where a = 0, A = 0 corresponds to the standard diffusion equation, a = 1, A = 0 yields the telegrapher's equation. Pauls et al (1993) suggested a cross-derivative term (A ^ 0) to account for a possible dominant helicity in the random component of the Heliospheric magnetic field. Gombosi et al (1993) pointed out that a modified a, which depends on the actual form of Dju gives better approximation.
The right hand side of Eq. 2.23.2 includes adiabatic focusing owed to the possible divergence of field lines (A(z) is the area element, and L is the focusing length, with 1/L = dln Adz ).
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