We have examined, by means of numerical simulation, the effects of the magnetic mirror, the backscattering primary electrons, and the magnetospheric hot ions on the field-aligned scale length (L) of one-dimensional double layers. In our simulation model, the external magnetic field is allowed to converge radially with spherical symmetry and a constant potential difference is applied across the system. The results show that the double-layer scale length varies from : (1) localized (L>10&lgr;D) in the absence of the backscattered primary electrons and the magnetospheric hot ions; (2) partially extended (L~100&lgr;D< system length) if only the backscattered primary electrons are present; to become (3) fully extended (L≲ system length and increase as system length increases) if both the backscattered primary electrons and the magnetospheric hot ions are present.