ON CURRENT SHEET APPROXIMATIONS IN MODELS OF ERUPTIVE FLARES

We consider an approximation sometimes used for current sheets in flux -rope models of eruptive flares. This approximation is based on a line ar expansion of the background field in the vicinity of the current sh eet, mid it is valid when the length of the current sheet is small com pared to the scale length of the coronal magnetic field. However, we f ind that flux-rope models which use this approximation predict the occ urrence of an eruption due to a loss of ideal-MHD equilibrium even whe n the corresponding exact solution shows that no such eruption occurs. Determination of whether a loss of equilibrium exists can only be obt ained by including higher order terms in the expansion of the field or by using the exact solution.