tokens: in the center of the sheet, and decays toward the rear of the current layer. This transpires from the shape and separation of the red equipotential contours in Figure 1 . Beyond the center, there is more energy in the stress-release motion than in the residual shear component. This means that a Poynting flux, , builds up behind the leading edge of the current front that subsequently converges, thus feeding the electromagnetic energy into the particles by maintaining . Associated with the first increasing and then decreasing stress-release motion is an inertial current, j in , pointing away from the sheet's center line. This component leads to concentrations of the current along the leading and trailing edges of the progressing sheet. Such refinements have not been incorporated in the cartoon of Figure 1 . Underlying this figure is the assumption that the current front propagates in a stationary fashion. In this case, the perpendicular potential drop equals and can be approximated byTwo aspects differ from the mechanical analogy of the fracture process: the magnetic field lines become newly connected, and part of the converted energy remains inside the fracture zone as thermal energy. However, a fraction, say about 10% of the electrons, is running through much of the parallel potential drop without losing its energy to the wave field and escapes along B , carrying perhaps up to 50% of the converted energy out of the sheet. These are the runaway electrons, candidates for producing the hard X-rays in solar flares. Figure 1. Oblique current sheet propagating with normal velocity, v n . Red lines represent equipotential contours for a quasi-stationary situation. The transverse electric field is related to the ongoing reduction of the shear field. The associated inertial current, j in , leads to current concentrations along the leading and trailing edges (not shown). The dashed magnetic field lines indicate the presence of a shear component, B ⊥ , that decays upon transit through the current sheet. An interesting problem is that of the reverse current. The question whether it is cospatial with the flux of runaway electrons has occupied many authors (Holman 1985 ). Emslie Hénoux ( 1995 ), for instance, argue against cospatial return currents. Figure 2 shows that the obliqueness of the current sheet facilitates maintaining charge neutrality. Charge neutrality requires that the flux of high-energy and low-density runaway electrons is balanced by counter-flowing low-energy background electrons. This begins outside the current sheet and thus outside the strong electric field, which would have a repulsive action on the return current, and continues all along the respective magnetic field lines. Because of the transverse propagation of the emitting current sheet, this lasts for a very limited time, τ CS , to be defined in Section 4.3 . It must be noted that the individual fluxes of runaway and reverse current electrons are by orders of magnitude higher
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