Abstract.The classical vacuum gap model of Ruderman & Sutherland, in which spark-associated sub-beams of subpulse emission circulate around the magnetic axis due to the E × B drift of spark plasma filaments, provides a natural and plausible physical mechanism explaining the subpulse drift phenomenon. Moreover, this is the only model with quantitative predictions that can be compared with observations. Recent progress in the analysis of drifting subpulses in pulsars has provided a strong support for this model by revealing a number of sub-beams circulating around the magnetic axis in a manner compatible with theoretical predictions. However, a more detailed analysis revealed that the circulation speed in a pure vacuum gap is too high when compared with observations. Moreover, some pulsars demonstrate significant time variations in the drift rate, including a change of the apparent drift direction, which is obviously inconsistent with the E × B drift scenario in a pure vacuum gap. We attempted to resolve these discrepancies by considering a partial flow of iron ions from the positively charged polar cap, coexisting with the production of outflowing electron-positron plasmas. The model of such a charge-depleted acceleration region is highly sensitive to both the critical ion temperature T i ∼ 10 6 K (above which ions flow freely with the corotational charge density) and the actual surface temperature T s of the polar cap, heated by the bombardment of ultra-relativistic charged particles. By fitting the observationally deduced drift-rates to the theoretical values, we managed to estimate polar cap surface temperatures in a number of pulsars. The estimated surface temperatures T s correspond to a small charge depletion of the order of a few percent of the Goldreich-Julian corotational charge density. Nevertheless, the remaining acceleration potential drop is high enough to discharge through a system of sparks, cycling on and off on natural time-scales described by the Ruderman & Sutherland model. We also argue that if the thermionic electron outflow from the surface of a negatively charged polar cap is slightly below the Goldreich-Julian density, then the resulting small charge depletion will have similar consequences as in the case of the ions outflow. We thus believe that the sparking discharge of a partially shielded acceleration potential drop occurs in all pulsars, with both positively ("pulsars") and negatively ("anti-pulsars") charged polar caps.

A large sample of pulsars was observed as part of the Meterwavelength Single-pulse Polarimetric Emission Survey. We carried out a detailed fluctuation spectral analysis which revealed periodic features in 46% pulsars including 22 pulsars where drifting characteristics were reported for the first time. The pulsar population can be categorized into three distinct groups, pulsars which show systematic drift motion within the pulse window, the pulsars showing no systematic drift but periodic amplitude fluctuation and pulsars with no periodic variations. We discovered the dependence of the drifting phenomenon on the spin down energy loss (Ė), with the three categories occupying distinctly different regions along theĖ axis. The estimation of the drift periodicity (P 3 ) from the peak frequency in the fluctuation spectra is ambiguous due to the aliasing effect. However, using basic physical arguments we were able to determine P 3 in pulsars showing systematic drift motion. The estimated P 3 values in these pulsars were anti-correlated withĖ which favoured the Partially Screened Gap model of Inner Acceleration Region in pulsars.

We propose a new, self-consistent theory of coherent pulsar radio emission based on the non-stationary sparking model of Ruderman & Sutherland (1975), modified by Gil & Sendyk (2000) in the accompanying Paper I. According to these authors, the polar cap ( with a radius r p ≃ 10 4 P −0.5 cm ) is populated by about (r p /h) 2 sparks of a characteristic perpendicular dimension D approximately equal to the polar gap height scale h ∼ 5 × 10 3 P 3/7 cm, separated from each other also by about h. Each spark reappears in approximately the same place on the polar cap for a time scale much longer than its 10 µs life-time and delivers to the open magnetosphere a sequence of e − e + clouds which flow orderly along a flux tube of dipolar magnetic field lines. The overlapping of particles with different momenta from consecutive clouds leads to effective two-stream instability, which triggers electrostatic Langmuir waves at the altitudes of about 50 stellar radii. This is the only known instability which can develop at the low altitudes, where the observed pulsar radio emission originates. The electrostatic oscillations are modulationally unstable and their nonlinear evolution results in formation of "bunchlike" charged solitons. A characteristic soliton length along magnetic field lines is about 30 cm, so they are capable of emitting coherent curvature radiation at radio wavelengths. A perpendicular cross-section of each soliton at radiation altitudes follows from a dipolar spread of a plasma cloud with a characteristic dimension near the star surface of about D ≈ h ≈ 50 meters. The net soliton charge is about 10 21 fundamental charges, contained within a volume of about 10 14 cm 3 . For a typical pulsar, there are about 10 5 solitons associated with each of about 25 sparks operating on the polar cap at any instant. One soliton moving relativisticaly along dipolar field lines with a Lorentz factor of the order of 100 generates a power of about 10 21 erg/s by means of curvature radiation. Then the total power of a typical radio pulsar can be estimated as being about 10 27−28 erg/s. The energy of the soliton curvature radiation is supported by kinetic energy of secondary electron-positron plasma created by the primary beam produced by the accelerating potential drop within the polar gap. A significant fraction of kinetic energy generated by sparks is radiated away in form of the observed coherent radio emission.

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We consider the curvature radiation of the point-like charge moving relativistically along curved magnetic field lines through a pulsar magnetospheric electron-positron plasma. We demonstrate that the radiation power is largely suppressed as compared with the vacuum case, but still at a considerable level, high enough to explain the observed pulsar luminosities. The emitted radiation is polarized perpendicularly to the plane of the curved magnetic filed lines coincides with that of extraordinary waves, which can freely escape from the magnetospheric plasma. Our results strongly support the coherent curvature radiation by the spark-associated solitons as a plausible mechanism of pulsar radio emission.

A detailed analysis of nulling was conducted for the pulsars studied in the Meterwavelength Single-pulse Polarimetric Emission Survey. We characterized nulling in 36 pulsars including 17 pulsars where the phenomena were reported for the first time. The most dominant nulls lasted for a short duration, less than five periods. The longer duration nulls extending to hundreds of periods were also seen in some cases. A careful analysis showed the presence of periodicities in the transition from the null to the burst states in 11 pulsars. In our earlier work fluctuation spectrum analysis showed multiple periodicities in 6 of these 11 pulsars. We demonstrate that the longer periodicity in each case was associated with nulling. The shorter periodicities usually originate due to subpulse drifting. The nulling periodicities were more aligned with the periodic amplitude modulation indicating a possible common origin for both. Most prevalent nulling lasts for a single period and can be potentially explained using random variations affecting the plasma processes in the pulsar magnetosphere. On the other hand, the longer duration nulls require changes in the pair production processes that need an external triggering mechanism for the change. The presence of periodic nulling puts an added constrain on the triggering mechanism which also needs to be periodic.

In this study we propose a classification scheme for the phenomenon of subpulse drifting in pulsars. We have assembled an exhaustive list of pulsars which exhibit subpulse drifting from previously published results as well as recent observations using the Giant Meterwave Radio Telescope. We have estimated detailed phase variations corresponding to the drifting features. Based on phase behaviour the drifting population was classified into four groups : coherent phase-modulated drifting, switching phase-modulated drifting, diffuse phase-modulated drifting and low-mixed phase-modulated drifting. We have re-established the previous assertion that the subpulse drifting is primarily associated with the conal components in pulsar profile. The core components generally do not show the drifting phenomenon. However, in core emission of certain pulsars longer periodic fluctuations are seen, which are similar to periodic nulling, and likely arise due to a different physical phenomenon. In general the nature of the phase variations of the drifting features across the pulsar profile appears to be associated with specific pulsar profile classes, but we also find several examples that show departures from this trend. It has also been claimed in previous works that the spin-down energy loss is anti-correlated with the drifting periodicity. We have verified this dependence using a larger sample of drifting measurements.