Pulsar radio emission is composed in varying proportions of microstructure, subpulses and amorphous emission. Each component has its own time and frequency behaviour. The average pulse profile changes its width with frequency, typically as ν−0.26. The location (pulse phase) of the subpulses refered to a fiducial point within the profile follows the same ν−0.26 law as the width of the average pulse profile. The fiducial point in the pulse profile follows the interstellar plasma dispersion relation exactly: the source emission at this phase is simultaneous at all frequencies. The subpulse width is typically nearly constant with frequency. Emission of a subpulse may be systematically absent at certain frequency ranges, the pulse profile may be different there. Micropulses are typically very wideband: ∼1GHz or more. Individual micropulse widths are constant with frequency. Its time width distributions have upper limits (0.1-1 ms). The time separation (phase) of any micropulse with respect to the fiducial point is constant with frequency, i.e. it is simultaneous at all frequencies at source. The spectrum of individual micropulses appears to have breakpoints: it is observable at ν ≤ ν(μ co), but not above it. This effect (and not a steep spectrum) seems to account for the smaller number of micropulses at higher frequencies: at ν we see micropulses with ν(μ co) > ν only, micropulses with ν(μ co) < ν are not visible, at low frequencies we will see more micropulses than at high frequencies. Micropulses prefer the conal emission zones, and are very weak or absent in the core emission zones. More than one micropulse can be observed at a given time. Micropulses occasionally appear in trains with a nearly constant time separation, up to 20 micropulses have been observed in such quasi-periodic sequences. Sequence phase and period are variable within a train, they may have discrete values. Such trains can be observed simultaneously at widely separated frequencies. Ocasionally more than one quasi-periodic train may appear at the same time. Subpulses in general follow the linear polarization angle of the average pulse profile, with occasional 90° jumps. Within a micropulse the polarization angle can be constant, or can have a fast swing of ∼ 50°. Micropulse polarization angle can be widely different from that of the subpulse on which it is superimposed.

The distribution of millisecond pulsars, just as normal pulsars, is affected by a death line. Since a large fraction of millisecond pulsars lie close to the derived death line, a significant number of neutron stars may exist which are not observed as millisecond pulsars. This makes the birth rate discrepancy between millisecond pulsars and their assumed low mass x-ray binary progenitors more acute.

Polarization observations show the existence of a well-defined group of young pulsars with highly linearly polarized mean pulse profiles. Many of these pulsars have interpulses. I suggest that both the main pulse and the interpulse in these pulsars are part of a very wide conal beam emitted along field lines emanating from a single magnetic pole. Such wide beams may be responsible for all interpulse emission.

Pulsar radio intensities contain a rich range of timescale variations ranging from months down to nanoseconds. The shortest of these fluctuations have been called microstructure. The observational properties of microstructure and the astrophysical value of microstructure are discussed.

Recent observational data of short time-scale fluctuations in the radio emission of pulsars offer the opportunity for new tests of proposed emission models. For this purpose, the existing models must be developed to make predictions regarding the temporal characteristics of the emission. A computer solution of the time evolution of plasma wave turbulence details the properties of the nonlinear plasma emission mechanism. As a consequence of ponderomotive nonlinearity in the plasma medium, two-stream growing modes develop modulational instability, leading to the onset of strong, two-dimensional plasma turbulence. The turbulence exhibits explosive spatial collapse of regions of high electric field, and the escape of bursts of radiation.

We discuss observations of giant pulses from the millisecond pulsar PSR B1937+21 at 430 MHz, with a baseband sampling system at Arecibo (Shrauner et al., these proceedings) and coherent dedispersion techniques. About one pulse or interpulse per hundred thousand is stronger than 100 times the mean flux density, and the giant pulse strengths follow a power law distribution somewhat shallower than that of the Crab giant pulses. The individual giant pulses appear consistent with impulses shorter than a few microseconds, convolved with an exponential due to interstellar scattering. They are systematically delayed with respect to the average emission, and many are nearly 100% circularly polarized.

Although individual radio pulses from pulsars vary in amplitude from pulse to pulse, their height distribution in general does not extend to amplitudes more than 10 times the mean. Two notable exceptions are the Crab pulsar and PSR B1937+21 (Lundgren 1995, Cognard et al. 1996 and references therein) which occasionally emit single radio pulses that have amplitudes more than 100 times the mean. Here we report on the detection of short time-scale, extremely large amplitude radio pulses from the nearby millisecond pulsar PSR J0437–4715. The events we have observed are distinguished by having peak flux densities in excess of 10 times the average pulse amplitude, and occur only within a very narrow (80 µs) window centered on the main pulse.

We present the modelling results of full polarization data of the PSR J0437-4715, recently published by Manchester & Johnston (1995). This pulsar presents a real challenge for pulsar modelling because of seven clear components in its mean profile. In terms of the conal model of pulsar emission this is the first case indicating three distinct cones surrounding the central core beam. We have attempted to model this pulsar and obtained a very satisfactory fit to the data. We succeded to reproduce the complexities of the multiple component profile and polarization properties.

To model the radiation from PSR J0437-4715 we use the method presented in Gil et al. (1995 and references therein). We assume that subpulses in single pulses correspond to the subpulse-associated plasma columns distributed within the overall pulsar beam. Each column represents a gaussian intensity distribution. These columns rotate more or less regularly around the magnetic axis. The exception is the central column directed almost along the magnetic axis defined by the local surface magnetic pole. This column corresponds to the core beam and consequently to the core component in our model. Outer columns rotating at some distances from the core axis will form a number of coaxial conal beams corresponding to the conal profile components. Figure 1a represents a projection of the subpulse-associated columns onto the polar cap while Fig. lb represents schematically a corresponding multiconal structure of the average beam. The initial positions, the characteristic dimensions and the relative amplitudes of the subpulse spots (Fig. 1a) have been estimated from the mean profile (Fig. 2b). We assume that the subpulse spots are separated from each other by distances comparable with their sizes (Fig. 1a). The very large polar cap with the radius rp = 1.4 · 104P−1/2 cm ≈ 1900 m as compared with the characteristic subpulse spot dimension D ≈ 300 m allows three conal rings surrounding the central core beam (Fig. 1b).

Average pulse profiles are generally separable into several components whereas individual components can be described by a Gaussian shape. One generally distinguishes between core and conal components, although it is yet unclear whether core and conal components originate from the same (Lyne & Manchester 1988) or a different emission mechanism (Rankin 1983). Investigating this problem, we studied the behaviour of individual components observed in about 200 profiles of 18 pulsars measured at three different frequencies, i.e. at 1.4 GHz, 4.75 GHz and 10.55 GHz. We unambiguously determined the number, location, width, flux density and spectral index of individual components using a special technique (Kramer et al. 1994). The applied method ensures that only intrinsic features of the pulse profiles are fitted by comparing off-pulse noise statistics and post-fit residuals as a goodness-of-fit estimator. As a first result, we note that all high S/N pulsar profiles can be accurately described by using only a small number (usually ≤ 5) of gaussian components, although we also find an example of a seven component pulse shape (B0740–28). Fitted components determined for different observations of the same pulsar were averaged, producing a grand mean profile which is completely noise-free and thus suitable for a reliable flux density measurement of individual components, for an accurate determination of the pulse width to calculate opening angles, ρ, of the emission cone, or for use as a template for obtaining pulse arrival times.

We performed the multifrequency time aligned measurements and the component structure analysis of the integrated profile of PSR 0329+54 in frequency range from 0.1 to 10 GHz. The result is that a commonly adopted five-component structure of the integrated profile does not provide a good match to the observed profiles. Only a six-component structure fits observations well. This result calls into question the validity of the two conal and core zones emission model. We suggest that the emission region inside of the cone of the open magnetic field lines represents a mosaic bunch of the discrete outflows of relativistic charges along magnetic field lines, injected by mosaic group of localized sparks in the polar cap. Our suggestion is based on the Rutherman and Sutherland belief that the polar gap discharges through a group of localized sparks, spaced at distances nearly equal to the height of the polar gap.

Mode changing may be produced by the change of an activity of some sparks pattern without rearranging the structure of the emission region.

If each significant component in a single pulse from a radio pulsar represents emission from a single site and the intensity at longitudes other than the peak arises from the forward beaming of this emission, the angular distribution of intensity within a component is given by (Zheleznyakov, & Shaposhnikov 1972),

Here, , with ν the velocity of the emitting particles at the emission site, α is the spectral index of emission in the rest frame of the emitting particles, and is also the spectral index as seen by the observer, ν is the observed frequency of the radiation. Kνα is the intensity in the rest frame of the emitting particles. ϕ0 is the angular co-ordinate corresponding to the direction of motion of the emitting particles, and ϕ is the angular co-ordinate at different regions in a single component.

Pulsar 0943+10 demonstrates very obviously that the mode switching phenomenon includes simultaneous and dramatic alterations of all principal properties of radioemission, i.e. intensity, polarization (both linear and circular) and drifting behaviour of individual pulses over a broad frequency range. On the basis of observations of PSR 0943+10 at the frequencies 40 and 430 MHz, a complete scenario of the transition process between two modes of radiation has been revealed. The ‘pre-switching transition process’ is found which manifests itself as attenuation of the pulse intensity of the current mode during hundreds of the pulse periods preceding a switch.

There are three mechanisms to cause pulsar inner gap breakdown: the inverse Compton scattering (ICS) of the high energy particles off the thermal-peak photons, off the resonant-frequency photons and the curvature radiation (CR). The pulsar mode-changing phenomenon can be interpreted as a switching effect between the resonant ICS sparking mode and the thermal ICS sparking mode.

Rankin (1990) attempted to distinguish between one- and two-pole interpulse models for PSR 1929+10, and, despite the evidence to the contrary (Lyne & Manchester 1988; Phillips 1990), argued that it was probably a two-pole interpulsar on the basis of its narrow components. If it is a two-pole interpulsar, its interpulse has just the right width to be a core component—that is, , the angular width of the polar cap. However, PSR 1929+10 was virtually the only well studied pulsar of any profile class which could not be comfortably fitted into the quantitative description of the conal emission geometry as discussed by Rankin (1993 a,b). The problem arose due to the narrow dimensions of the conal component pair in this pulsar, which do not fit well with the inner and outer cone properties followed by many pulsars. In order to explore this issue further, we use the technique of constructing partial ‘mode-separated’ profiles corresponding to the primary and secondary polarisation modes.

We have analysed the dipole magnetic field geometry for the general case of an oblique rotator and have found that open field lines which define the polar cap divide into two branches (Kapoor and Shukre 1996) which appear naturally relevant for distinguishing the core and conal emissions. The polar cap shape is actually determined by a quadratic equation having two roots leading to two values of the polar angle, θ+ and θ− with respect to the magnetic axis for a given azimuth φ. For the north pole both θ+ and θ− branches are shown as polar plots in Fig. 1 for various inclination angles α and a typical pulsar period . The discussion of pulsar polar caps hitherto (e.g. Biggs 1990) had not distinguished between the θ+ and the θ− solutions. The region defined by the θ+ solution is completely contained inside the polar cap. It has a peculiar triangular shape whose lowest vertex is always on the magnetic axis. This naturally suggests an identification of the θ+ and the θ− regions with the core and conal emission zones.

We present a comparison between the average radio pulse profiles of millisecond pulsars (MSPs) in the field and in globular clusters. Our sample consists of 20 field MSPs and 25 cluster MSPs for which observations exist at 400 - 600 MHz.

We find that 6 of the 20 field MSPs, or about 30%, have a comparable interpulse at a phase offset of 180 ±30 degrees. None of the cluster objects shows this feature. Here we define a ‘comparable interpulse’ as one whose intensity is at least 10% of the primary pulse. This lower limit is well above the noise level of all the profiles in our sample. While the cluster MSPs have much more poorly resolved profiles at present than field MSPs, it is unlikely that a strong interpulse near 180 degrees could be ‘hidden’ within the primary peak.

Pulse shapes of pulsars have been observed at many frequencies with the Effelsberg 100-m radiotelescope since 1972. The rationale behind these surveys was to use the great sensitivity of the 100-m telescope of the Max-Planck-Institut für Radioastronomie in particular at high radio frequencies for a complete study of pulse profiles.

The recent advent of HEMT amplifiers has improved the observational sensitivity enormously. Using these new systems it was possible to obtain integrated profiles of 183 pulsars at 1.4 GHz, 46 pulsars at 4.75 GHz and 24 pulsars at 10.6 GHz (see Seiradakis et al. 1995). Furthermore eight pulsars have been studied at 9 mm wavelengths (Kramer et al. 1996).

This paper discusses observational and analytical questions pertaining to the pulsar emission problem. A short outline of the area is given for those new to the subject. Some of the literature pertinent to pulsar polarization, emission and beaming, which has appeared over about the last five years is mentioned. There is a short discussion of efforts to carry out polarimetric observations of higher quality, and finally, there is a short discussion of recent work by the author and her colleagues.

Over a two year period the polarization properties of the largest sample of millisecond pulsars yet observed have been regularly monitored with the Effelsberg 100-m radiotelescope at λ21cm. A comparison of the observed emission properties with those of the population of long period pulsars is presented here. Most of the sources exhibit wide, often multiple-component profiles, with complex polarization features, not easily interpretable in the context of the well established phenomenological models used to describe the properties of long period pulsars. While both groups share common emission features suggesting similar emission processes, the implied field structures as revealed from polarization information appear substantially different from a dipolar structure. Generally, short period pulsars exhibit a relatively large percentage of circularly polarized emission while at λ21cm they tend to be less polarized than long period pulsars.

To date there have been few polarimetric observations of millisecond pulsars, mainly due to the instrumental challenge of simultaneous high time resolution and large bandwidth. Such observations provide our most direct clues to the structure of the magnetic fields around pulsars, and are especially important in the case of millisecond pulsars, where radio emission necessarily originates very close to the neutron star surface.

We have observed the bright millisecond pulsar J0437–4715 at the Parkes Radiotelescope with the Caltech Fast Pulsar Timing Machine, at several radio frequencies and in full polarimetric mode. Our analyses show significant deviations from the standard dipole field geometry.