Accretion by magnetic neutron stars. II - Plasma entry into the magnetosphere via diffusion, polar cusps, and magnetic field reconnection

A variety of entry modes were investigated to determine whether most of the accreting plasma enters the magnetosphere as a result of hydromagnetic instability or via other means. It is shown that diffusion is never important under the conditions of interest, nor is the loss-cone entry through the polar cusps when the plasma is collisionless. Although the loss-cone entry rate can be significantly increased if the plasma in the cusps cools and becomes collisional, this cannot stabilize the magnetosphere. The descent of the cusps cannot be the dominant entry process if the star has a persistent luminosity greater than about 10 to the 36th erg/s and a substantial fraction of the magnetosphere is illuminated. This, however, can be a significant entry process for much lower luminosities or strongly anisotropic illumination. The possibility that plasma entry via reconnection can stabilize the magnetosphere is also unlikely.