On the Formation of Inner Vacuum Gaps in Radio Pulsars

The problem of formation of the Ruderman-Sutherland type inner vacuum gap inneutron stars with ${\bf\Omega}\cdot{\bf B}<0$ is considered. It is argued bymeans of the condition $T_i/T_s>1$ (where $T_i$ is the critical temperatureabove which $^{56}_{26}$Fe ions will not be bound at the surface and $T_s$ isthe actual temperature of the polar cap surface heated by the back-flow ofrelativistic electrons) that the inner vacuum gap can form, provided that theactual surface magnetic field is extremaly strong ($B_s\gtrsim 10^{13}$ G) andcurved (${\cal R}<10^6$ cm), irrespective of the value of dipolar componentmeasured from the pulsar spin down rate. Calculations are carried out forpulsars with drifting subpulses and/or periodic intensity modulations, in whichthe existence of the quasi steady vacuum gap discharging via ${\bf E}\times{\bfB}$ drifting sparks is almost unavoidable. Using different pair-productionmechanisms and different estimates of the cohesive energies of surface ironions, we show that it is easier to form the vacuum gap controlled by theresonant inverse Compton scaterring seed photons than by the curvatureradiation seed photons.Comment: 14 pages, 3 figure