Tangential discontinuities and the optical analogy for stationary fields. v. formal integration of the force-free field equations

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Abstract
This paper demonstrates the appearance of tangential discontinuities in deformed force-free fields by direct integration of the field equation ▿ x B = αB. To keep the mathematics tractable the initial field is chosen to be a layer of linear force-free field Bx = + B 0cosqz, By = — B 0sinqz, Bz = 0, anchored at the distant cylindrical surface ϖ = (x 2 + y 2)1/2 = R and deformed by application of a local pressure maximum of scale l centered on the origin x = y = 0. In the limit of large R/l the deformed field remains linear, with α = q[1 + O(l 2/R 2)]. The field equations can be integrated over ϖ = R showing a discontinuity extending along the lines of force crossing the pessure maximum. On the other hand, examination of the continuous solutions to the field equations shows that specification of the normal component on the enclosing boundary ϖ = R completely determines the connectivity throughout the region, in a form unlike the straight across connections of the initial field. The field can escape this restriction only by developing internal discontinuities. Casting the field equation in a form that the connectivity can be specified explicitly, reduces the field equation to the eikonal equation, describing the optical analogy, treated in papers II and III of this series. This demonstrates the ubiquitous nature of the tangential discontinuity in a force-free field subject to any local deformation.

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