A model for a stable coronal loop

A plasma-physics model of a stable active-region arch is presented which corresponds to the structure observed in the EUV. Pressure gradients are seen, so that the equilibrium magnetic field must depart from the force-free form valid in the surrounding corona. The data and the approximate cylindrical symmetry are exploited to develop a modified form of the commonly assumed sheared-spiral structure. The dynamic MHD behavior of this pressure/field model is then evaluated by the Newcomb criterion (1960) from controlled-fusion physics, and the results show short-wavelength stability in a specific parameter range. For pressure profiles with widths of the order of the magnetic-field scale, the possibility is demonstrated that such arches can persist for reasonable periods. Finally, the spatial proportions and magnetic fields of a characteristic stable coronal loop are described.