outflowing equatorial wind is dissipated to become the source of synchrotron emission for the compact nebula extending to the boundary of the “bean”. One reason that the arcs are not complete rings might be that the emission is from outflowing particles which Doppler boost their emission in the forward direction.

This is essentially the picture for the similar arcs surrounding the Crab pulsar first suggested by Aschenbach and Brinkmann (1975) and later elaborated by other authors (Arons et al. 1998 and references therein). The main difference is that the dark cavity which contains the unshocked pulsar wind in the Kennel & Coroniti (1984a) model of the Crab is small compared with the volume of the Crab Nebula, while the radius of the Vela nebula (the bean), is barely twice as large as its pulsar wind cavity (see Figure 4). We assume that the two rings straddle the equator symmetrically, and suppose that the deficit of emission exactly in the equatorial plane is related to the fact that this is where the direction of a toroidally wrapped magnetic field changes sign; i.e., the field may vanish there. The semimajor axis of the ring $a = 25.7^{\prime\prime}$, and the ratio $a/b = 