Using a refractive scintillation event of PSR 1237+25, Wolszczan & Cordes (1987) inferred a projected displacement of the emission region of order 108 cm across the pulse. Gupta (these proceedings) inferred a lower separation limit of a few times 107 cm from a similar event for PSR 1133+16. In both cases the projected displacement was also a nonmonotonic function of pulse phase.

Neither these large separations nor the nonmonotonic behavior can be explained in a simple model which assumes that the emission originates parallel to the field lines of a magnetic dipole, at a fixed altitude across the open field zone, and propagates in a straight line to the observer. There is considerable circumstantial evidence for dipolar fields from polarization sweep and pulse morphological studies. Time delays due to different emission altitudes could produce asymmetries in pulse profiles and projected displacements, but cannot explain the large separations if the emission originates well within the light cylinder.

Mestel et al. (1985; MRΩ2) introduced an axisymmetric pulsar magnetosphere model in which electrons leave the star with non-negligible speeds and flow with moderate acceleration, and with poloidal motion that is closely tied to poloidal magnetic field lines, before reaching SL, a limiting surface near which rapid acceleration occurs. As well as these Class I flows, there exist Class II flows which do not encounter a region of rapid acceleration (Burman 1984, 1985b). The formalism introduced by MRΩ2 to describe the moderately accelerated flows can be interpreted in terms of a plasma drift across the magnetic field, following injection along it (Burman 1985a).

The MRΩ2 formalism fully incorporates the toroidal magnetic field generated by the poloidal flow. The general formalism leaves the poloidal magnetic field unspecified, but, in the detailed development of MRΩ2, and in my papers, that field was taken to be the dipolar field of the star.

Numerical work by Fitzpatrick & Mestel (1988a,b) suggested that the dipole approximation is inadequate. They developed a numerical technique for incorporating the modification to the poloidal magnetic field that is generated by the toroidal motions throughout the magnetosphere. They based their treatment on the hypothesis that those motions are such as to cancel the dipole field of the star, leaving a sextupole poloidal magnetic field at large distances.

We present a scenario of magnetic field (MF) evolution of newly-born neutron stars (NSs). Numerical calculations show that in the hot phase of young NSs the MF can be amplified by thermoelectric effects, starting from a moderately strong seed-field. Therefore, there is no need to assume a 1012 G dipole field immediately after the gravitational collapse of the supernova (SN) event. The widely accepted scenario for such a field to be produced by flux conservation during the collapse is critically discussed. Instead, it can be generated by amplification and selection effects in the first 104 yrs, and by the subsequent fast ohmic decay of higher multipole components, when the NS cools down.

The successful theory for radio propagation through the interstellar plasma is reviewed, where the density spectrum follows the Kolmogorov model. However, there are also several observations indicating more refraction than expected. A particular model of enhanced refraction is proposed in which isolated regions of the warm ionized medium support filaments of enhanced density on a scale of 1 AU, which do not couple to a turbulent cascade.

Pulsars are excellent probes of the galactic free electron layer. Interstellar dispersion and scattering measurably affect the observed pulsar signals, thereby providing information on the distribution and density of the free electrons causing these phenomena.

Primary calibration of galactic electron density models is achieved through adjusting their parameters to fit the observed dispersion of pulsars having independently measured distances. The distances are determined via kinematic analyses of HI absorption spectra, through angular or timing parallax measurements, and from associations with other objects of known distances.

The models have become steadily more refined as the body of data upon which they are based has grown. Independent distance measurements continue to accrue. The discovery of pulsars in globular clusters provided high latitude lines of sight for probing the z-distribution (Reynolds 1989). Additional calibration has been provided through incorporation of interstellar scattering measurements into the modelling process (Cordes et al. 1991). Individual spiral arms are now explicitly modelled (Taylor & Cordes 1993).

While great progress has been achieved with these models, there are still uncertainties in modelling the electron density of the local region and the inner Galaxy, and in the z–distribution of the electron layer. Currently anticipated observations will help to resolve some of these issues.

We have monitored the 610 MHz flux density of 21 pulsars on a daily basis for five years. The flux density time series for these pulsars range from nearly constant for the most distant and heavily scattered pulsars to rapidly varying, saturated time series for more nearby pulsars. The measured stability of the flux density from the most distant pulsars (variations less than 5%) implies that the average radio emission from pulsars, before it has been affected by propagation through the ISM, is constant in strength over five years. The flux variations for 12 of the pulsars are consistent with a Kolmogorov turbulence spectrum over a range of more than three orders of magnitude in scattering strength, with no detectable presence of an inner scale (si ≥ 107cm). The flux variations are greater than predicted by this model for five pulsars – including the Crab and Vela – but this group is consistent with a Kolmogorov spectrum and an inner scale of ∼ 1010cm.

We use radio-wave scattering in the Vela supernova remnant, surrounding the Vela pulsar, as an AU-scale lens to study the Vela pulsar’s emission region.

Refraction of pulsar radiation by electron density irregularities in the interstellar medium can sometimes produce multiple imaging of pulsars. Such events can be used as tools to resolve the emission regions in pulsar magnetospheres. Results from recent observations of a double imaging event for PSR B1133+16 are described. Significant fringe phase variations are seen across the pulse. The minimum value for the transverse separation of the emitting regions at the two edges of the pulse is inferred to be ≈ 5 × 105m. These results are compared with earlier values reported in literature and the implications are discussed.

High-precision timing of the millisecond pulsars PSR B1937+21 and PSR B1821-24 are conducted at 1.4 and 1.7 GHz with the high rate of observations of 10-15 times per month at Nançay. Time Of Arrival (TOA) and pulsar flux density are both measured. The rms of the daily post-fit TOA residuals are at the level of 0.5μs and 3.0μs, respectively. A characterization of refractive scintillation events produced by the Ionized Interstellar Medium (IISM) both in the TOA residuals and flux density variations is discussed.

Timing at Nan^ay is conducted with a pre-detection dedispersion system. The analysis of the TOAs was done with our software AnTiOPE solving for the classical pulsar parameters. The flux densities of the pulsars are averaged over ~30 or ~70 minutes. Calibration of the system temperature and efficiency of the telescope is good at the 10% level and diffrative scintillation limits this calibration to a comparable level (~35 scintels for PSR B1937+21 over 30 minutes and 7.5 MHz).

The spectra of radio flux from pulsars show deep modulations, over a typical time scale Δtd and frequency scale Δfd. These random diffractive interstellar scintillations are caused by inhomogeneities in the interstellar plasma on scales of 105m to 106m; on occasion there are also refractive modulations caused by very much larger scales. We report here an episode of periodic modulations of the dynamic spectrum of PSR B0834+06 seen in the sequence of observations centered at 408 MHz reported by Gupta et al. (1994 - GRL). There were 12 observations of about 2 hours covering 5 MHz spaced over 14 months. An episode of very fine periodic fringes was detected, though the period 45 kHz in frequency was barely resolved by the 19.5 kHz channel bandwiths; the period in time was about 5 minutes and was fully resolved. The normal Δfd was about 4 MHz (GRL) - ≈90 times the fringe period.

Refractive and diffractive scintillation are thought to be two related effects of scattering in the interstellar medium. The predicted modulation of diffractive scintillation parameters by refractive effects has not been thoroughly tested, however. We report 24 days of scintillation observations of the radio pulsar PSR B0329+54 from September 1993 to June 1994 with a 26-m telescope operating at 610 MHz. Dynamic spectra were obtained on each day, from which the characteristic bandwidth and time scale of the diffractive scintillation pattern were determined. A relative flux value for the pulsar was also obtained on each day. The cross-correlation coefficient for each unique pair of the observables — flux, δF, bandwidth, δν and time scale, δt — was formed from these data. The observed values of these coefficients and their 67% confidenceintervals are: CδFδν = −0.4 ± 0.2, CδFδt = −0.7 ± 0.1, and Cδνδt = 0.5 ± 0.2. Theoretical expectations are that CδFδν = −0.8, CδFδt = −0.6, and Cδνδt = 0.8. We conclude that we have detected the expected modulation of diffractive parameters by refractive scintillation. This adds confidence to the interpretation that diffractive and refractive scintillation of compact radio sources are caused by the same underlying scattering process.

Pulsars are very small objects and to resolve a pulsar magnetosphere you would need an interferometer with a baseline of 14 AU at a frequency 100 MHz. If the angular size of the diffraction pattern at the Earth is less than the angular size of pulsar: θc < θLC then emission regions separated by alight-cylinder radius will produce scintillations that are independent. The interstellar scintillation method presents a unique oppotunity for measuring the angular size of sources with a resolution up to 10−8 arcsec at meter wavelengths.

There were different attempts to resolve pulsar magnetospheres by the scintillation method (Cordes, Weisberg and Boriakoff, 1983, Wolszcan and Cordes, 1987; Smirnova and Shishov, 1989; Smirnova, 1992). Here we present new observations of 4 pulsars: PSR 0834+06, 1133+16, 1237+25 and 1919+21 at a frequency 102 MHz from which we can see evident decorrelation of the pulsar spectrum with increasing space separation between sources in the pulsar magnetosphere.

Interplanetary Scintillation (IPS) is a diffraction phenomenon in which coherent electro-magnetic radiation, from a distant radio source, passes through the solar wind, which is a turbulent refracting medium, and suffers scattering. This results in random temporal variations of the signal intensity (scintillation) at the Earth. IPS observations to monitor the interplanetary medium (IPM), to measure solar wind velocities in the directions of a number of compact extra-galactic radio sources and to estimate the angular diameters of their compact components have been carried out for many years at 103 MHz using the IPS facility (Alurkar et al. 1989) of the Physical Research Laboratory (PRL), Ahmedabad, India. Over a period of approximately 10 years of such observations, it has been seen that PSR 0950+08 has always remained well within the noise level. We observed a sudden enhancement in its flux (Deshpande et al. 1994) on 29 July, 1992. The ionospheric observations which are being carried out on a regular basis at PRL using an ionosonde also recorded a steep increase in the absorption index Fmin, which is an indicator of the excess ionization produced due to X-rays. During this period, the Sun was exceptionally quiet, thereby raising the question about the origin of the X-ray flux responsible for the observed steep rise in Fmin during the transit of PSR 0950+08.

We have observed 21-cm absorption and emission spectra in the direction of about 20 southern pulsars with the Parkes telescope to derive their kinematic distances and to study the interstellar medium. In each case we thoroughly investigated the environment of each pulsar as well as sources along the line of sight which may contribute to the pulsars’ dispersion measure (DM).

Some of the results, which are published in Koribalski et al. (1995) and Johnston et al. (1996), are included in the compilation of Hi absorption measurements by Joel Weisberg in this volume. In the following we present a detailed discussion on pulsars toward 1) the Gum Nebula and 2) the Carina Arm.

Many papers were in the past devoted to the investigation of the interstellar medium (ISM) with the help of pulsars, most of them studying the time and frequency structure of pulsar flux variations in the strong scintillation regime. Weak scintillation is typically found at cm- and dcm-wavelengths, but depends on distance so that the critical frequency (νc) increases with distance. Pulsar intensities weaken, however, at high frequencies, since as a rule pulsar spectra have spectral indices steeper than −1.5. This explains why there are practically no measurements of pulsars in the weak scintillation regime.

We report in this paper on first measurements of the correlation function of pulsar intensity fluctuations, of the decorrelation time τd and of the scintillation index md at high frequencies, i.e. at 4.75 and 10.55 GHz in the weak scintillation regime. The detail data analysis, theoretical relations and comparison with theory will presented by Malofeev et al. (1996).

One of the most fascinating features of the polarization of pulsar radio emission is the occurrence of orthogonal modes of polarization. The origin of the modes is not clear, but, whatever their underlying nature, the modes will affect observables such as the instantaneous linear polarization. We derive an idealized distribution of fractional linear polarization assuming that the modes are superposed and compare our result with observations. We discuss how superposed modes may affect the position angle distributions.

Consider a subpulse composed of superposed modes of orthogonal polarization. We assume that each mode is completely linearly-polarized with no dispersion in position angle. If the flux densities of the individual modes are represented by the independent random variables X1 and X2, then the linear polarization of the subpulse is related to X = X1 − X2 and its total intensity is Y = X1 + X2. We denote the means of the mode distributions by μ1 and μ2 and their standard deviations by σ1 and σ2. The means are generally not the same because observed distributions of position angle (e.g. Stinebring et al. 1984) show that the modes generally do not occur with equal frequency. If the individual modes are normally-distributed with identical standard deviations, X and Y are also normally-distributed, independent random variables. In this ideal case, the distribution of fractional polarization, Z =|X/Y|, is

RMs of all 262 pulsars are used to see the large scale magnetic field of our Galaxy. We find that: (1). RMs of 91 pulsars at high galactic latitude (|b| > 8.6°) show two toroidal magnetic rings antisymmetric with respect to galactic plane. These data shouldn’t be used for modelling the large scale field in Galactic disk; (2). pulsars at low galactic latitude are concentrated on the spiral arms; (3). the directions of fields have a similar pitch angle to that of spiral arms.

I review what properties of the white-dwarf companions of recycled pulsars can be inferred from optical observations, and discuss how these can help us understand the characteristics and evolution of these binaries. I focus on spectroscopic observations, describing results obtained recently, and looking forward to what may come.

In observations of six binary millisecond pulsars with the Hubble Space Telescope, we have discovered white dwarf companions to PSRs J0034-0534, J1022+1001, J1713+0747, and J2019+2425 and improved photometry on PSRs J1640+2224 and J2145-0750. Three of the white dwarfs are among the coolest and oldest known. We have determined that the masses for the helium companions are consistent with the expectation based on the core mass of a progenitor that filled its Roche lobe. The cooling times for many of the white dwarfs are much less than the characteristic spin-down times, implying that the spin period at the end of the accretion stage was close to the current period. The initial spin periods calculated are used to place limits on the accretion rate at the end of the low-mass X-ray binary phase. The accretion rates are found to be over an order of magnitude less than the Eddington rate.

I was fortunate enough to have been involved with unravelling the secrets of PSR B1259-63 right from the start. The major part of my PhD thesis was a large survey of the southern galactic plane at an observing wavelength of 20 cm. Confirmation of a further batch of good-looking pulsar candidates from the data processing took place in November 1989 at the Parkes radio telescope in central New South Wales, Australia. My fellow student Jeremy Lim and I were in the control room when the 47.7 ms periodicity of PSR, B1259–63 was confirmed. At first, the slight increase in period from the original detection led us to believe we had discovered a ‘Crab-like’ pulsar. However by the end of the following year we realised we had something much more exciting on our hands.