= Radial magnetic-field Fokker-Planck equation
{title2=$F_t=D\partial_B(B^2F_B-2BF),\quad D=\kappa_2/4$}
For an isotropic initial ensemble in <Gaussian white-noise magnetic stretching>, $F(B)=4\pi B^2P(B)$ is the magnitude <probability density function>. The radial vector equation $P_t=D(B^2P_{BB}+4BP_B)$ gives the displayed conservative <Fokker-Planck equation>. Its integral is conserved under zero endpoint probability flux. For an anisotropic initial vector distribution, use the angularly averaged magnitude density instead; isotropic noise alone does not make arbitrary initial data isotropic.
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