An estimate of the effect of light bending and redshift on pulsar beam characteristics has been made using a weak Kerr metric for the case of a 1.4 M⊙ neutron star with a radius in the range 6-10 km and rotation periods of 1.56 ms and 33 ms, respectively. Assuming that the pulsar emission has the form of a narrow conical beam directed away from the surface and is located within two stellar radii, the beam is found to be widened by a factor of ≤ 2 and to suffer a reduction in the intensity (flattening of the profile) by an order of magnitude or less. The effect is largest for the most rapidly rotating the neutron stars. For an emission region located beyond 20 km, the flattening is generally insignificant. The pulse profile is slightly asymmetrical due to dragging of the inertial frames. For millisecond periods, aberration tends to reverse the flattening effect of space-time curvature by narrowing the pulse and can completely overcome it for emission from a location beyond ≃30km. Although the pulse must slightly brighten up, a large redshift factor overcomes this effect to keep the pulse flattened for all neutron star radii considered here.

We investigate several instabilities which give different characteristic times for nonstationary processes. Several instabilities are connected with the mechanism of plasma generation in the polar cap gap region. Another nonstationary process is due to the nonlinear phenomenon arising in the magnetosphere during the propagation of the flux of electromagnetic radiation.

At present there exist several well-grounded models of pulsar radiation which do not exactly coincide with each other [e.g. Ruderman and Sutherland (1975), Cheng and Ruderman (1980), Arons and Sharlemann (1979), Arons (1981)]. The creation of a dense, relativistic, electron-positron plasma in the polar regions of rotating neutron star magnetospheres is the point of similarity between these models. Surely the pulsar radiation should be generated in such a plasma. The plasma density near the stellar surface is np ≃ 1016 e to 1017cm–3, and the average Lorentz-factor of the particles is γp = 3 to 10. The plasma is penetrated by the beam of “primary” electrons, extracted from the stellar surface and accelerated by the electric field. The beam parameters are as follows: nb = 7 x 10–2B0P–1, where P is the pulsar period and B0 magnetic field at the stellar surface and γb = 3 x 106 to 107.

As noted previously in this meeting, any millisecond pulsar that is not perfectly aligned must obviously be strongly charged electrostatically simply because the wave zone is so close to the surface that any trapped neutralizing electrosphere over the polar caps will be driven away beyond 6 or 7 neutron-star radii. Acceleration of stray charges (e.g., from the ISM or a nearby orbiting disk) in such a highly charged system automatically produces coherently radiating bunches of downward moving particles (Michel 1990), this coherent radiation simply reflecting from the surface (or missing the star entirely, as an alternative possibility). This mechanism, suggestive of how pulsars might operate, we will call a pair-production avalanche.

The vast majority of pulsar profiles at meter wavelengths are dominated by core components (Rankin 1983, 1990; Lyne and Manchester 1988), but in the usual polar cap models of pulsar emission, it is difficult to get central beam or “core” radiation. In this paper, we present a calculation for both the “core” and hollow “cone” emission beams, as well as model pulse profiles in an inverse-Compton scattering (ICS) model. Both “core” and hollow “cone” emission beams axe obtained naturally in the calculations. Examples of pulse profiles of pulsars at different radio frequencies are presented.

The theoretical shapes of the pulse profiles agree very satisfactorily with actual observations, which means that the mechanism suggested here may be the actual one.

The physical conditions and locations of the emission regions for core and hollow cone emissions are very important in understanding the mechanism of radio pulsars. We present two related methods in an Inverse-Compton Scattering (ICS) model in this paper, which give a clear scenario for determining the location of the emission regions and are consistent with the results given by Cordes et al. (1984) and Rankin (1990).

The flow of e+e− plasma ejected by an axisymmetrically rotating magnetized neutron star is considered in a hydrodynamical approximation. It is shown that in the vicinity of the light cylinder a helical discontinuity is formed. The transformation of toroidal magnetic field energy into plasma energy takes place at this discontinuity. Particles are accelerated to an energy of 10TeV for a neutron star with the characteristics of the Crab pulsar.

The induction of the electric fields near a rotating neutron star is considered within the framework of General Relativity. It is demonstrated that within the open magnetic field region, filled by relativistically moving charged particles, a sufficiently strong component of the electric field is generated. This component is due to the effect of dragging of inertial frames of reference and predominates in the case when a neutron star is not exactly an orthogonal rotator. Finally, we discuss some implications of our results on the theory of radio pulsars.

Magnetospheric issues are discussed as they relate to radio intensity variations, spindown, rotational noise, and emission altitudes. Radio fluctuations trace the overall state(s) of the magnetosphere in spite of the fact that Lradio ≪ IΩ. Five methods for estimating emission altitudes re are discussed. Four measure either the transverse width or radial depth of the emission region; assumption of a dipolar field and, in some cases, a radius to frequency mapping, yields estimates for the absolute radius of the emission region. The fifth method (v/c effects in Stokes parameter waveforms) provides direct altitude estimates. Most results suggest that re ≲ 0.02 RLC at v = 0.4 GHz. A radius to frequency mapping appears viable for some but not necessarily all objects. The mapping need not be one-to-one. Prospects are discussed for using simultaneous radio and high energy observations to make further progress in our understanding of magnetospheres.

We have studied the individual pulses from PSR 1055–52, a pulsar which has a strong interpulse. Their properties, and those of the average pulse profile are consistent with an emission geometry in which the magnetic and rotation axes are nearly perpendicular. That is, we are observing both magnetic poles of this pulsar. A correlation between the emission from the two magnetic poles was found wherein a strong interpulse is followed by a strong main pulse. Also, the main pulse appears to undergo mode changes that are primarily characterized by a change in the peak flux density of component I.

We have sought correlations between the fraction of null pulses with other pulsar parameters for an ensemble of 72 pulsars using survival analysis methods. The strongest correlation was found between the null fraction and pulse period. Correlations were also found between other parameters that typically have strong dependencies on pulse period, and this tends to indicate that the null fraction increases with age as was first suggested by Ritchings (1976). However, no explicit correlation was found between pulsar characteristic age and null fraction. A significant anti-correlation was found between the angle subtended by the magnetic and rotation axes and the null fraction.

Many of the pulsars presented here were found to null. In particular, all pulse profile classes in the scheme devised by Rankin (1983a) have members that null. Differences in the mean age of these pulsar classes are not very pronounced, and the influence of class on pulse nulling statistics is probably less than that suggested by Rankin (1986), but cannot entirely be ruled out. Also, there is considerable variation in the fraction of null pulses from pulsars within each class, but generally class St pulsars null the least. Of special note is the fact that two pulsars PSR 0833-45 and PSR 1556-44 apparently do not null. The upper limit for PSR 0833–45 is quite low; no nulls were detected in observations of over 120,000 pulses.

The similarity of the nulling parameters of pulsars observed at two frequencies near 400 MHz and 843 MHz suggests that the pulsar emission mechanism is wide band over this frequency range.

Single-pulse radio emission from pulsars has been observed to vary both in intensity and position inside the pulse window. In some cases the variations show a rather regular structure (drifting subpulses). Such a regular structure can clearly be studied by fluctuation spectral analysis. The analysis has identified some particular peculiarities in the fluctuation spectra at different longitudes of the pulse window in many pulsars (Backer 1973, Page 1973, Popov and Smirnova 1982). In pulsars with complex integrated profiles (PSR 1919+21, PSR 1237+25) narrow features in the fluctuation spectra vary strongly with longitude, at some longitudes being quite undistinguishable.

We present an analysis of high signal-to-noise ratio, 430-MHz total intensity single pulse observations from PSR 2303+30 recorded at the Arecibo Observatory in August 1986. We believe that the regular subpulse modulation observed in this pulsar can be explained only in terras of subpulse-associated emission beams rotating around the axis of the pulsar beam. This idea was first introduced by Ruderman and Sutherland (1975) in their spark model. We use the quantitative prediction of this model to calculate the pulsar magnetic field. For this reason we need to estimate the rotation period of a spark around the magnetic pole. We estimate this period by reproducing the observed pulse sequences within an angular beaming model of subpulse emission and fitting this model to the data.

This paper presents results on four radio pulsars observed at Arecibo. Three pulsars have been discovered as new mode switching pulsars. In two of them the switching effect is correlated with a dramatic change of the amplitude of the core component, suggesting that the change occurs not only in the location of the emission region, but also in the efficiency of the emission mechanism. In another pulsar evidence is presented for the existence of a third (possibly a core) component which is hidden between the two conal components of the average profile. Previously unknown drifting subpulses and at least two new components have been found in one of the three mode switching pulsars.

Regular structure with 10 to 50kHz-scale was detected in the radio scintillation spectrum of PSR 1919+21. This regular structure may be a simple interferometric pattern formed by refractive multi-ray propagation between the pulsar and observer. It was found that the pulsar PSR 1919+21 radio emission scintillation spectrum phase changes significantly across the average profile. Within the refractive scattering scenario this phase-longitude dependence corresponds to a transversal shift of the magnetosphere emitting region of about 6 x 108 cm. The radio emission altitude is estimated to be near the light cylinder.

Diffractive scintillations from large-scale irregularities of the interstellar medium were observed for 5 strong pulsars at 102.7 MHz. The shifts of the time cross-correlation function of intensity variations at different longitudes of the profile were obtained as a function of longitude separation (phase curve). The shift is defined by the transverse separation of the pulsar emission sources. The maximum values of the spatial separation are: 1-2 x 108 cm. We show that the phase curves are inconsistent with the dipole model of the magnetosphere and change from day to day not only in amplitude, but also in direction.

Correlation analysis of subpulse spacing P2, subpulse bands P3, subpulse drift rate with some quantities such as pulsar period P, profile width We, characteristic age r and others indicates some new casual dependencies not previously confirmed. Relations obtained in this paper possibly point out that interaction between pulsar radio emission and the interstellar environment may exert some influence on the drift appearance.

The subpulse drift phenomena—that is, periodic subpulse phase variations in the individual-pulse sequences—was discovered by Drake and Craft (1968) in the beginning of the pulsar era. Many observational programs have been carried out since then, and it has been shown a) that only a minority of pulsars exhibit the phenomenon, and b) that only some of these can be characterized by a definite drift rate (Huguenin, Taylor, and Troland 1970, Backer 1970, Taylor and Huguenin 1971).

We use a perturbation technique to compute the rotational corrections to the non-radial oscillation spectrum of a realistic neutron-star model. We compute, to first order in the rotation rate, the corrections to the normal mode eigenfrequencies and eigenfunctions. We find that l = l0 oscillations are coupled to l = l0 ± 1 oscillations by the Coriolis force. For the toroidal modes, this coupling introduces a non-zero radial component to the velocity field. We have used this result to compute the neutrino damping rates for several corrected toroidal modes. This damping mechanism is inoperative for toroidal modes in a non-rotating star because these modes produce no density nor temperature perturbations. The neutrino damping time can approach the gravitational radiation damping time in rotating neutron stars if the central temperature is high enough, (Tc ≥ 108 K). The rotationally induced coupling of spheroidal oscillations to toroidal modes can also produce significant displacements at the stellar surface. This may have interesting implications for channeling energy, e.g., that associated with a glitch, to the surface of the star. Perhaps this might produce observable effects in the pulsar emission process or a γ-ray burst event.

Coherent emission mechanisms may be classified as (i) maser mechanisms, attributed to negative absorption by resonant particles in a resistive instability, (ii) a reactive or hydrodynamic instability, or (iii) to emission by bunches. Known coherent emission mechanisms in radio astronomy are plasma emission in solar radio bursts, maser emission in OH and other molecular line sources, electron-cyclotron maser emission from the planets, and pulsar emission. Pulsar radio emission is the brightest of all known coherent emission, and its brightness temperature is close to the maximum conceivable in terms of energy efficiency. Three possible pulsar radio emission mechanisms warrant serious consideration in polar cap models; here these are called coherent curvature emission, relativistic plasma emission, and free electron maser emission, respectively.

1. 1. Coherent curvature emission is attributed to emission by bunches. There is a fundamental weakness in existing theoretical treatments which do not allow for any velocity dispersion of the particles. There is no satisfactory mechanism for the formation of the required bunches, and were such bunches to form they would quickly lose their ability to emit coherently due to the curvature of the field lines.

2. 2. Relativistic plasma emission is a multistage emission process involving the generation of plasma turbulence and the partial conversion of this turbulence into escaping radiation. In pulsars the dispersion characteristics of the relativistic electron-positron plasma determines the form of the turbulence, which may be in either longitudinal waves or Alfvèn-like waves. Various instabilities have been suggested to produce turbulence, and a streaming instability is one possibility. Alternatively, in a detailed model proposed by Beskin et al. (1988) the instability depends intrinsically on the curvature of the field lines, and in a theory discussed by Kazbegi et al. (1988), a cyclotron instability generates the turbulence relatively far from the neutron star.

3. 3. Free electron maser emission or linear acceleration emission requires an oscillating electric field, postulated to be due to a large amplitude electrostatic wave. A recent analysis of this mechanism (Rowe 1992) shows that it allows emission in two different regimes that provide a possible basis for the interpretation of core and conal emission in pulsars. Effective maser emission seems to require Lorentz factors smaller than other constraints allow.

Other suggested theories for the emission mechanism include one that arises from a loophole in the proof that curvature absorption cannot be negative, and another that involves a closed “electrosphere” in which the radio emission is attributed to emission by bunches formed as a result of pair production due to a primary charge accelerated towards the star by its Coulomb field.