A recently proposed gedanken experiment [G. Z. Machabeli and A.D. Rogava. Phys. Rev. A 50, 98 (1994)], exhibiting surprising behavior, is reexamined. A description of this behavior in terms of the laboratory inertial frame is presented, avoiding uncertainties arising due to a definition of a centrifugal force in relativity. The surprising analogy with the radial geodesic motion in Schwarzschild geometry is found. The definition of the centrifugal force, suggested by J.C. Miller and M.A. Abramowicz, is discussed.

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A B S T R A C TA theory of pulsar radio emission generation, in which the observed waves are produced directly by maser-type plasma instabilities operating at the anomalous cyclotron±Cherenkov resonance q À k k v k q B = g res 0 and the Cherenkov drift resonance q À k k v k À k ' u d 0, is capable of explaining the main observational characteristics of pulsar radio emission. The instabilities are due to the interaction of the fast particles from the primary beam and the tail of the distribution with the normal modes of a strongly magnetized one-dimensional electron± positron plasma. The waves emitted at these resonances are vacuum-like, electromagnetic waves that may leave the magnetosphere directly. In this model, the cyclotron±Cherenkov instability is responsible for the core-emission pattern and the Cherenkov drift instability produces conal emission. The conditions for the development of the cyclotron±Cherenkov instability are satis®ed for both typical and millisecond pulsars provided that the streaming energy of the bulk plasma is not very high g p < 10. In a typical pulsar the cyclotron± Cherenkov and Cherenkov drift resonances occur in the outer parts of the magnetosphere at r res < 10 9 cm. This theory can account for various aspects of pulsar phenomenology, including the morphology of the pulses, their polarization properties and their spectral behaviour. We propose several observational tests for the theory. The most prominent prediction is the high altitudes of the emission region and the linear polarization of conal emission in the plane orthogonal to the local osculating plane of the magnetic ®eld.Key words: plasmas ± radiative transfer ± waves ± pulsars: general. I N T R O D U C T I O NMore than 25 years have passed since the discovery of pulsars and there is still no consensus on the basic emission mechanism. At the present time, there are about 12 competing theories, which differ both in the physical effects responsible for the radiation and in the locations where they operate (Melrose 1995). Probably the only point of agreement between all these theories is the association of pulsars with magnetized, rotating neutron stars. By contrast, there is so much observational data available that none of the existing theories can explain all the main observational facts. A useful observational framework for discussing the theory is a description of pulsar radio emission given by Rankin (1990). The main feature of this model is the division of emission into two main classes: core and cone. There may be many cones of emission. In each pulsar the averaged pro®le may be a combination of core and/or cone emissions (Fig. 1).To date, the most widely discussed theory attributes the emission to coherent curvature emission by bunches of particles. Although this theory can explain a broad range of observed pulsar properties by the careful arrangement of the magnetic ®eld geometry and the form and size of bunches, 30 years of theoretical efforts have failed to explain the origin of these bunches (Melrose 1995). This ...

Abstract. An investigation of the kinematics of a plasma stream rotating in the pulsar magnetosphere is presented. On the basis of an exact set of equations describing the behavior of the plasma stream, the increment of the instability is obtained, and the possible relevance of this approach for the understanding of the pulsar rotation energy pumping mechanism is discussed.

A novel model of particle acceleration in the magnetospheres of rotating active galactic nuclei (AGN) is constructed.The particle energies may be boosted up to 10 21 eV in a two step mechanism: In the first stage, the Langmuir waves are centrifugally excited and amplified by means of a parametric process that efficiently pumps rotational energy to excite electrostatic fields. In the second stage, the electrostatic energy is transferred to particle kinetic energy via Landau damping made possible by rapid "Langmuir collapse". The time-scale for parametric pumping of Langmuir waves turns out to be small compared to the kinematic time-scale, indicating high efficiency of the first process. The second process of "Langmuir collapse" -the creation of caverns or low density regions -also happens rapidly for the characteristic parameters of the AGN magnetosphere. The Langmuir collapse creates appropriate conditions for transferring electric energy to boost up already high particle energies to much higher values. It is further shown that various energy loss mechanism are relatively weak, and do not impose any significant constraints on maximum achievable energies.