The width of a resonant peak in the spatial distribution of the ULF Pc5 pulsation amplitude at high latitudes is controlled by the dominant dissipation mechanism of Alfven waves in the ionosphere-magnetosphere system. Among possible mechanisms, mostly Joule dissipation and dispersive effects are considered to terminate the growth and narrowing of a resonant peak pumped by an external source. We estimate the width of the resonance which may be provided by the field-aligned electric field in a thermal, mirror-confined collisionless plasma. We consider a simple case, in which a potential drop is confined within a narrow region above the ionosphere, called the auroral acceleration region. Theoretical estimates show that the mirror-force mechanism can dominate over the ionospheric dissipation and dispersive effects in auroral regions with a sufficiently large field-aligned potential drop. Therefore ground-based monitoring of the resonance width (e.g., with the gradient method) may be used as an indicator of the occurrence of large-scale parallel electric fields in the outer magnetosphere. ¿ 2001 American Geophysical Union