Estelle Asseo

The role of multipolar magnetic fields in pulsar magnetospheres

ABSTRA C T We explore the role of complex multipolar magnetic fields in determining physical processes near the surface of rotation powered pulsars. We model the actual magnetic field as the sum of global dipolar and star-centred multipolar fields. In configurations involving axisymmetric and uniform multipolar fields, ‘neutral points’ and ‘neutral lines’ exist close to the stellar surface. Also, the curvature radii of magnetic field lines near the stellar surface can never be smaller than the stellar radius, even for very high-order multipoles. Consequently, such configurations are unable to provide an efficient pair-creation process above pulsar polar caps, necessary for plasma mechanisms of generation of pulsar radiation. In configurations involving axisymmetric and non-uniform multipoles, the periphery of the pulsar polar cap becomes fragmented into symmetrically distributed narrow subregions where curvature radii of complex magnetic field lines are less than the radius of the star. The pair-production process is only possible just above these ‘favourable’ subregions. As a result, the pair plasma flow is confined within narrow filaments regularly distributed around the margin of the open magnetic flux tube. Such a magnetic topology allows us to model the system of 20 isolated subbeams observed in PSR B0943a 10 by Deshpande & Rankin. We suggest a physical mechanism for the generation of pulsar radio emission in the ensemble of finite subbeams, based on specific instabilities. We propose an explanation for the subpulse drift phenomenon observed in some long-period pulsars.

Boundary Effects in the Pulsar Magnetosphere

The source of coherent pulsar radio emission has often been related to the development of plasma instabilities in pulsar polar cap models1. These instabilities have been studied either for a relativistic beam of e(or e+), or for a relativistic plasma of eand e+ , or for the interaction between both, mainly by using straight geometry and an approximation of infinite and homogeneous beam and plasma. For ;I tentative interpretation of radio observations, according to Rankins classification of pulsar profiles which show a hollow cone or a full cone of radio eiuission2, we include in our analysis curved geometry and finite extent of the beam3. Indeed, relativistic particles are constrained to move along the extremely strong pulsar magnetic field that is dipolar and thus curved. They form a flowing beam that has a finite extent, being limited to the bundle of open magnetic field lines present i n the emission region. This beam is bounded by external plasmas that have different characteristics, namely the plasma that lie on closed field lines and eventually the plasma inside the hollow cone.

Propagation of Large Amplitude Pulsar Waves

Large amplitude waves (hereafter l.a.w.) have been studied mainly in connection with pulsars. A rotating neutron star, with an intense non-aligned magnetic dipole field, surrounded by a vacuum, radiates beyond the light cylinder distance, large amplitude electromagnetic vacuum waves of very low frequency (Ostriker and Gunn, 1969). In such a rotating configuration electromagnetic effects completely dominate and imply the existence of a relativistic plasma in the pulsar magnetosphere (Goldreich and Julian, 1969). Because the oblique vacuum model predicts a value of 3 for the braking index of the pulsar whereas the observed or computed values are different, a realistic model has to include both the relativistic plasma outflow and the electromagnetic wave emission. The vacuum wave model has been changed to include self-consistent plasma effects (Asseo et al., 1975; Asseo et al., 1978) in plane geometry and inhomogeneities linked to spherical geometry (Asseo et al., 1981). This results in very restrictive conditions for the possibility of propagation of the l.a.w.