Electrodynamics of Magnetars III: Pair Creation Processes in an Ultrastrong Magnetic Field and Particle Heating in a Dynamic Magnetosphere

We consider the details of the QED processes that create electron-positron pairs in magnetic fields approaching and exceeding 10^{14} G. The formation of free and bound pairs is addressed, and the importance of positronium dissociation by thermal X-rays is noted. We calculate the collision cross section between an X-ray and a gamma ray, and point out a resonance in the cross section when the gamma ray is close to the threshold for pair conversion. We also discuss how the pair creation rate in the open-field circuit and the outer magnetosphere can be strongly enhanced by instabilities near the light cylinder. When the current has a strong fluctuating component, a cascade develops. We examine the details of particle heating, and show that a high rate of pair creation can be sustained close to the star, but only if the spin period is shorter than several seconds. The dissipation rate in this turbulent state can easily accommodate the observed radio output of the transient radio-emitting magnetars, and even their infrared emission. Finally, we outline how a very high rate of pair creation on the open magnetic field lines can help to stabilize a static twist in the closed magnetosphere and to regulate the loss of magnetic helicity by reconnection at the light cylinder.