Two-dimensional Configuration and Temporal Evolution of Spark Discharges in Pulsars

Abstract

We report on our investigation of the evolution of a system of spark discharges in the inner acceleration region (IAR) above the pulsar polar cap. The surface of the polar cap is heated to temperatures of around 106 K and forms a partially screened gap (PSG), due to thermionic emission of positively charged ions from the stellar surface. The spark lags behind corotation speed during their lifetimes due to variable E × B drift. In a PSG, spark discharges arise in locations where the surface temperatures go below the critical level (T i ) for ions to freely flow from the surface. The spark commences due to the large drop in potential developing along the magnetic field lines in these lower temperature regions and subsequently back-streaming particles heat the surface to T i . Regulation of the temperature requires the polar cap to be tightly filled with sparks and a continuous presence of sparks is required around its boundary since no heating is possible from the closed field line region. We estimate the time evolution of the spark system in the IAR, which shows a gradual shift in the spark formation along two distinct directions resembling clockwise and anticlockwise motions in two halves of the polar cap. Due to the differential shift of the spark pattern in the two halves, a central spark develops representing the core emission. The temporal evolution of the spark process was simulated for different orientations of a non-dipolar polar cap and reproduced the diverse observational features associated with subpulse drifting.