Slow pulsars show a great deal of qualitative and quantitative regularity in the structure of their radio emission beams as described by the core/double-cone model; however, millisecond pulsars (MSPs) have shown little. It is thus arresting to encounter a 2.7-s MSP with what appears to be a double-cone/core profile—and even more so to find that the arrangement of the cones around the core suggest aberration/retardation emission heights that are very reasonable. This and several other pulsars then represent rare opportunities for analysis and raise questions about why a few MSPs show such orderly beam structure while so many do not.

Pulsars were discovered on the basis of their individual pulses, first by Jocelyn Bell and then by many others. This was chart-recorder science as computers were not yet in routine use. Single pulses carry direct information about the emission process as revealed in the detailed properties of their polarization characteristics. Early analyses of single pulses proved so dizzyingly complex that attention shifted to study of average profiles. This is turn led to models of pulsar emission beams—in particular the core/double-cone model—which now provides a foundation for understanding single-pulse sequences. We mention some of the 21stC single-pulse surveys and conclude with a brief discussion of our own recent analyses leading to the identification of the pulsar radio-emission mechanism of both slow and millsecond pulsars.

Using the proper-motion as a proxy for a pulsar’s rotation-axis orientation, the emission polarization can be related to the magnetic field direction in the emission region, providing a much more physical foundation for understanding orthogonal mode radiation.

We present high-sensitivity XMM/LOFAR observations that show for the first time that mode switching extends from radio to X-ray. In pulsar B0943+10, the known changes in radio profile and drift rate are confidently tied to simultaneous changes in X-ray emission.

In mode switching, seen in many pulsars, profile and subpulse-drift behavior change almost instantaneously. The mechanism for these drastic changes, or for the bi-stable emission behavior, is not understood; while even for the basic emission mechanism different families of theories (e.g. vacuum gap, or space-charge limited flow models) exist.

To discriminate between such models, we carried out a campaign of 42 hours of simultaneous LOFAR/XMM-Newton observations on PSR B0943+10. Through LOFAR's unparalleled low-frequency sensitivity, mode changes were pinpointed to 10-second accuracy. XMM X-ray photons were next separated by the two radio modes. We discovered large modal differences in the X-ray pulsations, flux, and spectral shapes: in the radio-bright mode, no X-ray pulsations are detected – but in the radio-quiet mode, B0943+10 is strongly pulsating in X-ray.

Multi-frequency, single-pulse polarimetric observations of the Vela pulsar have been made using the VLA. These high time resolution measurements allow us to probe the geometry, and the time dependent characteristics of the radio emission region. Evidence for multiple emission components with differing spectral indices is found.

Building on the analysis of the previous paper by one of us, we here turn first to a discussion of B0943+ 10’s chaotic “Q” mode and then to the polarisation properties of its “B”-mode radiation. Here again, we see evidence that the plasma processes responsible for pulsar emission may be organized into a system of columns. In short, 0943+10 provides unprecedented insight into the phenomena comprising what might be termed pulsar polar-fluxtube “weather”.

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 explore the detailed polarisation behaviour of pulsar 0823+26 using the technique of constructing partial “mode-separated” profiles corresponding to the primary and secondary polarisation modes. The characteristics of the two polarisation modes in this pulsar are particularly interesting, both because they are anything but orthogonal (Backer & Rankin 1980), and because the secondary mode exhibits structure seen neither in the primary mode nor in the total profile.

Using the model-angle method of polarisation-mode deconstruction (Rankin et. al. 1989; Cordes et. al. 1978), we can take the following representation of the single-vector model to define the respective primary and secondary-mode PA traverses

where χ is the PA, ϕ the longitude, the maximum PA sweep rate, and A a constant which can be computed from the total PA traverse, χc between reference longitudes ±ϕc as

and which is formally equal to sin ζ/ tan α, where α and ζ are the angles that the magnetic axis and the sight line make with the rotation axis, respectively.

A marked anti-correlation between the degree of linear polarisation dL and the intensity I, has been reported for some pulsars (Manchester, Taylor & Huguenin 1975; Xilouris et. al. 1994). A detailed investigation of the variation in the total polarisation of single pulses for some pulsars has been done, with proper statistical correction applied to the linearly polarised power. This study, then, reveals very different behaviour as well as some remarkable correlations in many pulsars, which reflect directly on the nature of pulsar radio emission.

The most important correction to be applied to linear polarisation data arises from the differences in the statistical properties of the Stokes parameters Q and U, defining the linear polarisation, and the linearly polarised power The measured value of the linearly polarised power is larger than the true value, with the difference being , where σ is the measured standard deviation in the off-pulse region in any of the Stokes parameters (Rankin, Campbell & Spangler 1975).

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.

We have developed a method to understand the detailed structure of the mean pulse profile of pulsars, namely, the Gaussian fit separation of the average profile (GFSAP) method, in which we assume the total profile is the superposition of several individual Gaussian-distributed components. Linear polarization data is used to simplify the fitting. The components of PSR 1451–68 are separated and their spectral indices calculated. We find that the profile of PSR 1451–68 is actually quintuple instead of triple, which is in agreement with the idea that quintuple is the prototype shape of pulsar profiles. The core component is shown to have the steepest spectrum, while the inner cone has the flattest. Another conclusion from PSR 1451–68 is that the core width does not change with frequency.

We present average polarization profiles for 20 pulsars at 1720 MHz made with the 100-m Effelsberg radio telescope. All four Stokes parameters were appropriately delayed in a linear dedisperser to correct the interstellar dispersion on-line. Our sample includes a number of complex profiles which were observed polarimetrically for the first time at a high frequency, with the aim of describing their frequency evolution. Classification of the corrected profiles is attempted according to the empirical pulsar model described earlier by Rankin(1990).

Pulsar 0301+19, whose two well-separated components provide a textbook example of the conal double profile, apparently evolves to ajeingle form at frequencies above 5 GHz. PSR 2016+28, by contrast, exhibits a single form above 200 MHz, but bifurcates strongly and very asymmetrically below 100 MHz. In few pulsars can we see the full possibilities of conal profile evolution so clearly manifested.

Some 15 members of the class of profiles with five components have been identified. These profiles have a central core component and two pairs of conal outriders, thus exhibiting the highest degree of formal complexity observed. No good examples of pulsars with six or more profile components are known.

Members of the five-component class have very similar physical parameters and emission-region geometries. An analysis of their emission geometry indicates both that the core and conal beams scale as P–1/2 and that the 1-GHz conal emission comes from a height of some 100-200 km.

A system of pulsar profile classification is used as a starting point to study the emission characteristics of pulsars. Two types or mechanisms of pulsar radiation are identified which combine geometrically to produce five major species of profile. The core emission, which forms a pencil beam of radiation, is apparently produced close to the stellar surface throughout the entire polar cap region by low γ particles. The conal emission, which consists of a hollow conical beam, then seems to be emitted at heights of 10 to 20 stellar radii by currents of high γ particles travelling along some of the most peripheral of the “open” field lines.

Polarized, pulsar profiles consist of two distinct types of emission units, core and conal components. Various configurations of these two basic units then underlie the 5 principal species of profile: conal and core single, conal double, triple, and multiple (5 comp.) profiles. Conal single profiles typically evolve through bifurcation into conal double forms at low frequency; whereas core single profiles evolve into triple forms at high frequency by adding pairs of conal “outriders”. Conal components and outrider pairs exhibit polarization properties compatible with a hollow-conical emission zone which spreads weakly at low frequency. The central core components in each profile species, by contrast, often exhibit circular polarization of symmetrically alternating sense. The triple profile is then most generally prototypical of pulsar emission, and the physical distinction between its outriding conal components and its central core emission feature are primary–implying two distinct corresponding radiation mechanism. (Ap. J. 274, 333/359).

The isolated core components of stars with core single profiles exhibit little ordered modulation, nor do they null or evince of any sort of mode changing; many have featureless fluctuation spectra, while others exhibit low-frequency (P3 = 15–50 periods/cycle) features with which no orderly “drift” is apparently associated. The core components in triple, double (“saddle” region), and multiple profiles also exhibit these stationary, low-frequency fluctuations. “Drifting” subpulses are then an exclusively conal phenomenon. Systematic subpulse modulation (P3 = 2–15 periods/cycle) is associated with the conal components of stars with double, triple, and multiple profiles, but progressive, orderly drifting is observed only in conal single stars. The ensemble P3 values associated with the conal components of the various profile species seem indistinguishable, again suggesting that these species are geometry-specific manifestations of a single physical configuration. Mode changing and pulse nulling are exclusively associated with neither core nor conal emission, but rather connect the two. Mode changing is typically manifest as a reorganization of the conal constituents of emission about the profile's core component. Finally, pulse nulling is not so closely associated with spindown age as has been thought. Pulsars which null are members of older profile species to be sure, but within a given species, those stars which null are no older than those that do not. (1986 Ap. J. 301, 901)