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F. C. Michel, I. A. Smith (Rice Univ.)
Recent observations of the Crab and Vela by HST and Chandra have revealed that the winds seem to form polar jets and equatorial outflows. Features in the Crab winds flow with velocities comparable to c, which is no surprise theoretically. But the theoretically assumed wind patterns typically were more or less uniform outflows (e.g., Melatos and Melrose 1996, Fig. 1), unlike the jet/equatorial patterning. Interestingly, however, this geometry exactly parallels the expected distribution of trapped plasma around an aligned rotator (Krause-Polstorff and Michel 1985): plasma of one sign of charge is concentrated over the magnetic polar caps while the opposite sign plasma is concentrated in the equatorial regions. Unfortunately, it is not obvious how the wind is formed. We have examined several mechanisms (Smith, Thacker, and Michel 2000) and so far are forced to conclude that inclination of the dipole is essential and that plasma must be forced away by the resultant large-amplitude waves in the wave zone (a.k.a. light-cylinder). The KPM simulations showed that the popular Goldreich-Julian (1969) model was incorrect because two incompatible assumptions were made: (1) that the charged particles come from the neutron star surface and (2) that the magnetosphere is entirely filled by these particles. The KPM simulation initially adopted only assumption (1). Recently Contopoulos et al. (1999) found numerical solutions adopting assumption (2). Neither solution is GJ. However, assumption 2 would require ionization processes at implausible distances for typical pulsars.
These results are of particular relevance to the popular assumptions that
1. Radio emission comes from the magnetic polar caps,
2. Plasma is accelerated from the magnetic polar caps,
3. Inclination of the dipole simply rotates the beam,
4. An aligned rotator would function as an "unpulsed" pulsar, and
5. Plasma is centrifugally ejected at the light-cylinder.
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