Natural logarithm of the ratio of r_ap of the outermost field line to r_ap of the innermost field line, ﹩t_{\mathrm{out}\,}-t_{\mathrm{in}\,}\equiv \mathrm{log}\,[ r_{\mathrm{ap}\,}( \Psi _{\mathrm{out}\,}) / r_{\mathrm{ap}\,}( \Psi _{\mathrm{in}\,}) ] ﹩, as a function of the twist ﹩u_{1}﹩.