItr-1)
	if (0 .lt. P .and. Q .le. 1) then
	    ThomsonPDistance = ElSun*sqrt((1-Q)/(1+P))
	else
	    ThomsonPDistance = BadR4()
	end if

	return
	end