Comment [on “The Parker Challenge”]



Fundamental to considering the relative merits of a (B, V) versus an (E, j) description of a plasma, one must first define the bulk fluid velocity and characterize the dissipation mechanism responsible for fluid cohesion. While the equations of magnetohydrodynamics (MHD) owe a heritage to studies of Coulomb-collisionally dominated plasma, many geospace regions of interest are, in fact, Coulomb-collisionless. Yet, the fluid equations of collisional MHD give a remarkably good and predictive description of this plasma. Still, collisionless plasma have qualities not captured by the collisional MHD paradigm, and their recognition requires an interpretation of the bulk fluid velocity that distinguishes the frozen-in frame and the flux-defined fluid velocity of conventional MHD.