In a magnetospheric condition, particles are often precipitated into the ionosphere of the host planet. When a flux tube is not interacting with those neighboring it, charge lost through its open ends to the ionosphere must vanish to ensure charge quasi-neutrality in the flux tube as a whole. To achieve this, a parallel electric field must be present to impede the faster electron motion. Electric fields so arisen are usually referred to as ambipolar fields. In this paper, we discuss a number of specific aspects of the ambipolar coupling, including the value of the ambipolar potential drop, the effects of loss cone depletion, the outflow of ionospheric electrons extracted by the ambipolar field, and point-to-point distribution of the potential. Although the noninteractive assumption for the flux tube restricts the validity of the treatment to cases where field-aligned currents are very weak, the construction of the model sheds new light on an important problem in magnetospheric physics. ¿ American Geophysical Union 1993