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Charge Densities above Pulsar Polar Caps

The ability of the neutron star surface to supply all or only part of the charges filling the pulsar magnetosphere is crucial for the physics prevailing within it, with direct consequences for the possible formation of pair-creation regions. We evaluate the conditions for e- emission from pulsar surfaces for a simple Goldreich-Julian geometry taking both thermal and field emission processes into account. Using recently published estimates for the equation of state at the neutron star's surface, we show that, for a large range of Tsurf,B, and P, the liberated charges will fully screen the accelerating B-parallel electric field E∥. For surface temperatures Tsurf < 2 × 105 K a balance between field emission of electrons and shielding of the field will occur. Even in the overidealized case of Tsurf = 0 one can expect a prodigious supply of electrons, which will weaken the accelerating E∥. We calculated the motion of electrons along selected polar field lines numerically for the low-temperature, field emission scenario yielding their Lorentz factors, as well as the produced radiation densities. Inverse Compton and later curvature losses are seen to balance the acceleration by the residual electric fields. We found that the conditions for magnetic pair production are not met anywhere along the field lines up to a height of 1500 pulsar radii. We did not a priori assume an "inner gap," and our calculations did not indicate the formation of one under realistic physical conditions without the introduction of further assumptions.