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J. S. Kern (NRAO / NMIMT), J. A. Eilek (NMIMT)
Pulsar radio emission comes from plasma which has been accelerated to relativistic speeds in the open field line region. We do not know the density of this plasma. It is often assumed to be in steady flow at the corotation density, but this assumption contradicts observations of variable, low-frequency radio emission. We do know that a charged, outflowing plasma carries a current. The current density should be determined by properties of the global circuit, and may well differ from the corotation value.
We have begun a program to carry out time dependent numerical simulations of the pulsar current flow. We use a one-dimensional particle-in-cell code to model the plasma and a variety of signal processing techniques to study the time signatures of the simulation results. We find that the response of the system depends on the current density. Currents which are slightly below the corotational density develop strong, low-frequency oscillations. The oscillation frequency depends on the current density. We find frequencies ~0.01 - 0.1 \nup in our preliminary runs (where \nup is the local plasma frequency).
We believe the radio emission from this plasma will also show this strong, low-frequency modulation. We expect it to appear as ``micro-structure'' or ``nano-structure'' in the pulsar signal. There is no satisfying explanation to date as to why some pulsars exhibit microstructure and others do not. We speculate such microstructure becomes observable when the local current is close to, but below, the corotation value. The current is determined by the global circuit, including effects of the pulsars environment. Thus observing microstructure may be an indirect observation of both the current density in the star's magnetosphere, and the physics of the pulsar-ISM interface.
The author(s) of this abstract have provided an email address for comments about the abstract: jkern@nrao.edu
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.